An Investigation of Green Roofs to Mitigate Air Pollution With Special Reference to Tehran, Iran Dissertation

Abstract

Many inquiries done into the use of green roofs as pollution control technologies have shown significant relationship between green roofs and pollution control. This inquiry focuses on the use of green roofs to mitigate air pollution when retrofitted to conventional roofs on any building in many parts of the world today.

The paper draws on lessons learnt by retrofitting green roofs to building sin urban areas that have limited space to accommodate conventional gardens. Thus, the study investigates the use of green roofs as measures to mitigate air pollution with special reference to Tehran, Iran.

In addition to that, the current research synthesizes information from literature review son the history of green roofs and types of green roofs. Further inquiries continue into the environmental benefits gained from the use of green roofs in different perspectives and the limitations of green roofs as pollution control technologies.

The paper also examines case studies of green roofs and models the cost benefit analysis of green roofs on the energy and quality of air from a green roof. The model provides a guide on the installation costs and energy saving benefits of a green roof. A further inquiry into the impact of mitigating air pollution on the environment with Tehran as a case study informed the inquiry.

The study culminated in to both external and internal analyses of IKIA, the proposed site for the airport facility. In addition to that, the study has focused on energy consumption activities to understand the chemical processes of carbonCO₂ cycles, carbon related emissions, and other pollution emissions affecting the environment.

A regional and site analysis of the proposed site yielded plenty of information on prevailing climatic conditions at the proposed site for airport construction. This study has also established weather conditions, the rate of airflow, and prevalent wind directions in the proposed construction site for an airport facility.

On the other hand, the paper examines air pollution levels, different types of plants that thrive in desert and semi-desert areas, carbon footprints experienced due to air pollution in the proposed area, and the likely benefits gained from the use of green roofs as a pollution control technology.

Aims

• The main aim of the study is to conduct an investigation into green roofs as methods to mitigate air pollution with special reference to Tehran, Iran.
• To address the question of the effectiveness of green roofs in Tehran, Iran as a method to manage CO₂ emissions into the environment

Objectives

The study objectives were:

• To investigate the history of green roofs.
• To investigate the different types of green roofs.
• To examine the benefits gained from the use of green roofs.
• To conduct an inquiry into the use of green roofs as methods to mitigate air pollution.
• To mathematically model and analyze the benefits of using green roofs for mitigating the effects of air pollution.
• To analyze the characteristics of a proposed site for the construction of an airport (IKIA) in Teheran by identifying plants that can be grown in desert and semi-desert conditions for use on green roofs.
• To analyze the theories that underlie the use of green roofs and green roofs carbon management systems in order to establish “best practices “for green roofs carbon management systems.

The main aim of the study is to conduct an investigation into the methods used in green roofs to mitigate air pollution with special reference to Tehran, Iran.

Thus, the aim of the research is to inquire into the basic information on the concept of green roofs, to answer the research questions on different attributes of green roofs, methods used to construct green roofs, and the resulting benefits from green roofs.

The study provides detailed information about green roofs particularly for interested parties either in the private or in the public sector on the right choice to make when selecting a green roof with specific design attributes for specific end-user needs. The research is a baseline upon which any interested party can select a green roof.

Typically, any interested party can select a green roof from either of the two options. These options include intensive green roofs and extensive green roofs. On the other hand, the method forms the basis upon which one can identify the benefits of installing a green roof in Tehran and specifically in IKIA city.

In addition to that, the method would also form the baseline to enhance the development pace of a green roof pilot project at the Imam Khomeini Airport City by the Ministry of transportation, the main area of focus. From a historical perspective, green roofs have been in existence since antiquity, with archeological evidence showing their existence to date back to Babylonian times.

At the time, a number of green roofs designed with different objectives in mind with many of them having a lot of aesthetical significance. In addition to that, the purpose of green roofs was to fulfill the views of philosophers, the regard and esteem held for kings and highly regarded people in the Middle East societies.

It was, however, in the modern times that hidden benefits associated with green roofs reinforced the need to study about their construction. These benefits became more apparent with new and emerging challenges associated with climate change mainly due to several activities that lead to air pollution.

That has made governments and individuals in the recent past to realize the importance of incorporating green roofs as pollution control mechanisms on conventional roofs.

Literature review

The history of green roof dates back to many centuries. This research shows that different types of green roofs, constructed in different regions of the world, date back many centuries. These roofs range from ordinary brown roofs to green roofs. Among the regions with documentary evidence on green roofs are the Scandinavian countries and many parts of the European continent.

However, in 1960, the Germans were the first to develop new forms of green roofs. At that time, the purpose of green roofs was to attract tourists and to serve as air pollution mitigating technologies. Research shows that green roofs exist in two categories. These include intensive and extensive green roof. Each of the types of the green roofs has specific and unique attributes making it more applicable to specific environments.

The literature review conducted in this paper details specific and general methods of integrating green roofs to conventional roofs. In addition to that, different construction methods, different material requirements, and maintenance and operation requirements of green roof systems form a significant part of the study.

The methodology of the study is a literature review, which is also a qualitative study of green roof systems. On the other hand, a quantitative study in this paper had its basis on document analysis about the area around Tehran, Iran. The aim is to study the use of green roofs to mitigate air pollution with special reference to Tehran, Iran.

History of green roofs

Modern green roofs are a concept that had its beginning many centuries ago when the construction of green roofs had started in many parts of Europe. According to Basset al, (2003), green roofs were mostly prevalent in the Northern Scandinavian countries with the first modern green roof developed in Germany in 1960.

It was after this, according to Grant et al, 2003, that “the development trends for green roofs spread out to other countries in Europe and USA” p.2. Basset al, (2003) and Grant (2006) suggest a common ground to support the argument that green roofs had been constructed for aesthetical purposes before later developments were designed to address the issue of environmental pollution.

Several authors have investigated the origin and significance of green roofs both in history and in the modern world. The results support the conclusion that green roofs have a history that dates back many centuries. It was to meet the esteem and regard held for kings in the past that many people constructed green roofs. At the time, there was no need to control CO₂ emissions into the environment.

Some of the hidden benefits gained from constructing green roofs, according to studies byGrant, Engleback, Nicholson, Gedge, Frith& Harvey (2003) include the capacity of a green roof to control runoff water and mitigate upon the polluting effects of CO₂ emissions. Oberndorfer, Lundholm, Bass, Coffman, Doshi, Dunnett, Gaffin, Köhler, Liuk and Rowe (2007) have also established similar facts.

On the other hand,Victoria Transport Policy Institut (2011), Grant et al (2003), and Gedge& Kadas (2004) have additionally suggested that the ever diminishing sizes of land is another reason to develop green roofs in many parts of the world and in particular in Tehran, Iran. Studies by Gedge and Kadas (2004) suggest that the use of green roofs is partly due to increasing population densities in urban areas.

On the other hand, the realities of environmental pollution and the resulting impact on the environment and global climate change are other contributing factors. Not many authors have written about the history of green roofs. However, a couple of authors have provided some examples of the history of green roofs including the green roofs of the Romanian Herculaneum sites and the hanging gardens of Babylon(Basset al, (2003).These studies show that green roofs thrived in different parts of the world centuries back.

In addition to that, these studies also show other areas with archeological evidence that points to prior existence of green roofs to include the Scottish Island with architectural designs that revolve around earth-sheltered huts, pitch covered roofs, and the American mid-west with green roofs made from turf (Grant et al, 2003).

Fig. 1,Grant, 2010.

One of the outstanding examples of green roofs, with significant contributions to the modern green roofs was the hanging roof of the Semiramisians. These roofs are among the Seven Wonders of the World. On the other hand, modern green roofs are the most compelling replicas of the Seven Wonders of the World with many features and benefits derived from the complex design and manner of construction.

Additional benefits gained from green roofs include the roofs, which serve as tourist attractions besides serving as methods to mitigate upon the polluting effects of CO₂ into the environment (Basset al, (2003). The type of component used to construct a green roof defines the type of vegetation grown on a green roof.

Among the materials used in their construction of a green roof includes waterproofing materials, the medium top surface for the plants, the concrete bricks used to make drainage patterns, and the filtering patterns of the roofs. These and other features, according to Rosenfeld, Akbari, Bretz,Fishman, Kurn,Sailor, Taha (1995) add up to the extra features used to define a green roof. In addition to that, the standards used in their design and development are important issues to consider.

Green roofs

Liu& Minor (2005) and Liu (2004) define a green roof as an architect constructed on a building with a variety of vegetation to fulfill aesthetical appeals of different people. In addition to that, green roofs are-constructed purposely to meet environmental conservation needs, such as air pollution control besides a number of other benefits gained from their use.

Green roofs fall into intensive green roofs, semi-intensive green roofs, and extensive green roofs. These categories rely on the purpose of the green roof, the substrate type, the cost benefits of the green roof, maintenance requirements, and pollution control benefits (Grant et al, 2003).

Research into some of the benefits of green roofs show these roofs to have a great potential as pollution control mechanisms due to their capacity to enhance the energy performance of a building, the ability to improve air quality inside and outside building, and the capacity to provide improved watershed capabilities particularly in urban areas.

In addition to that, another benefit is the ability to absorb carbon dioxide from the atmosphere that is a carbon management and control mechanism (Villarreal, Semadeni-Davies & Bengtsson, 2004).Villarreal et al, (2004). Many authors agree that environmental issues particularly air pollution control is a key benefit gained by constructing green roofs on buildings.

Trumper, Bertzky,Dickson, van der Heijden, Jenkins, and Manning(2009)have shown scientific evidence which suggests that the release of large quantities of CO₂into the atmosphere as a result of industrial activities is one of the major sources of air pollution. Scientific evidence shows that the problem is prevalent in many parts of the world today.

Trumper, Bertzky,Dickson, van der Heijden, Jenkins and Manning (2009) have concluded these studies that releasing large quantities of CO₂ into the environment is the main cause of climate change and greenhouse gas effects. Estimates show that up to tenfold and even more quantities of CO₂ released into the environment every year is due to air pollution activities.

The blame on the principal sources of CO₂ has been a matter of exchange between developing and developed counties. Developed countries blame developing countries due to the poor and unrefined technologies used in developing countries. On the other hand, developing countries hold a strong view that developed countries are the principal sources of CO₂.

While the rage and debate goes on, greenhouse gas effects are being felt (Trumper, Bertzky, Dickson, van der Heijden, Jenkins&Manning, 2009). Thus, the need for a pollution control mechanism in both situations compels the option to use green roof systems.

Research into the principal sources of CO₂, which is the principal cause of greenhouse gas effects, indicates significant contributions from developed and developing countries in their different production capacities. Developing countries are facing exponential population growth, rapid industrialization like China, Brazil, Iran, and many others.

In addition to that, developing countries use unrefined technologies in their quest to industrialize and to compete in the global scene. That has further reinforced need for green roofs as air-pollution mitigation technologies. To keep CO₂ emissions and the resulting effects minimal, different strategies and measures include the use of green roof systems are in place.

Other measures in place include governments formulating greenhouse gas policies, use of environmental management methods, and other methods that include constructing green roofs on buildings. Additional methods include the use of carbon capture methods and clean technologies, methods that contribute significantly to carbon management strategies.

Each of these technologies has proven successful to significant levels reinforcing their use as pollution control mechanisms. However, one of the technologies proven successful, both in theory and practice, is the construction of green roofs on conventional roofs. These are facts established with significant support from different authors including (Trumper et al, 2009; Grant et al, 2003; Wong, Chen, Ong& Sia, 2003).

Different Types of Green roofs

As briefly mentioned above, green roofs fall into extensive and intensive green categories. According to Ong and Sia (2003), many authors view green roofs from different perspectives depending on the aim of constructing a green roof and its area of application. The following study is a review of literature on different types of green roofs and the benefits associated with each type of green roof.

Extensive Green roofs

Ong and Sia (2003) view extensive green roofs as a technology that provides a range of benefits to end user and the environment in general. Thus, both public and private users experience these benefits. Public benefits have potential implications as air pollution mitigating methods while private benefits are for aesthetical appeals.

Other benefits associated with the environment include improved air quality, wide-ranging biodiversity, and enhancement of landscape views. On the other hand, Grant et al, (2003) associates these benefits to the environment with particular focus on carbon mitigation capabilities. Thus, it is important to examine some of the characteristics of green roofs.

Wark, Christopher, Wark&Wendy (2003) determined that high porosity substrates are some of the characteristics of extensive green roofs. Further calculations show that the substrate types extend to a depth of between 2 cm and 20 cm (Grant et al, 2003). The Rainwater absorption capacity of an extensive green roof is 75% of the incident rainwater.

In addition to that, extensive green roofs have a water retention capacity of approximately 25% of the rainwater incident on them when studied over a period of 2 months. On the other hand, intensive green roof systems have shown a water absorption capacity of 60% of the rainwater falling on it.

On the other hand, Wark, Christopher, Wark, and Wendy (2003) later established that extensive green roofs constructed on roof required an installation angle of 33% to the horizontal plane. One important attribute of an extensive green roof is its retrofit characteristics. The retrofit characteristics of an extensive green roof are that the roof does not require additional structural support.

On the other hand, Grant et al, (2003) and Wark, Christopher, Wark and Wendy’s (2003) findings indicate extensive green roofs to be light in weight compared with other types green roof. However, Wark, Christopher, Wark and Wendy (2003) and Villarreal, Semadeni-Davies and Bengtsson’s(2004) findings indicate that extensive green roofs demand little or no irrigation for the vegetation to thrive on them.

Typically, that is partly due to the water absorption capacity of the green roof and drought resistant vegetation grown on the roofs. The substrate type on extensive green roof systems has high porosity, lightweight, and low organic composition characteristics. Well-designed and constructed extensive green roofs have shown a high storm-water management retention capacity of 50% of the incident rainwater.

A detailed study by Villarrealand Bengtsson (2005) indicates that a50 mm deep gravel bed, receiving rainwater at a rate of 50 liters per square meter of the green roof surface enables an extensive green roof to acquire a water retention capacity of 50% of the incident rainwater. That is also in consideration of the fact that 100 mm of rainfall fell in that locality in the period under consideration (Grant et al, 2003).

Based on general knowledge, the 100 mm of water readings are from a rain gauge. A rain gauge is a scale commonly used by meteorologists to determine the numerical value of the amount of rainwater falling in a given area. The rain gauge is calibrated in mm from which readings are taken and recorded for a given period. Gradually, the records are analyzed to show the actual estimate of the amount of rain falling in a specific area.

A detailed analysis of rainwater measurements and analysis is beyond the scope of this study. From an economic perspective, extensive green roofs are comparatively cheaper to maintain. Other benefits include the lower cost of construction since extensive green roofs use single layer construction methods.

On the other hand, extensive green roofs require low mowing and weeding activities and can accommodate additional weights. These weights lie between 70 and 170 kg for every square meter of the surface of a green roof (VanWoert, Rowe, Andresen, Rugh, Fernandez, R. T. & Xiao, 2005).

From the “perspective of the community of plants grown on extensive green roofs, it is important to note that drought tolerant plants are the best choice for extensive green roofs” (Grant, et al 2003). A cross sectional design profile of the green roof is illustrated in figure 2 below.

Fig 2, Grant, 2003.

Studies by Toronto and Region Conservation (2006) and The Green Roof Code (2011) have shown that soil requirements for a green roof, the growing medium should be less dense compared with natural soils. Natural soils therefore are much denser.

A comparative analysis of the soil used in the extensive roof systems with natural soils indicates that the planting medium for the weight of an extensive roof lies between 10 and 25 pounds per square meter compared with natural soils, which weigh 100 pounds in a similar size. The weight factor is a benefit for extensive green roofs since they retrofit on conventional roofs without any reinforcements.

A detailed examination of the above extensive green roof shows that the roof is made of a container, a filter layer, a steel dock at the bottom of the structure, an insulation mechanism, a protective layer between a membrane, and a drainage layer.

Several studies by Theodosiou (2003) have shown that extensive green roofs can accommodate drought resistant plants as mentioned elsewhere in this study. Many of the species of plants grown on extensive green roofs belong to the meadow family. These include herbs, turf plants, and the sedum family of plants (Theodosiou, 2003).

Another element of the extensive green roof system is the filtrate layer. According to Takebayashi and Moriyama (2007),the functionality of the filtrate layer is to capture and keep essential components and prevent roots from the growing plants penetrating into the roof that might damage the roof below (Wark & Wark, 2003).

The protective layer prevents chemicals and water from penetrating into the roof below. Deeper penetrations may cause the roof dampen from seeping water. On the other hand, the drainage layer is another component that forms part of the structure of the extensive green roof. Purposely, the drainage layer provides an exit for excess water that has not been absorbed into the entire green roof system.

According to Takebayashi and Moriyama (2007), any design of an extensive green roof system must reflect different characteristics. These include the protective, membrane and protective layers as shown in fig 3 above. According to the design, the green roof stands on steel rollers with different functions.

Steel rollers provide the flexibility to move the green roof system according to user requirements and installation guides. In addition to that, steel rollers integrated into the design of a green roof system provide contact with the conventional roof on which the entire roof system rests. That makes the green roof offer additional functionalities and other benefits(Mentens, Raes& Hermy, 2005).

Another outstanding feature of the green roof system shown in fig 3 is the steel container. The container accommodates all materials used in the construction of the entire green roof system, a fact observed from practical applications of the roof components used in its construction (Mentens, Raes, and Hermy, 2005).

Thus, the overall structure is an assembly of plants, planting media, planting container, filtering layer, drainage layer, protective layer, the membrane, the insulation, and the steel roof deck. The need to use extensive green roofs is due to the benefits accruing from their use on conventional roofs.

Thus, it is important to make informed decision on the choice of a green roof based on the benefits realized from their use as discussed elsewhere. In addition to that, the flexibility of using these green roofs on different types of conventional roofs and the type of plants grown on them are additional considerations to make.

One outstanding attribute of an extensive green roof is that it requires little or no irrigation, making it suitable for semi desert and desert conditions. In addition to that, installing a green roof demands a background area that meets the requirements of an extensive green roof. Technical details and material requirements for extensive green roofs rely on the locality where the roof is to be constructed.

On the other hand, design considerations play a central role in deciding on the type of materials to use in that specific area. Research shows that common design considerations tend to be general irrespective of the area where the green roof system is constructed. Morikawa, Takahasi, and Kawamura (1998) developed a list of design considerations which include:

1. The structure of the building the green roof is to be constructed.
2. The load carrying capacity of the building on which the green roof system is to be constructed.
3. The load capacity of the green roof system.
4. Policy and standard requirements adhered to by the user installing the green roof system.
5. Water retention and drainage patterns.

On the other hand, study reports by Morikawa, Takahasi, and Kawamura (1998) further show that several quality issues need critical considerations before commencing the construction of a green roof on a conventional roof. These quality issues, according to Morikawa, Takahasi, Kawamura (1998), and Liu (2004) include:

1. The process of installing membranes is the responsibility of qualified and experienced technical staff.
2. There should be undivided attention particularly when installing each component of the entire green roof system.
3. Procurement of primary water proofing materials should be from certified sources.
4. It is important to engage a qualified inspector to identify the appropriateness of the membranes and other materials used in the construction and attachment of the green roof to the conventional roof.
5. Materials used in the construction should not be opened before reaching the construction site to authenticate their sources and integrity.
6. Care should be taken to ascertain that job conditions qualify the fact that the health of the workers involved in the work is taken care of.
7. Water proofing membranes should be 0.036 inches thick. In addition to that, the physical properties of the material used should pass verification tests against standard specifications contained in construction manuals.
8. Any roofing material should measure to a nominal value of 0.043 inches or as best defined by different specifications fit to specific environments. Other requirements according to the following details in table 1 below.

Table 1. Requirements for a green roof.

Liu, 2004

Membrane installation should follow well-defined steps and careful installation procedures to avoid the possibility of puncturing or damaging the membrane. A damaged membrane has the chance of negating the water retention capabilities of the green roof.

Membrane installation should follow instructions outlined in the specifications document of the manufacturer. In addition to that, materials such as rolls used throughout the structure should also follow specification installation standards outlined by the manufacture and other general industry requirements.

In their investigations, Morikawa, Takahasi, Kawamura (1998), and Liu (2004) have established the requirement to flash all penetrating walls in the entire structure of a green roof. That includes drainage pipes and other components of the system. However, Morikawa, Takahasi, Kawamura (1998), and Liu (2004) have established that facts that pitch does not form part of the waterproofing membrane structure.

According toFarzaneh (2005), Liu (2004), and Foxon (2002), water tests occur before declaring a green roof as complete and commissioning it for use. General test requirements are that the area for planting should continuously have a water presence of 24 hours to a minimum depth of 2 inches.

Semi-Intensive Green roofs

Semi-intensive green roofs are a hybrid of intensive and extensive green roofs. Thus, these kinds of green roofs commonly referred to as green roofs are rarely constructed. However, there is little or no literature on these kinds of green roofs (Wark, Christopher& Wark, Wendy, 2003).

Intensive Green roofs

Intensive green roofs are another functional method used to minimize the polluting effects of carbon on the environment. A critical analysis and evaluation of intensive green roofs from a functional point of view shows intensive green roofs to be aesthetically beneficial. In addition to that, space utilization and leisure need are additional benefits gained from green roof systems.

On the other hand, intensive green roofs are characterized by demanding attributes, which include 20 cm substrate depths, frequent irrigation, and restricted access based on imposed legal requirements. On the other hand, maintenance requirements are similar to maintenance requirements for extensive green roofs (Kadas, 2006).

One typical characteristic of intensive green roofs is the communities of plants grown on the roofs are restricted by prevailing weather and climatic conditions. To grow the plants on these roofs, substrate requirements included good exposure, a well-exposed garden, and the use of irrigation facilities, which vary significantly from the requirements of an extensive green roof.

The following diagram represents a typically detailed view of an intensive green roof (Grant et al, 2003).

Fig 3, Grant, et al, 2003.

A general overview of green roofs indicates a modular design advantaged by the use of specialized planting trays (Grant, et al, 2003).

Construction Methods

It is important to consider the secondary functions of green roofs before starting the construction processes.Mentens, Raes and Hermy (2005) argue that the main reason driving behind the construction of a green determines the choice of a green roof. Reasons for constructing green roofs are diverse. These, include aesthetical appeals that people view for enjoyment, environmental benefits including an approach to mitigateCO₂ emissions into the environment, and other benefits realized from the use of green roofs.

In addition that, it is important to take into consideration the purpose of a green roof by considering important design aspects and specific requirements particularly aimed at addressing environmental issues to mitigate CO₂ emissions into the environment. Different authors provide different figures and standards for design and development of a green roof with some of the requirements shown in table 2 below.

One approach is to identify common components that make up a green roof as demonstrated below. That provides a flexible choice when procuring construction materials prior for the construction process (Mentens, Raes& Hermy, 2005). The results are in table 2 below.

Table 2. Reference values for a green roof.

Mentens, Raesand Hermy, 2005.

On the other hand, other factors to consider are in table4 below for each type of green roof system (Beattie&Berghage, 2004).These factors include the depth of the substrate type, the porosity requirements, the respective size of the porosity, the water holding capacity, air content in the soil, water permeability requirements, and the organic content, and plants content.

Once these facts have been established, construction steps outlined in the table below are followed beginning with step 1 all through to step 4 (Bass&Baskaran, 2003).Research results recommend the need to conduct a cost benefit analysis prior to the installation of a green roof on a building.

Green Roof Cost Analysis

In the repository of general knowledge, it is always necessary to conduct a cost benefits analysis before investing in a project. A cost befit analysis always provides the economic rationale to invest in any particular venture as it minimizes the susceptibility of the potential risk of loss and other unanticipated project problems.

Larson, Matthes and Kelly (2000) view a cost benefit analysis for a green roofs as an essential component particularly when making informed decisions about the kind of roof to construct. That also depends on the locality where project implementation occurs and the potential benefits of investing in the development of the green roof system.

A lot of research on the benefits of green roof systems has dwelt on thermal benefits from green roofs with little research on environmental benefits, thus, making it all more important to conduct an investigation into the monetary benefits of green roofs. The investigation, based on a probabilistically model could provide valuable information to private and public developers on green roofs in terms of the cost savings resulting from several benefits from green roofs (Clark, Adriaens&Talbot, 2007).

Studies conducted byClark, Adriaens&Talbot (2007), Wark, Christopher, Wark, and Wendy (2003), Victoria Transport Policy Institut (2011), and The Green Roof Code (2011) have provided comparative details about the cost of installing a green roof and conventional roofs.

Findings from the studies and other sources of literature have shown that the cost of a conventional brown roof to be $167 with the cost of a green roof shows a standard deviation of $ 28 from the conventional roof. Observations and data analysis of the deviations shows green roofs to be more costly due to variations in design and the additional costs due to the components used to develop a green roof, creating a cost gap between a brown roof and a green roof.

On the other hand, the argument that green roofs are costly compared with brown or conventional roofs rely on established facts based on research findings. Among the benefits gained from the installation of green roofs, include a gap in the installation costs, cost reductions due to storm water management, and the aggregate effects of energy savings from the use of green roof systems on conventional roofs compared with brown roofs.

In addition to that, the thermal efficiency of green roofs is an additional benefit gained from green roof systems. That is particularly due to the efficiency with which green roofs minimize the loss of heat into the surrounding environment. The rate of heat retention by a green roof is mathematically expressed in the following thermal equation:

In the equation, “ΔT is the difference between the ambient temperature and the temperature of the interior of the building under consideration” (Clark, Adriaens&Talbot, 2007). Q represents the flow of heat through the structure under consideration, and A represents the total surface area of the roof in question. The following tabulation shows a clear comparison of the conductance of green roofs between the brown roofs and green roofs (Clark, Adriaens&Talbot, 2007).

On the other hand, cost, social, and private benefits contribute significantly to the number of benefits experienced from the use of green roofs. Further findings based on a comparative analysis of three models are tabulated in table 4 below:

Table 4.

Mentens, Raes and Hermy(2005).

Another cost benefit analysis conducted based on an evaluation of air quality improvements is based on the mitigating effects of NO_X compounds and a sensitivity and financial analysis of the roof systems. The results obtained from the research are in the table45 below (Clark, Adriaens&Talbot, 2007). NPV is the net present value.

Clark, Adriaens&Talbot, 2007.

From the tabulations shown above, the “R-Value” is the mean value of the storm water management of a brown roof and exceeds that of a green roof by 23.72 percent. That shows that a green roof system has more and better storm-water management benefits compared with brown roofs. In the analysis both high and low storm water management methods were basic in evaluating the comparative benefits of brown and green roofs.

In addition to that, comparative values showing in general that green roof systems offer more benefits than brown roofs were also tabulated. Data from the above table provides a brief view of research findings conducted to establish the benefits derived from either type of roofs.

The results indicate gross benefits, which result from either direct or indirect uptakes by plants grown on green roofs, a typical air pollution control mechanism through the green roof systems. However, some limitations such as behavioral characteristics of have limitations on the study.

The baseline of the above study is an analysis of a green roof with a 40-year lifespan to determine or estimate the lifespan of a green roof. From the above data, the NPV varied between a low of 25% and a high of 40% with the green roof experiencing an extra lifespan of 40 years compared with a conventional or brown roof (Clark, Adriaens&Talbot, 2007).

According to Clark, Adriaens, and Talbot (2007), green roofs provide additional insulation benefits compared with conventional roofs. Further research findings indicate additional cost benefits associated with green roofs to include a higher life expectancy compared with traditional roofs. In addition to that, green roofs provide a prolonged lifespan on the water proofing membrane with little or no need to re-roof (Clark, Adriaens&Talbot, 2007).However, the locality where the green roof installation occurs may also influence the price of a green roof.

It is however important to conduct a thorough evaluation of government policies and other related issues to address specific guidelines for constructing green roofs before starting any construction.

Green Roof Maintenance

Studies by Eugene (2008), Carter and Keele (2007), Celik, Morgan, Retzlaff and William (n.d), and Clark, Adriaens andTalbot (2007) show that green roofs require specialized maintenance requirements. However, the operation and maintenance of green roofs depend on the design and the secondary purpose of the green roof. According to Clark, Adriaens, and Talbot (2007), intensive and extensive green roofs have different maintenance requirements depending on their use.

In general, the purpose of a green roof and maintenance requirements ensure that plants and wildlife habitats remain healthy while vegetation grows and gains momentum towards stability. One approach used in the maintenance of green roofs is by sustainably keeping a specified level of moisture content in the soil and ensuring long-term water retention capabilities of the green roof are enforced.

That in effect is to ensure appropriate management of runoff water. To achieve the latter fact, it is important to determine beforehand irrigational requirements of a green roof to ensure a sustained supply of moisture and water needs for the plants and wildlife before the construction and installation of a green roof begins (Eugene, 2008).

However, it is important to formulate a method of minimizing maintenance costs and requirements of green roofs by the use of low–growing plants that limit the need to prune the plants while satisfying the aesthetical needs of the people. It is important to emphasize on the frequency of inspections of the green roof membranes to ensure they do not leak.

If leakages are detected, specialized personnel should take corrective action at an earlier stage to stop the leakages. Personnel should use electrostatic tools to detect fine pinholes to ensure no leakages occur while taking corrective actions where necessary (Eugene, 2008). Eugene (2008) maintains that the frequency of maintaining a green roof system should be monthly or at an interval of three months, or as might be deemed fit by the technical experts based on the type of green roof and its maintenance requirements.

According to study findings by Eugene (2008), Carter and Keele (2007), initial selection of plants on the green roof should provide a basis in decision making on the frequency with which green roof maintenance occurs.

Beattie and Berghage (2004) support the view by different authors that one of the most common maintenance requirements of a green roofs is weeding. Weeding should follow recommendation guidelines contained in the instruction manual developed by the experts.

Other maintenance requirements include plant replacements at the correct time interval. Beattie and Berghage (2004) further affirm regular maintenance requirements should to be done to remove the debris on a routine basis to keep the roof clean.

Green roof Examples

Various examples of green roofs abound. The construction of each type of green roof is based on different concepts based on the objective for which it is constructed. Typical examples of different types of green roofs are shown in the following pictures.

The above picture illustrates an extensive green roof system in Brazil. Details about the roof system and its characteristics as discussed elsewhere in the paper.

Cutlip, 2006

The main objective of the above roof system is provision for a surface for growing plants spontaneously.

Coffman and Davis, 2005

The above example is an extensive green roof, which incorporates a solar heating system specifically for the parking lots. Plants on the roof belong to the Delosperma othona family.

Asner, Scurlock, Hicke,2003

The pictureshown above illustrates an intensive green roof found in Rio de Janeiro. The main objective is to thrill an observer aesthetically.

Bass and Baskaran, 2003

The green roof shown above is in the semiarid north country of Brazil. It is in Campina Grande. The foliage is desert and semi-desert tolerant. They belong to the shrubs and succulents families. Typically, the area covers a substrate are equivalent to 10-20 cm.

Baskaran, 2003

Bass and Baskaran, 2003

The above example of a small and modern green roof incorporated into a city mall. It is meant to fulfill provide aesthetical fulfillments.

Images of green roof examples, n.d.

The above is an illustration of a green roof at a Manhattan island at the Rockefeller Center.

Coffman and Davis, 2005

A typical example of a green roof system in an urban dwelling is in Manhattan on top of the story building as viewed on the picture for aesthetical and air quality management purposes.

Images of green roof examples, n.d.

The above picture represents a typical example of a green roofthat is integrated into the underground area of a garage which includes a number of playground and other physical facilities.

Images of green roof examples, n.d.

The picture shown above is an example of a green roof with sufficient open space for various functions including to satisfy the aestheticsof the viewer

The GRO Green Roof Code, 2011.

Green roofs, Benefits and Limitation

The rapid development of green roofs in many parts of the world today is due to the qualitative and quantitative benefits realized from constructing green roofs on conventional roofs. A number of benefits discussed elsewhere bear evidence to the benefits that include mitigating air pollution on the environment (Dixon, Butler& Fewkes, 1999).

From the environmental conservation perspective, green roofs have been studied and known to provide invaluable benefits when used in densely populated areas that are particularly prone to high levels of carbon pollution. Other additional benefits include areas that are devoid of fauna and flora, leading to the conservation of energy and leading to higher energy performance.

Balancing the imbalances that occur on the ecosystem due to the discharge of waste products into the environment is another benefit. The importance of using green roofs on conventional roofs is examined in detail in the following discussion. Based on the benefits gained from the use of green roofs particularly in densely populated areas, the value of a green roof as a pollution mitigating method cannot be underestimated (Eugene, 2008).

Environmental Benefits

One of the benefits gained from the use of green roofs lies on the impact of air pollution on the environmental. One of the environmental benefits includes storm water management besides being a carbon control mechanism among other benefits. In theory and practice, energy conservation is another environmental benefit associated with the construction of green roofs (Takahashi, Konaka, Sakamoto & Morikawa, 2005).

Energy Conservation

Celik, Morgan and Retzlaff (n.d) provides a mathematical model for calculating the energy conservation of green roofs based on the mathematical expression discussed below.

Mathematical Modeling

One approach upon which the mathematical model is developed is based on the conservation of energy due to the use of a green roof on which a variety of plants is grown on the growth medium. In this case, one outstanding method green roofs prevent heat loss is through evapo-transpiration. From the energy conservation point of view, evapo-transpiration is direct shading used to curb evaporative cooling and the effects of direct cooling.

One distinguishing characteristic in the conservation of energy is the ability these plants have to radiate energy previously absorbed back into the environment. On the other hand, another attribute of these plants is their ability to conduct heat through the growth medium on a green roof. It is important to equip one with a mathematical tool or model to analyze the thermal performance of green roofs before a further discussion of additional thermal benefits of green roofs concludes the inquiry (Celik, Morgan& Retzlaff, n.d).

Energy and Environmental Benefits

Having developed a mathematical model upon which the thermal performance of a green roof can be calculated, it is important to develop a detailed view of the benefits that are gained from the use of green roof vegetation in relation environmental performance and benefits of green roofs(Liu, 2002).

According to the Journal of Roof Consultants Institute (2004), thermal benefits result from the heat energy efficiency experienced due to the enveloping effect of a green roof on a building. Comparing brown roofs with green roofs in terms of thermal performance, green roofs have better thermal performance effects than brown roofs.

In addition to that, green roof foliage offers better thermal performance by dumping the thermal effects of the solar radiation incident on the foliage that forms the roof. One of the factors used in the analysis of the thermal benefits of a green roof is to examine the temperature profile of a green roof. It is important to view the temperature profile of a green roof as graphically presented in graph 1 below:

Journal of Roof Consultants Institute, 2004.

A graphical representation shown above displays the temperature profile of green roofs. Deducing from the above temperature profile, the accumulating effect is positive where the temperature of the green roof system indicates a rising trend (Cooper-Marcus& Barnes, 1999).

According to Cooper-Marcus and Barnes (1999), some of the important benefits realized from green roof include the effect of reducing the amount of greenhouse gas emissions into the environment. Asner,Scurlock and Hicke (2003) have examined in detail one of the methods of reducing the effects of carbon dioxide on the environment and have found that the strategy for reducing CO₂ emissions is through the sequestration of carbon.

From a practical point of view, green roofs have significant CO₂ reduction effects on the environment. In addition, to that, space conditioning with an aggregate effect of reducing the quantities of CO₂ gas emissions are some of the significant impacts due to carbon sequestration from the environment.

According to Taha, Akbari, and Rosenfeld (1991), different methods used to fulfill the effects of carbon sequestration are discussed later. One such is a built in method. The built in method for reducing CO₂ emissions include preventing heat loss into the environment is due to the insulating effect on the environment. The results are a reduction of energy demanded in a building.

On the other hand, evapo-transpiration effects and the aggregate impact on the urban heat island effect are also experienced. Typically, green roofs plants work by absorbing heat energy from the sun when exposed to solar radiation. As night approaches, the environmental temperature drops significantly leading the plants to radiate the absorbed heat during the day.

However, it is important to note that the thermal performance of a plant due to thermal stress experienced by plant membranes leads to the poor performance of the plant. The study indicates that membranes are damaged due to the whole cycle of absorption and radiation of solar energy from and into the environment Taha, Akbari & Rosenfeld (1991) and (Liu, 2002) through thermal stress.

Despite the ecosystem benefits gained from green roof systems, it is worth noting that the cost for developing ecosystems is prohibitive, and the structural requirements place further emphasis and further demands on the cost of developing a roof that can withstand the additional effects of green roof loads (Liu, 2002).

Quality of Water

Among the benefits realized from green roofs is the ability to retain high quality water captured from a green roof. The benefit due to the good quality of Water reinforces the rationale to construct a green roof.

Among the typical capabilities of green roofs is the ability to reduce the amount of load accumulating as contaminants from the roof through the water absorption and drainage ducts. That includes plant uptakes, evapotranspiration methods, and a variety of microbial activities. However, the green roof attenuation activities lower the concentration of contaminants through chemical and physical processes (Toronto and Region Conservation, 2006).

Another capability due to the green water conservation system is the ability to slow down the rainwater, an ability attained by slowing down the impact of falling raindrops, allowing the rainwater to percolate slowly into the roof, then into the filter and eventually into the drainage cell below. Typically, that is a strong structural benefit of green roofs compared with brown and conventional roofs (Saiz, Kennedy, Bass& Pressnail, 2006).

It is important to note that the aggregate effect of green roofs on runoff water and the chemical effects due to chemical pollutants significantly remain minimal in the process. On the other hand, leaching, one of the adverse effects experienced on green roofs occurs through the building materials used for the construction of green roof.

Overall effects include chemical contaminations particularly from treated materials such as timber used in the construction of green roofs (Toronto and Region Conservation, 2006). Analytically, it is important to formulate a mathematical model of the quality of water to allow its calculation assigned on numerical values.

Storm Water Management

Storm water management is one of the benefits realized from the construction of green roofs. Typically, the retention of runoff water is an additional benefit besides slowing down the speed at which rainwater penetrates into the ground, which helps to reduce the overall impact of rainwater and the resulting effects.

As an additional benefit in controlling the flow of surface water, the volume of restricted water flow on the surface has the overall impact of reducing the volume of runoff water and dissolved substances besides other runoff precipitates (VanWoert, Rowe, Andresen, Rugh, Fernandez & Xiao, 2005).

From the leaching effects due to runoff water, studies indicate that the amount of nitrogen leach decreases with time while phosphorous remains significantly the same. The chemical compositions are typically influenced by the amount of saturation in the target soil rainwater infiltrates the ground (VanWoert,et. al, 2005).

Mitigation of the Urban Island Effect

The urban island effect is the warming effect that causes urban areas to thrive at higher temperatures compared with their surrounding areas (Lundholm, 2006). The outstanding cause for the heat effect is the low reflectivity of urban areas, low vegetative cover, and the effect of arresting solar radiation due to the particles trapped in polluted air.

In addition to that, the “large amounts of heat released due to industrial activities, from buildings, and from automobiles contribute significantly to the urban island heating effect” (Morikawa, Takahasi&Kawamura, 1998).The overall impacts are adverse effects on human health and adverse environmental effects (Rosenfeld, Akbari, Bretz, Fishman, Kurn,Sailor, Taha, 1995) and (Bass,Krayenhoff, Martilli, Stull& Auld, 2003).

Other adverse effects include accelerated chemical activities such as smog and higher energy demands to cool buildings, which eventually increases the amount of CO₂discharged into the environment (Morikawa, Takahasi&Kawamura, 1998).

On the other hand, an analysis of the behavior of vegetated surfaces indicates that cooler temperatures can be optimized the use of vegetation cover. Typically, green roofs provide cover and reduce the aggregate effects on the temperature of roofs thus reducing the heating effect of incident solar radiation on green roofs.

Air Quality

Another benefit arising from the use of green roofs is improved quality of air. The quality of air is attainable by the removal of dirt particles from the air by the filtering effect of plants. CO₂, smog, and related discharges into the environment are some of the causes of global warming and the resulting detrimental effects. Typically, the removal of such dirt particles by the uptake mechanism of plants and through contacts (Grant, et al, 2003).

Biodiversity Benefits

Cities develop and expand through a range of destructive effects on biodiversity. That causes significant losses of wildlife sanctuaries and plant habitats (Cottingham, Brown& Lennon, 2001). A green roof plays a critical role in providing sanctuary for wildlife and natural habitats. One limitation of green roofs is that they do not provide the kind of habitations typical of natural habitats.

However, the importance of green roofs as natural habitats for plant and animal life such as vertebrates cannot be underestimated. It is important, as an approach in keeping green roofs to discuss in details about the operation and maintenance of green roofs (Grant, et al, 2003). It is also important to present a comparative view of the advantages and disadvantages of green roofs from various perspectives.

Advantages and Disadvantages

The following discussion crystallizes the merits and demerits of the green roofs to inform any interested party on an appropriate choice of green roof to adopt when selecting a green roof to construct. Many authors have studied the advantages and disadvantages of both types of green roofs and established the facts here discussed.

Advantages of extensive green roofs

Extensive green roofs have characteristic advantages by being light in weight with little demands on additional reinforcements, thus making the overall cost much lower. On the other hand, extensive green roofs are generally suitable for large surfaces and have a dutiable inclination of 30°(Grant, et al, 2003).

In addition to that, extensive green roofs are largely good for retrofit undertakings, are comparatively less expensive, take the natural aesthetics of the environment, and allow the growing of vegetation to occur spontaneously. Detailed research into the benefits of green roofs indicates that they require less technical expertise and approval by various government authorities are easy (Dobson, 1995).

Disadvantages

Extensive green roofs have various de-merits due to the use of limited varieties of plants and are less appealing aesthetically. In addition to that, extensive green roofs provide limited or no access to public recreations and are less thermal efficient and diminishing water retention abilities (Grant, et al, 2003).

Advantages of intensive green roofs

Extensive green roofs have the advantages of the ability to accommodate diverse biodiversity and wildlife, have good thermal insulating properties, have the republication capabilities of natural habitats, and optimize membrane lives. In addition to that, intensive green roofs have greater capacities to retain storm water, higher energy efficiencies, and are flexible in meeting the aesthetical requirements of a green roof. A typical benefit of intensive green roofs is that they fulfill recreational needs of a city (Grant, et al, 2003).

Disadvantages

Intensive green roofs lay greater structural demands due to higher weight loads. In addition to that, intensive green roof systems lay additional irrigation and water requirements therefore leading to higher energy consumption. On the other hand, intensive green roofs require higher capital investments and require constant maintenance, are more complex, and require technical expertise in their design and construction when compared with brown roofs (Cooper-Marcus& Barnes, 1999).

Challenges

This discusses in details the challenges that the current generation is facing particularly due to high levels of carbon emissions into the environment and methods that have been used to mitigate the effects of carbon emissions into the environment. One most important challenge is to mitigate the effects of CO₂ a gas known in scientific cycles and now is common known as the gas that is the major cause of global warming and the devastating effects experienced.

On the other hand, governments and other sources on methods of reducing the emission of CO2 into the atmosphere have experienced many challenges. These include the impact on the environment due to climate change, a fact that is evidently affecting both developed and developing nations. A number of impacts due to climate change have already been evident in developing countries.

Among the effects are prolonged droughts due to rising global temperatures, which have changed rain patterns, unpredictable rain patterns, dropping sea levels, and floods besides other effects that have started appearing. On the other hand, developed nations have experienced high levels of pollution, smog and associated effects on the environment.

That has led nations to formulate policies to mitigate the effects of air pollution on the environment. One such method is to construct green roof on conventional roofs.

Pollution and CO₂ Emission

In the modern age, pollution particularly due to emissions of carbon dioxide into the environment due to industrial and human related energy consumption activities is a critical cause for concern for governments. An increasing body of evidence indicates rising levels of CO₂ as one of the single most significant contributors to global warming with devastating consequences.

Countries, after the painful experience and anticipated effects of large quantities of CO₂ in the atmosphere, have been compelled to lay strategies to optimize any available technology to curb and reduce CO₂ emissions into the environment(Victoria Transport Policy Institute, 2011).

Scientists have established that CO₂ emissions are a major global pollutant on a global scale. The polluting effects are both quantifiable and unquantifiable and present serious challenges to governments to search for ways to curb the devastating quantifiable and unquantifiable adverse effects (Grant, 2006).

Quantifiable effects include high mortality rates from the adverse effects of the CO₂ pollutant, asthma attacks, and adverse effects on pulmonary functioning among others. On the other hand, unquantifiable effects include lung inflammations among others. Typically, these are adverse health effects (Victoria Transport Policy Institute, 2011) and (Oberndorfer, Lundholm, Bass, Coffman, Doshi, Dunnett,Gaffin, Köhler,Liuk & Rowe,2007).

Environmental effects have so far, by many researchers been identified to be the single challenge faced by many countries in the world due to large quantities of CO₂ emissions into the environment. Over and above, these emissions result to climate change, commonly known as greenhouse gas effects.

Research into different emitters of CO₂ indicates that motor vehicles are the greatest contributors of CO₂ into the environment (Huang& Franconi, 1999). Various assessments by different governments conducted show detailed evidence on the risks and uncertainties associated with the impact on climate change.

Formal economic models used in the inquiry provide significant amount of data and information on the potential impact of global warming due to carbon emissions. Results of the studies estimate the overall impacts on costs associated risks to be 15% of the GDP of world countries, and expected to rise to 20% with associated rise in the impacts due to risks and costs (Kolb& Schwarz, 1993).

According to the Victoria Transport Policy Institute (2011) report, another challenge revolves around the contentions by other countries and firms, which contest fiercely that CO₂emission does not have any impact on global warming. The argument is that little statistical data and scientific research show support for the argument that CO₂ is the direct cause of global warming.

With the grim statistics promising worse eventualities in case CO₂ emissions are not curbed, governments have to take it as an obligation to formulate methods to address the issue (Farzaneh, 2005). Governments, working with scientists and other experts have concluded that the effects of global warming are due to CO₂ emissions and other pollutants.

In light of the results, policy formulations, carbon capture strategies, use of clean technologies, and in the more recent past, use of plants to reduce CO₂ presence in the air particularly in the cities has taken the day. The following discussion provides views on climate change and government goals in mitigating the effects of CO₂emissions into the environment (Victoria Transport Policy Institute, 2011).

Climate Change Mitigation & Government Goals

A report compiled by the Information Services of the UNFCCC secretariat (2007) shows that climate change results from the effects of CO₂ emissions compelling countries to develop policies to address the carbon pollution issue. Estimates show that developing countries will require significant sums of money, in the tune of US $ 67 billion to address the effects of CO₂emissions by 2030 (Information Services of the UNFCCC secretariat, 2007).

Governments have come up with strategies and set goals to mitigate upon the effect adverse effects of CO₂emissions on the environment. It is important to note that developing countries are highly vulnerable to the effects of climate change. Typically, the social framework of the countries, political, and geographic orientation (Takebayashi & Moriyama, 2007) drives the vulnerability of each country.

Further, the Information Services of the UNFCCC secretariat (2007) is one particular approach used in the mitigation of the adverse effects of climate change and large quantities of CO₂ emissions is adaptation. Adaptation allows human beings to change with the changing climatic needs and environmental demands.

That is particularly because the frequency with which the effects of global warming impact on the environment are increasing with time. Estimates indicate a drastic rise in global temperatures particularly in the last 25 years to be results of human activities such as the burning of fossil fuels (Information Services of the UNFCCC secretariat, 2007).

A diagrammatic representation of the effects of climate change and the chain of activities leading to the changes occur in the following discussion. There is significant evidence from the diagram showing that human activities are the prime causes of global warming (Wong, Chen, Ong & Sia, 2003).On the other hand, the diagram shows in detail climate change processes, the carbon cycle and resulting enhancement of greenhouse gas effects, and the resulting threats due to the human activities.

One of the main goals of many Governments is to formulate methods to address the emerging issues of climate change. These included cutting emissions to tolerable levels, encouraging personal responsibility in responsible use of energy, investing in research and development particularly by encouraging the use of clean technologies.

In addition to that, governments have endeavored to invest in research and development in the use of renewable technologies, and encouraging and investing in the use of plants to absorb CO₂ from the atmosphere (Information Services of the UNFCCC secretariat, 2007).

Climate Change and Developing Countries

The impact of climate change and associated risks had most implications on developing countries. According to Grant (2006), developing counties experience different conditions with specific impacts on their environments. U.S. EPA (1998) and Gaffin et la, 2006 have established that the potential impact of climate change as being influenced by issues such as the geographical orientation of the specific country under consideration.

In addition to that, Gaffinet la, 2006 asserts that different measures specific-to-specific conditions of each country can be adopted to address the effects of climate change and particularly due to air pollution. On the other hand, Gaffinet la, 2006 and Hastaie (2000) have emphasized that climate change has wide-ranging effects on the environment, which include adverse effects on agriculture, socio-economic activities, and effect on rainfall patterns.

A summary of the negative effects of climate change are shown in the following figure along with other threats and consequences mentioned above (Grant, 2006) as illustrated in the fig 4 below

Grant, 2006.

Among the changes observed due to climate change, based on the above diagram include greenhouse effects which result from CO₂ cycles. These effects are directly due to urbanization, deforestation, transportation, agriculture, and many other sources. The potential impact of climate change processes include changes in precipitations, melting ice caps, average temperature changes, abrupt climate changes, and rising sea levels among other effects.

Gaffin, Rosenzweig, Parshall, Hillel, Eichenbaum-Pikser,Greenbaum, Blake,Beattie and Berghage (2006) have studied in detail the effects on carbon as one of the major causes of global warming and the need to mitigate the polluting effects on carbon in the environment. Among the causes are an increasing number of effects such as rising global temperatures, rising levels of smog, and adverse health implications.

According to Grant (2006), typical of the effects that come with rising levels of temperatures, include rising sea levels threatening cities lying close to the sea. In addition to that, climate change is bound to come with unpredictably harsh weather conditions and unpredictable surging level of storm, situations evidently observed in the recent past.

Developing countries are the main cause of geographic distributions of diseases, typically interacting with other vulnerabilities particularly malaria and HIV/AIDS, lowered life expectancy, and threats from the impact of prolonged droughts.

A study by U.S. EPA (1998)shows that climate change subjects the highly vulnerable natural habitats in developing countries to extinction with adverse effects on the entire ecosystem. Some of the effects include rising water levels. In addition to that, developing countries are susceptible to a decrease in annual precipitation, destruction of the terrestrial ecosystem, and rising temperatures consequent of the effects (Gaffin, Rosenzweig, Parshall, Hillel, Eichenbaum-Pikser,Greenbaum, Blake,Beattie& Berghage, 2006).

There is need for developing countries to plan for sustained development and invest in capacity building and capacity adaptation to the impacts of climate change. In addition to that, developing countries need to work with developed nations, non-governmental agencies, and other organizations to plan and strategies on the best approaches to minimize the impact on global warming (Emmanuel, 2005). A typical example of a country affected by CO₂ emissions is Iran, and particularly the city of Tehran, as discussed below (U.S. EPA, 1998)

Tehran Case Study

A study by Asadollah-Fardi (n.d) shows Tehran as one of the most heavily polluted cities in the Middle East. AccordingAsadollah-Fardi’s (n.d) study, one of the contributing factors to the high levels of pollution include the dense population estimated at 10 million people whose energy intensive activities lead to high CO₂ emissions.

The similarity of the city’s geographical orientation to that of other cities such as Los Angeles in USA contributes to the study. Due to its topological orientation, the city does not experience any flow of wind as either parts of the city surrounded by mountains capped with ice. Restrictive movements of the wind on the city are one of the contributing factors to high thermal concentrations within the city.

In addition to that, the restricted movements of the wind over the city denies it the benefits of moving dirty air out of the city casing a further rise in pollutant concentrations with the consequent destructive effects (Asadollah-Fardi,n.d).

High pollutant emissions are a direct result of burning energy from the environment. In addition to that, a growth rate of emissions experienced in the recent past due to ever-increasing number of vehicles on the roads has been recorded (Asadollah-Fardi, n.d). To compound the seriousness of the pollution problem, climatologically factors have shown significant contributions, making the city to suffer from higher ozone levels and associated effects (Hastaie, 2000).

Further research shows that obsolete machines due to the long standing economic and technology sanctions have made the situation worse. Other contributing factors are a rising number of private vehicles on the roads, poor urban planning methods, poor vegetation cover within the city, cheap and poor quality fuels, and a rising population with social and economic consequences (Hastaie, 2000).

Among the mitigating strategies laid by the Iranian government to curb the rising levels of CO₂emissions, include conducting inspections on vehicles in use, active participation in international forums on the reduction of the greenhouse gas emissions, using solid waste management technologies, relocating industries besides investing in to develop green spaces.

Green spaces include the planting of tress as analyzed in the following section (Asadollah-Fardi, n.d). In order to lay strategies for mitigating the impact on the environment due to pollution effects, an environmental committee constituted the elements as illustrated below with the main goal of looking for strategies to curb urban pollution (Hastaie, 2000) as illustrated in the chart 1 below.

The theoretical framework upon which the inquiry relies focuses on the CO₂lifecycle within the plant system and within the environment. These studies rely on effects caused by plants and their overall effects on the environment especially in relation to the control and emissions of CO₂.The study examines the option of using green roofs as pollution mitigation methods based on the carbon flow lifecycle in plants since these plants can grow on these roofs.

According to studies based on the carbon lifecycle, carboxylation of CO₂ is a process that works through the acceptor molecules by reducing component substances into two separate molecules. Typically, the process takes place in the plant in the form of advanced or primitive forms. The process fixes CO₂into other systems as one of the metabolic processes.

The fixation of CO₂ in plants is one of methods used to control and amount of CO₂ emissions into the atmosphere. Typically, that is a carbon management strategy integrated into green roof systems. Many researchers have studied the benefits gained from green roof systems based on different models.

These benefits have led to the conclusion that constructing green roofs into conventional roofs has shown significant energy reductions with a 2% reduction in energy consumption and a 9 % decline in the overall consumption of natural gas within a building. In addition to that, other benefits include lower greenhouse gas effects, which calculations estimate at 702 g C per m².

Other studies however show that the carbon costs incurred during the construction and installation of green roof systems could take a significant period to offset (Getter et al, 2009). This study examines the theoretical framework upon of the carbon lifecycle is studied and the use of green roof systems as a pollution mitigating measure.

Plants and CO₂ Emission

Different studies on the vegetation cover in many cities shown that much of the cover consists of exotic plants and native plants. In addition to that, green roofs in densely populated areas in certain sections of different cities with a diversity of trees, contribute to the preservation of wild life and other animal species. A spot check of one of the cities in North America shows a variety of tree species prevalent in the region as tabulated in table 5 below.

Table 5.

Sources: Urban Forest Effects and Values, 2008.

Several studies by many researchers to understand the effects of plants and their overall implications on the environment in relation to CO₂ emissions have yielded several of results. These studies show how the problem associated with CO₂ is to the environment. Thus, it is important to examine the mechanism through which plants interact with CO₂ in their intake and discharge of the gas into the environment and their contribution to reducing the gas (Lee& Kim, 1994) and (Hartig, Mang& Evans, 1991).

Typically, green roofs offer a strong promise on the reduction of CO₂ emissions into the atmosphere. Therefore, it is important to look at the theoretical processes through which CO₂ is absorbed and discharged into the environment. To understand the rational of using plants as one critical approach to address problems of pollution, which lead to global warming, Theodosiou (2003) has examined the issue in detail.

From a theoretical point of view, CO₂ reactions take place in the environment through photosynthesis as illustrated below.

Photosynthesis: Carbon Reactions, n.d.

Theoretically, the conceptual framework modeled after the Calvin cycle shown above shows the carbon cycle. Typically, the cycle consists of three steps. Carboxylation starts the first cycle of the whole process, followed by the second reduction step in the cycle and concluded with a regeneration process (Photosynthesis: Carbon Reactions, n.d).

In the cycle, carboxylation of CO₂ works in tandem with the acceptor molecules reducing the component substance into two separate molecules. Typically, the process occurs in all forms of vegetation in their advanced or primitive forms. The process leads to the fixation of CO₂inone of the metabolic processes (Photosynthesis: Carbon Reactions, n.d).

Water and CO₂ fixations into the atmosphere are a process that occurs through enzyme activities resulting into two molecules mentioned above. 3-phosphoglycerate resulting from the process is further reduced by substances generated photo-chemically into carbohydrates leading to a further generation into 5 more CO₂ acceptor molecules at the end of the first phase. Several other chemicals cooperate in the process (Photosynthesis: Carbon Reactions, n.d).

In practice, during photosynthesis, plants take large amounts of CO₂ from the atmosphere and release it later by burning it. When the amount of CO₂ released into the atmosphere and absorbed from the atmosphere is similar, a biotic balance is established. However, when both the absorption and release of CO₂ is not balanced, there is no biotic equilibrium, creating regions of carbon sinks as has been experienced in cities (Photosynthesis: Carbon Reactions, n.d).

A study by the Photosynthesis: Carbon Reactions (n.d) paper reports that various plants show significant changes with changes in the environmental. As environmental conditions change, most plants indicate an adjustment in their uptake mechanism. One important point about plants is that they have an inbuilt mechanism of adjusting to increasing amounts of water and CO₂uptakes from the atmosphere, a very critical point when establishing a green roof.

Plants convert CO₂ into sucrose and other substances and keep the captured CO₂ in that form for a long time. In effect, that is a strong point to consider when constructing a green roof as one of the critical environmental benefits. That achievement occurs by way of enzyme reductions and other forms of reactions involving the absorption and conversion of CO₂ into different forms.

That makes it important to translate the above theoretical conclusions into practice to help answer the inquiry on the role of plants in reducing CO₂ emissions into the atmosphere and gradually reduce global warming (Earth Pledge, 2005). One argument in favor of the use of plants and pointing to their role as CO₂ management approaches is that plants have an integrated mechanism that allows them to adjust and adapt easily to climate changes (Photosynthesis: Carbon Reactions, n.d).

Another argument is one that views plants as being able to adjust to upward temperature changes due to global warming by changing their perspirations accordingly. In addition to that, plants correspond to carbon releases by decreasing the amount of CO₂released due to climate change. Typically, the characteristics of the plants are a deciding factor when selecting the type of soil to use in creating a green roof (Theodosiou, 2003). According to Theodosiou (2003), the families of plants also influence the level of carbon emissions into the environment.

Green Roofs and CO₂ Management

Stabilizing the amount of CO₂ emissions into the atmosphere is critically important particularly in cities with high population densities and high levels of carbon emissions. Among the methods used to reduce the amount of CO₂into the environment, which is, also a carbon management strategy is the use of green roof systems (Trumper, Bertzky, Dickson,van der Heijden, Jenkins & Manning, 2009) and (Del Barrio, 1998).

Researchers and scientists have proposed many approaches of reducing CO₂ into the atmosphere. Two of the methods include reducing the amount of CO₂ discharged into the environment or increasing the absorption rate from the environment. While different approaches in addition to the supporting policies on carbon management are in place, their effects have not caused significant impacts in reducing the effects of air pollution on the environment (Trumper et al, 2009).

Thus, the need to use green roof as a pollution control mechanism is overwhelming. Among the areas that green roof systems have found significant use are in cities. The use of green roof systems is a good option since it is a proven and successful technology in reducing CO₂emissions into the environment(Goward, et al., 1985). Typically, the reason for use in urban areas has been primarily that these areas suffer heavily from high-level concentrations of CO₂ due to industrial activities and high population densities (Trumper et al, 2009).

One of the methods generally accepted as a carbon management strategy is a well-designed green roof system. A green roof is a natural a carbon management approach that comes with a significant number of environmental benefits. One benefit of green roofs is due to the characteristics of the ecosystem. Research has shown that a wide biodiversity of plants on green roof systems offers ecosystem services.

These include the capacity to store large quantities of carbon depending on the characteristics of the plants grown on the green roof (Liu, 2004). To optimize the use of green roofs for the capture of carbon and as a carbon management strategy, low growing plants the roofs attain that goal. In addition to that, plants with a low growth rate and low decomposition rates are used (Trumper et al, 2009; Villarreal& Bengtsson, 2005).

On the other hand, restoration of the degraded environment usually contributes significantly in reducing the amount of CO₂ emissions into the environment Getter& Rowe, 2006) and (Boivin, Lamry, Gosselin &Dansereau, 2001).

Estimates indicate that a carefully designed and installed green roof has the capacity to capture large amounts of CO₂ and restrict the discharge and exit of CO₂ into the environment from buildings insulated by the green roof. Calculation indicates that close to 55,000 tons of CO₂ can be captured using green roofs in an area equivalent to one million people.

However, recommendations on a further study on carbon quantification and the overall impact of a CO₂ management strategy be done and the results from such a study examined to further inform the inquiry (Trumper et al, 2009). The results are based on a research conducted on different types of green roofs, different plant specifies, resulting biomass, and conducted on different locations as tabulated below (table 2) (Theodosiou, 2003).

Different species planted on each plot of the green roof had different characteristics. These included the variants of sedum including the acre, spurium Bieb, sedum, and album species. The seeds, carefully germinated to grown over a number of years provided good plants. Maintenance activities for the green roofs should be carried out at regular intervals on a three months period cycle.

Maintenance included roof irrigation to keep the water saturation levels in the soil to established standards, and weeding done according to requirements (Trumper et al, 2009). Many samples taken and analyzed from the proposed site provided a series of results. The samples collected and analyzed from the roofs to evaluate the carbon sequestration “a process through which CO₂ is removed for the atmosphere by plants” (Trumper et al, 2009) by the plants grown on the roofs in the proposed area.

The procedure included removing the roots from the soil sampled for examination to determine the amount of CO₂ sequestration. In addition to that, the roots pulverized into powder before analyzing for carbon content in the plants (Trumper et al, 2009). It was also important to determine the level and amount of carbon per a given unit area. The procedure used an ANOVA model in the study as detailed in table 6 below.

Table 6.

Rahshahr International, 2011

After conducting a study of the family of plants suitable for green roofs, it is important to discuss the potential benefits (Baumann, 2006) as illustrated in table 7 below.

Table 7.

Liu and Minor, 2005.

The amount of carbon capture with age and substrate depth considered in detailed in the study are shown above. However, the amount of carbon capture depends on the age and species of the plant. On the other hand, the amount of carbon above the ground varies with each species. Despite that effect, the overall findings concluded that different plant species contribute different amounts of carbon emissions with different carbon concentrations in a given quantity of waste missions (Getter, Rowe, Robertson, Cregg& Andresen, 2009).

Tabulated data from the above table shows the potential benefits of the relative amounts of CO₂ captured and the benefits from each type of green roof system(Givoni, 1998). In addition to that, the results based on practical findings show the lowest level of carbon sequestration to be 73±16.0 while the highest benefit registered was 276±28.0. The mean value obtained from the results was 162±11.7 indicating a standard deviation from the mean to be small from all the samples (Getter et al, 2009).

Conclusions from the study show that green roof systems generally have a carbon sequestration rate of 375 g C·m⁻² which is an accumulated value for above ground and biomass substrate carbon content. Noteworthy, it is important to note that a significant number of components that make up a green roof system experience a carbon cost in the production process (Getter et al, 2009).

A critical analysis of green roof systems of the constituent components shows that a green roof should incorporate a barrier to the roots on the roofing system. That is particularly the case to prevent damage from the roots that might penetrate into the roof. Other costs associated with the roofing system are maintenance and regulation of water flow in the roof and additional carbon costs due to the gravel used on the roof (Getter et al, 2009).

On the other hand, irrespective of the amount of energy consumption of the roof system installation and maintenance carbon costs, the benefits outdo the carbon costs mentioned above (Liu& Minor, 2005).

Different researchers have modeled further studies on the benefits of green roofs. Deductive conclusions from such research show that constructing green roofs into conventional roofs have registered an estimated 2% reduction in energy consumption and a 9 % decline in the overall consumption of natural gas within a building. The benefits include lower greenhouse gas effects, estimated at702 g C per m².

On the other hand, calculated estimates show that the carbon costs incurred in the construction and installation of green roof systems could take a significant period to offset, about nine years (Getter et al, 2009). Further research into the benefits and capacity of green roof systems for use in carbon management shows that they are empty spaces exploitable for good use to sequester carbon emissions.

Different cities have developed green roofs covering different surface areas. A typical example is Detroit Metropolitan region. Estimates show that the total area under green roofs in Detroit is less than 8399 ha. At the sequestering rate of 375 g C·m⁻²for green roofs, it is estimated that 55, 252 tons of carbon can be sequestered in a year, which is an equivalent of emissions produced by 10, 000 midsized trucks in a year (Getter et al, 2009).

It is worth noting that the above findings were susceptible to the orientation and design of a roof system. In addition to that, climatic characteristics of the locality in consideration exert a strong influence on the results on carbon management.

Methodology

The research methodology included a literature review of various perspectives and research by on literature by different authors on different categories of green roofs. The review mentioned three categories of green roofs including intensive, semi-intensive, and extensive green roofs. However, the research focused more on intensive and extensive green roofs in informing the research.

Various issues researched while conducting the literature review to inform the research included a history of green roofs, classification of green roofs, construction methods, types of green roofs, benefits particularly environmental benefits of green roofs, and narrowed down on Iran as a case study. In addition to that, the role of plants in carbon management, carbon costs, and carbon management benefits derived a practical and analytical study of different types of green roofs and carbon management approaches and findings(Wellburn, 1990)

From a practical point of view, the research focused on an airport proposed for construction near the imam Khomeini airport in Tehran. The aim was to make the project zero carbon rated in terms of emission into the environment. Typically, it was to serve a pilot project to provide required and detailed data characterizing a typical concept of a pollution control mechanism due to roof green systems (Köhler & Keeley, 2005).

One approach used in the study was a regional analysis of various issues such as the level of saturation in the soil, targeted for the construction of the proposed IKIA site. Various issues, with particular contribution to carbon emissions and other pollutants into the environment formed part of the study.

A site analysis was one of the critical elements contributing to the flow of air from and into the city, the level of pollution due to CO₂ emissions, and the percentage carbon reductions achieved. On the other hand, a detailed study of the type of green roofs worth constructing at the site as a pollution mitigating strategy formed part of the study. In addition to that, the study details the family of plants that to plant on green roofs, and the level of CO₂ reductions attained from the use of the plants.

A document analysis of details about the climatic conditions prevailing at the proposed site for the construction of IKIA airport have been examined in detail to inform this study. Further research conducted into landscape design and the overall effects on the carbon management and contributing elements such as the orientation to the sun, wind and soil characteristics, and saturation of water in the soil.

Case study result

An analytical study of the proposed area for the construction of the airport facility starts with a summary of the location of the proposed site, the weathers, flora, the fauna, site information about the plants growing at the site, and the available plants. Site characteristics include a close examination of the topological features that are prevalent on the northwestern part, which is dotted with small hills that are insignificant in terms of airflow control.

In addition to the studies concentrated on the southwestern side slopes with 5% of the land mass with an aspect on the windward side examined in the study. On the other hand, an ecological report which played an important role during the decision making process on the type of plants to grow to achieve zero carbon level carbon emission, a goal that may not be achieved in practice is discussed.

Other issues discussed in the report include the climate of the region and the bio-climate, which falls into the geo-botanic category prevalent on an arid-north climate and lies between Artimisia, Astrugalus genera, and Afghan-Anatol steppes.

The aim of the study is to provide detailed information to enable decision making on the use of green roofs as a carbon management method and a method to mitigate upon the effects of carbon related pollution, with Tehran, Iran as the case study.

Site Characteristics

The topology of the area, the direction of the wind, the location of the site, the source of electrical power, industrial centers, residential areas, usable areas, and zoning for flight routes examined to establish the carbon generating activities and the use of appropriate green roofs as a carbon control mechanism.

The site survey showed Tehran to sit in a semi-arid area, with much of the land showing a high aspect to the sun as an additional benefit to tap into the solar energy. A critical evaluation of various zones showed the western and Northwestern zones to be appropriate for supporting non-pollution activities. In addition to that, the Northwestern side proved highly dominant from the wind.

The Rood Shoor River bound the southern part with little ecological significance with some areas registering a slope of 5%although other areas register deeper gradients not fit for civil works. Deep gradients occur along the Southern boundary. To evaluate the site further, a significant portion of the region is suitable for civil works while alluvial deposits not suitable for civil works dominated the rest of the site. The Shoor River binds much of the useful areas.

Important reports show that some parts of the southwestern part overlapped with small geological fault lines dominated by alluvial deposits along the Riverbeds that are susceptible to liquefaction, emphasizing the need for buffer zones. On the other hand, the Southern part is highly susceptible to quick erosion with good water shed development capacity (Givoni, 1998).

A close study of the topological features shows the northwestern part to be dotted with small hills that have little significance in influencing the direction and speed of airflow. On the other hand, the southwestern side forms a slope of 5% of the land mass showing an aspect on the windward side. One feature dominating the northeastern part is a small water reservoir with a small catchment, which reduces the possibility of the likelihood of floods occurring.

On the other hand, the northeastern part dominates excellent soils, which are quite good for agricultural purposes while the rest of the land dominating arid and semi-arid landmasses. River Rood Shoor presenting little risks from floods with little ecological significance dominates the southern part. Further analysis of the site shows the region along which Rover Rood Shoor to be in a low-lying valley.

Statistical data shows residential areas to be Parand New Town with a population density estimated to be 40,000 occupants, Robat Karim City consisting of 750,000 occupants, and Hasan Abad city with 20, 000 people. Industrial concentrations are within the Parand Industrial Area occupying an approximate area of 210 ha, Parand Power plant, Tehran wheat distribution point, Nasirabad Industrial area, and Shamsabad Industrial Area occupying approximately 289 ha of land.

A summary of land use shows 7500 ha usable land of a total 13750 ha. In addition to that, a survey of the airport and land possibility of noise pollution shows different areas conforming to different noise level standards and zones. Zoning of noise levels with areas classified with a range sound below 60 dB, 65dB, and 70dB with respective activities conforming to the sound levels.

Typically, areas with sound levels below 65dB are equivalent to 6000 ha, below 70dB covered in equivalent to 7,200 ha, area with sound levels below 60dB cover a surface of 3,600 ha. In addition to that, the height of buildings is restricted due to flight of aircrafts.

Ecological findings

An ecological report was important to enable decision-making on the type of plants to grow to attain zero carbon emissions. In addition to that, the ecological report tables the findings about the ecology of the proposed site and the relationship between the plant species and the environment (Che-Ani, Shahmohamadi, Sairi, Mohd-Nor, Zain & Surat, 2009). According to Che-Ani, Shahmohamadi, Sairi, Mohd-Nor, Zain & Surat (2009), the ecological inquiry provides adequate information about the kind of roof to construction a particular area.

Ecological Characteristics On a larger Scale

The proposed area of IKIA airport lies in the Irano-Turanian area, which is with physo-geographical characteristics. It has abioclimate, whichfalls into the geobotanic category prevalent on an arid-north climate and lies between Artimisia, Astrugalus genera, and Afghan-Anatol steppes. In addition that, the region has low rainfall with 69% dominated by Irano-Turanian plants.

Its forest cover consists of a range of species including Juniperus, Hultemia persica, among several others on the Tehran side. On the other hand, the region split into nine sub regions with different types of vegetation cover typically classified into eight groups.

Meteorological Findings

Investigations into the climatic parameters of the proposed site reports an annual precipitation of 161.7 mm based on statistical findings over a long period of investigation. “Typically, the average precipitation for the entire country is 250 mm indicating the region to experience lower precipitations than the rest of the country with significant precipitate variability in the region”(Rahshahr International, 2011).

Statistical data shows that temperatures can rise to a high of 40°centigrade and a low of -20° centigrade with a mean daily temperature with an estimated value of 17.70 C. The region suffers from frost occurring between mid-Octobers to late April peaking in January with an average occurrence of 60 days for the year. The six of the remaining months do not experience frost.

Reportedly, the highest precipitation occurs between December and February with dry air occurring in the months of May and September with July registering the lowest precipitation. The availability of ground water is an additional element in the region. A hydrological report shows Karaj and Zarand-Zaviye to be hydrological sources of ground water. The relative amounts of ground water from both hydrological sources put in table 8 below.

Table 8. Soil characteristics.

Rahshahr International, 2011.

Soil Resources

One important aspect to consider when constructing a facility with zero carbon emissions is to examine in detail the characteristic of the soil that supports the vegetation intended to help to reduce carbon emissions. Much importance related to the percentage use of soils and land resources. Land use contributes a significant amount of carbon emissions into the environment due to the energy intensive activities (Foxon, 2002).

A survey of the land shows the land to fall into three regions. These include hills, which form the major feature in the region, plateaus, and alluvial plains and other characteristic features illustrated in table9 below.

Table 9. Land descriptions.

Rahshahr International, 2011.

The basic carbon generating activities, which are highly dominant in the proposed area, include agricultural activities and emissions due to airport activities such as aircraft and motor vehicle emissions. The rest of the land is either bare and of little or no use and is characterized by consisting a range of rocks and moderate ranges. The biological environment provided information about the appropriateness and strategies to reduce carbon emissions and falls into prevalent vegetation, plant types, and wildlife.

The Biological Environment

The area is deficient of tree cover and severely limited to a specific range of species. Artemisia seiberi is one type of plant that thrives on a surface area of 19,000hectares, with an elevation of 1000 m. In addition to that, the plant thrives in semi-desert areas. On the other hand, Artemisia seiberi- Pteropyrum sp. dominates a land cover of 52,000 ha with its variants.

Other species include Artemisia seiberi- Stipa sp that covers an area of 111,000 ha (Severinsen & Jager, 1998). Further research on the biological environment showed that the regional wildlife contributes significantly to the beauty and attraction of visitors into the area. Mammal species are 42 in number constituting 15 family species. Birds, on the other hand, contribute 122 species belonging to 31 families, each contributing a specific amount of CO₂ to the environment.

However, it may be difficult to quantify the amount of carbon emission into the environment due to the wild life in the area. Further still, nine families of reptiles are distributed the area consisting of 24 species. The distribution consists of amphibians and two species of fish. It is important to conduct an investigation into the ecological effects of protected areas and gradually the carbon emission contributions into the environment.

Typical areas under protection include Alborz, which is 23 km aerial distance. Kavir, which is located at a distance of 12 km of aerial measurements is a protected area and connects with jajrod. Typically, these areas are constituents of the satellite areas with ecological characteristics disconnected from each. The jajrod area measures close to 56625 hectares and has many mountains and hills covering the entire region.

In addition to that, the area experiences the effects of Roheden Rivers that passes through it, particularly in the eastern part of it. With an average temperature of 11 °C and an elevated area of 1150 hectares, the region experiences an average precipitation of 275 mm influencing the overall climate to be semi-arid to an arid area.

The attributes of the area are high concentration of biodiversity with 517 species of plants. In addition to that, animal species identified in the area count up to 192 animals with a concentration of the Astraglus and Artemisia tree species. Each of the animal and plant species contributes significantly to carbon management in the area. Moreover, it is important to note that the area is close to Tehran, the area proposed for construction of the new airport facility.

Another area of interest worth investigating is Kavir, one of the protected areas around the Tehran area. Located in the Ghome salt lake area and covering an area equivalent to 248957 hectare, the protected area covers three administrative provinces managed as a national park area.

Additional features of the area show plain like features with a temperature range of 10 to 25 centigrade and an elevation varies between 740 and 1360 meters. On the other hand, the precipitation level is 132 mm. The ecosystem is a landscape of swamps, salt ecosystems, and desert landscapes.

Location

The proposed site has a location, which is a hybrid between the industrially developed urban and agricultural region, which lies to the North of Tehran and Qom salt lake, and the desert region, which is largely a wilderness area. From a topological point of view, the area is approximately 1000 meters above sea level when measured from the eastern side.

Further topological results show the area to be flat with significant slope variations. The sloping of the landscape varies between 10 and 15%, with an average slope of 5% in the region. On the other hand, the greater percentage aspect of the area is towards the eastern side with the rest of the area presenting a clear view from different directions.

The area enjoys an annual precipitation of 161.7 mm compared with the prevailing annual value of 250 mm. That classifies the area into the category of a low rain receiving area and cold arid climatic characteristics. A critical analysis of the ambro-thermic diagrams reveals the region dominated by arid conditions with five wet moths in a year (Grime, 1998).

Wind strongly influences the flow and direction of the movement of dirt and smog particles in the air. The direction and speed of wind is therefore an important element to consider when setting up a city. Typical of the area under consideration is the wind speed and direction. The area experiences a wind flow of 23 knot (11.5 m/s) in the westerly direction (Hanson& Lindberg, 1991).

On the other hand, the Robat Karim and Shoor Rivers are the dominant rivers that within the region with the Shoor River being the most important of the entire Rivers in the region. However, no significant agricultural activities are suited to the salt water from the River. A distinguishing characteristic of the Shoor River is that it remains dry for a significant part of the year, though it receives water from many tributaries.

On the other hand, it is clear that underground water is unsuitable for agricultural activities as it maintains high levels of salinity (Gaffin, Rosenzweig, Parshall, Beattie, Berghage, O’Keeffe&Braman, 2005). The Kahrizak geological formation is especially of much importance with its lithology characterized by interacting sandstones, clay, and other conglomerates.

Activities that lead to the consumption of energy and eventually leading to high levels of carbon emissions within the region include airport activities and other land use activities. The region suffers from limited precipitation, consists of 42 mammal species,2 amphibian species, poor vegetation cover, saline underground water, and significantly low support to life and biodiversity.

Regional Analysis

A regional analysis of the proposed site informs the study of various issues such as availability of agricultural land. Agricultural land is under increasing threats from a rapid rate of urban development particularly from the construction industry and associated carbon emission due to energy consumption (Korhonen, 2005).

The diagram below shows the most suitable location of the proposed facility and the topological details on of the region. The main factor considered here are the settlement areas, wind flow, and proximity of the area to the mountain range and Parand town that provides some suitable perspective for the growth of the town.

Table 10. Area and land use.

To further analyze and inform the study, an analysis of the water flow and flood control and their effects in the region show the area to be earthquake prone with the highest risk being a high potential of floods from the streams of water that pass through the region. On the other hand, the liquefaction potential varies with each locality.

The eshtehard region is susceptible to a high potential of liquefaction while the rest of the region experiences frequently occurring liquefactions, which at other places occurs without notice. On the other hand, IKIA suffers from a combination of frequent liquefaction to no liquefaction at all. The following table illustrates the findings around the IKIA region in table 10 shown below.

On the other hand, a close study of the region shows that the region experiences solar insulation varying between the surrounding regions with varying intensities. Thus, the solar down gradient varies between 5000 and 5500 with IKIA lying in the above-mentioned solar gradient concentration.

Analytically, the climate of IKIA region is partially pleasant. The yearly cooling energy needs vary from mild levels, temperate, warm, and hot. Typically, cooling energy needs demands for a rise in energy consumption causing a rise in carbon emissions into the environment. On the other hand, energy requirements based on yearly heating needs vary between cold, ultra cold, and frost levels.

Each of the heating needs consume specific quantities of energy resulting from the burning of carbon and leading to carbon emissions into the environment. Historical sites and artifacts provide a basis for tourism and associated energy consumption and carbon emissions. Historical artifacts cover prehistoric, historical, Islamic, and non-specified historical attractions.

Another important factor contributing to carbon emissions is the trend and rate of population growth. A critical analysis of the population growth and trend shows an outwards inter-state migration trend driven by the availability and distribution of job opportunities, a major factor in attracting large population growth into Tehran city. The population growth and distribution is thus concentrated in the urban areas around Tehran.

That has changed the status of certain cities to principal cities and province centers. Typical examples include Qazvin and Qom. On the other hand, the railway network provides an enabling environment as a good communication network for the proposed construction site. The following illustration provides a clear view of the proximity of IKIA to other centers for cargo and other transportation needs (Felson& Pickett, 2005).

A review of cities in proximity of the proposed site with carbon emitting activities, which cause air pollution, is worth analyzing. One of these towns is the city of Aftab. Aftab is located in the southern part of Tehran with a total surface land area estimated at 4689 Hectares. When put to use, estimates show that the area can support 56% of the total surface area dedicated to green space.

On the other hand, Aftab has an area allocated to land use for exhibitions measuring 211 hectares. The 211 hectares are only 4.55% of the total land area proposed for use (Hien, 2002). Other uses of land include residential areas for hotel apartments constituting approximately 2.33% of the total proposed land use. Estimates show the total residential land can accommodate about 1200 residential units with an estimated capacity of five thousand people.

In addition to that, additional features include residence for college students estimated at 80,000 students. Many of the land use activities are according to the estimates summarized in the table above. It is important to conduct an analysis of the ecological footprints associated with each carbon and other pollutants generating activities (Dunnett& Kingsbury, 2004).

Typically, the ecological footprint provides estimates of the amount of consumption energy in each sector. The service sector is estimated to consume energy estimated between 1.43 M.J and 107 552 milliards MJ. On the other hand, per capita consumption specific to areas susceptible to flooding is 230906 MJ, which contributes an energy estimated at 30.787 MJ in the areas of crop production, among other agricultural activities.

In addition to that, estimates of per capital consumption of energy are in the range of 143 million GJ equivalents of 1.43 hectares (Durhman, Rowe, Ebert-May& Rugh, 2004). Estimates show over 171 million m² surface area to be under houses, a significant source of air pollution into the environment due to energy consumptions. Further estimates show residential areas to have an aggregate per capita energy consumption equivalent to 2 million MJ. On the other hand, it is important to note that each surface area is associated with energy of 100 GJ of energy.

Site Analysis

A site analysis reveals detailed information about prevailing weather conditions at the proposed construction site, which is a strongly influencing variable for the levels of energy consumption. Much detailed information about climate data specific to the proposed site shown in table 11 below.

Table 11. Summary of climate data.

Rahshahr International, 2011

The above table details the nature of the climatic conditions at the proposed site and the likely influence on carbon emissions. In addition to that, the climate allows for the implementation of strategies specifically tailored to mitigate and allow for appropriate carbon emission management at the site (Dunnett & Nolan, 2004).

On the other hand, the amount and rate of urbanization asociated with carbon burning activities contribute significantly to strategies formulated to manage carbon and other environmental pollutions.That includes construction projectsincludedin the development list around the IKIA corridor and the two North subways.

It is important to incoporate another access route into IKIA to help make the proposed region accessible, making the region expreince less congestion (Hogrefe, Lynn, Civerolo, Ku, Rosenthal, Rosenzweig, Goldberg, Gaffin, Knowlton& Kinney, 2004). Other issues to be considered during the contruction process activities related to carbon emissions when constructing access routes.

It is important to define an appropriate roads network particulalry with respect to the aspect of the wind, the mountains, and the rains as a strategy of reducing emissions into the emvironment (Asner, Scurlock,Hicke, 2003). At this point, it is important to examine in detail the carbon footprint for each of the carbon emitting activitiesthat consume energy in the following section.

Carbon Foot Print

From an investigation of energy intensiveand carbon producing activities, development needs and projections, significant amounts of energy will have to be burned and strategies have to be put in place to curb the final effects from the emissions. Typically, that also includes land use activities and the size of land in use besides proposed construction activities (Liu & Baskaran, 2003).

Typically, most of the land use is biast to the service industry which appears to be the largest single most carbon emitting source. Typically, the activities and the resulting effects due to the use of green roof systems are analysed and summarised in the following table (Che-Ani, Shahmohamadi, Sairi, Mohd-Nor, Zain & Surat, 2009) in table 12 below.

Table 12. Effects of different factors on Green roofs.

Source: Performance evaluation of an extensive green roof, 2005

Rahshahr International (2011)

Rahshahr International (2011)

It is important to consider a variety of plants known to thrive in the proposed facility. Typically, the plants respond well when planted in dry conditions. These are inherent characteristics of semi-arid and arid areas typical of the Tehran environment (Dunnett, Nagase, Booth& Grime, 2005). n addition to that, the plants demand low maintenance costs thus making the overall maintenance cost for green roof systems within reach of many residents in the proposed site (Köhler, 2003) and (Heinze, 1985) as illustrated in table 13 below tolerant plants drought

Table 13.

A variety of plants to incorporate into green roof system.

Source: Rahshahr International, 2011.

Analysis and Discussion

According to this study, both intensive and extensive green roofs demonstrate several benefits. An examination of the entire area where the planned construction of an airport and surrounding areas have shown a great potential for the construction of green roofs on the buildings to be erected within that region. That typically borrows from the modern concept of benefits of green roofs and the associated technologies.

Thus, borrowing from historical application of green roofs, it has been in modern times that the concept of green roofs as a pollution control mechanism has been studied as one of the mainstream methods as an air pollution control technology.

From theoretical and practical perspectives, green roof systems are one of the most important strategies for reducing pollution effects on the environment particularly specific to the proposed airport construction site. Typical of the green roofing systems are intensive and extensive green roof systems. Both systems in theory and practice, when evaluated come with several benefits, reinforcing the rationale for their implementation as pollution mitigation techniques in urban areas.

Accruing benefits include water quality and environmental benefits such as storm water management, reduction of urban island effects, improved air quality, improved biodiversity, and positive effects on climate change, aesthetical revelations, and a CO₂ emission control mechanism are other benefits associated with green roofs at IKIA.

In order to understand and model the benefits from green roof systems for any private and public investor at IKIA, it is important to conduct a mathematical analysis for various benefits as detailed below.

One of the benefits modeled mathematically is the water quality management. Water quality benefits arise from the capabilities of plant metabolic processes, evapo-transpiration mechanisms, and a number of microbial activities. In addition to that, green roof attenuation activities decrease the concentration of waste chemicals through chemical and physical processes (Lazzarin, Castellotti& Busato, 2005).

One method based on discussions from above is the unit water loading (UAL) that enables calculations on the amount of water loading in a specific unit area under consideration to be performed, based on a number of variables. The variables include EMCi for calculating concentrations when testing the quality of water at any given time during quality analysis and events (n) which provide a basis for the starting point of the calculation.

A is the catchment area which captures the entire IKIA area, and Vi denotes the amount of runoff precipitates that can be accumulated from the roofs. However, it may be difficult to quantify the amount of runoff water. However, the following mathematical expression estimates the amount of controlled water (Hall & Pfeiffer, 2000).

It is important to model the quality of water to determine the weighted mean, which is the volumetric concentration (MC_VW) of water, and the variables that significantly influence the outcome of the calculations.

The above mathematical expression provides a clear way to calculate variables that constitute the quality of water under investigation at any point in IKIA. However, further research in the mathematical model need to be done to address every runoff event in the investigation and for a long period.

Another benefit is storm water management. Analytically, one can perform calculations on water management benefits based on different mathematical expression explained elsewhere in the paper.

On the other hand, the aggregate values of the accruing benefits, the monetary cost of storm water management and the value of erosion mitigation of the green roof system during the construction process are strong benefits. On the other hand, the water retention capacity of the green roof system within the area of the green roof under considerations indicates the viability of using the green roofs to capture and retain runoff water.

Based on the above mathematical model, any public or private investor can calculate storm water benefits in monetary terms. As a pollution control mechanism, carbon sequestration values due to CO₂emissions, insulation of heat loss into the environment thus resulting in a reduction of energy demands for a building, evapo-transpiration effects, and an aggregate reduction of the urban heat island effect.

On the other hand, the ecological foot print lays further support to the need for the use of green roof systems as illustrated in the following where the service sector in Iran is estimated to consume energy estimated between 1.43 M.J and 107 552 milliards MJ. That is in particular within the IKIA region.

On the other hand, per capita consumption specific to areas susceptible to flooding is 230906 MJ, which contributes energy estimated at 30.787 MJ in the areas of crop production, among other agricultural activities. In addition to that, estimates of per capital consumption of energy are in the range of 143 million GJ equivalents of 1.43 hectares.

Thus, the amount of carbon generated from these activities is bound to generate significant amounts of CO₂and water into the environment calling for significant actions to formulate methods and in particular the use of green roofs as mitigation techniques. On the other hand, it is evident from the above study that significant emissions of carbon into the environment is bound to cause smog and other forms of environmental pollution that require specific technologies to clean up.

Conclusion

The main aim of the study is to conduct an investigation into green roofs as a method to mitigate air pollution with special reference to Tehran, Iran. Green roofs have been in existence since the time of antiquity. This study has focused on available literature and information on green roofs constructed in urban areas with limited space.

The space limit does not allow for the construction of large gardens on conventional roofs, reinforcing the need to make use of minimal space on conventional roofs. Thus, the study is an investigation on green roofs to mitigate air pollution with special reference to Tehran, Iran.

The paper examines many of the factors that influence carbon-generating activities. These factors include the topology of the area, the direction of the wind, the location of the site, the source of electrical power, industrial centers, residential areas, usable areas, and zoning for flight routes.

These carbon generating activities and the use of appropriate green roofs as a carbon control method in the area proposed for the construction of the IKIA airport and the need to construct green roof systems as a carbon management strategy and a pollution mitigation mechanism.

Green roofs have come with several benefits both to the user and to the environment. These include:

1. The green roof provides thermal insulation, a benefit consequently leading to energy conservation and CO₂ reductions into the environment.
2. Plants grown on green roofs provide a mechanism for reducing CO₂ emissions into the environment, thus mitigating carbon pollution into the environment.
3. Green roofs reduce urban island effects through a carbon absorption mechanism.
4. Green roofs have the capacity to mitigate the effects of storm water by reducing the rate of absorption of rainwater into the underlying surface.
5. Green roofs have the effect of fulfilling the aesthetical desires of people, thus acting as a tourist attraction.
6. Extensive green roofs have the capability to remove dirt particles from the air resulting in cleaner air.
7. Green roofs have a biodiversity benefit compared with brown roofs, making the roofs beneficial as a tourist attraction.

Typically, green roofs fulfilled aesthetical feelings of the highly regarded in the societies in the past. However, due to current pollution and other environmental challenges facing governments and individuals, green roofs are among the most critically important strategies for mitigating the effects of air pollution due to energy consumption and carbon emitting activities.

Green roofs fall into intensive and extensive roof systems and each system has specific benefits that distinguish each. Thus, the rationale to construct a green roof is due to several benefits among them being a pollution control mechanism. In addition to that, other benefits include storm water management and mitigation of urban island effects.

In addition to that, it is possible to tailor a green roof to fulfill aesthetical views that may lead to tourist attractions. That is the case with IKIA, a region proposed for the construction of an airport facility. A case study analysis of Tehran serves to inform the study at IKIA. Typically, the site proposed for the construction of the airport facility situated some kilometers from Tehran experiences semi-desert and desert climatic condition.

In addition to that, high-rise buildings are proposed for construction in the region typically due to job opportunities to accommodate rising population. Energy consuming activities that lead to the production of carbon and related compounds are bound to rise significantly, contributing to rising levels of pollution.

As a critical approach to overcome all the challenges associated with a myriad of CO₂ generation activities, it is the construction of green roofs as a pollution control mechanism that form one of the methods of addressing the pollution control problem. However, there is need to conduct further research to mathematically model the benefits from green roof.

It is, however recommended that detailed studies be done at the proposed region for the construction of the airport facility. Further, there is need conduct a detailed analysis of available documents detailing the topology of the region and other geographical details in other studies.

On the other hand, established facts show that the wind direction strongly influences the flow and direction of the movement of dirt and smog particles in the air. These movements have detrimental effects on the temperature of the environment within the IKIA region, where the proposed site for the construction of the airport is to occur.

The direction and speed of wind is therefore an important element to consider when setting up a city. Typical of the area under consideration is the wind speed and direction. Facts show that the entire area experiences wind flow of 23 knot (11.5 m/s) to the west, further urging the movement of smog and other dirt particles into the region, eventually increasing the effects of the resulting particles on the environment.

References

Asadollah-Fardi, G. R. Current Situation of Air Pollution in Tehran withemphasis on District 12. Web.

Asner, G. P., Scurlock, J. M. O.and Hicke, J., A., 2003. Global synthesis of leaf Areaindex observations: implications for Ecological and remote sensing studies. Global Ecol. Biogeography, 12, 191–205.

Bass, B.and Baskaran, B., 2003. Evaluating Rooftop and Vertical Gardens as an AdaptationStrategy for Urban Areas. Ottawa (Canada): National Research Council Canada, Institute for Research in Construction. Report no. NRCC-46737.

Bass, B., Krayenhoff, E. S. Martilli, A., Stull, R. B. and Auld, H., 2003.The impact of Greenroofs on Toronto’s urban heat island. Pages 292–304 in Proceedings ofthe First North American Green Roof Conference: Greening Rooftops for Sustainable Communities; Chicago. Toronto (Canada): Cardinal Group.

Baumann, N., 2006. Ground-nesting birds on green roofs in Switzerland: Preliminary observations. Urban Habitats 4: 37–50.

Beattie, D. andBerghage, R., 2004. Green roof media characteristics: The basics. Paper presented at the Second Annual Greening Rooftops for Sustainable Communities Conference,Awards and Trade Show ,Portland: Oregon.

Boivin, M-A., Lamry, M-P., Gosselin, A.and Dansereau, B., 2001. Effect of artificial substrate depth on freezing injury of six herbaceous perennials grown in a green roof system.HortTechnology 11: 409–412.

Carter, T. and Keele, A., 2007. Life-cycle cost–benefit analysis of extensive vegetated roof systems. Web.

Celik, S., Morgan, S. and Retzlaff, William, A., n.d. Energy Conservation Analysis of Various Green Roof Systems. Edwardsville: Southern Illinois’ University.

Clark, C., Adriaens , P.and Talbot, F. B., 2007. Green Roof Valuation: A Probabilistic Economic Analysis of Environmental Benefits. University of Michigan: Ann Arbor, VOL. 42, NO. 6, 2008.

Clements, J. A. and Sherif, S. A., 1998. Thermal analysis of roof spray cooling. International Journal of Energy Research 22: 1337–1350.

Climate change, n.d. Impacts, vulnerabilities and adaptation in developing countries. United Nations Framework Convention on Climate Change.

Che-Ani, A. I., Shahmohamadi, P., Sairi, A., Mohd-Nor, M. F. I., Zain, M. F. M. and Surat, M. 2009. Mitigating the Urban Heat Island Effect: Some Points without Altering Existing City Planning.European Journal of Scientific Research ISSN 1450-216X Vol.35 No.2, pp.204-216. Web.

Coffman, R. R. and Davis, G., 2005. Insect and avian fauna presence on the Ford assembly plant ecoroof. Paper presented at the Third Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show; Washington, DC.

Cooper-Marcus, C.and Barnes, M., 1999. Healing Gardens: Therapeutic Benefits and Design Recommendations. New York: Wiley.

Cottingham, K. L., Brown, B. L. and Lennon, J. T., 2001. Biodiversity may regulate thetemporal variability of ecological systems. Ecology Letters 4: 72–85.

Del Barrio, E. P., 1998. Analysis of the green roofs cooling potential in buildings. Energy and Buildings 27: 179–193.

Dixon, A., Butler, D. and Fewkes, A., 1999. Water saving potential of domestic water reuse systems using greywater and rainwater in combination. Water Science and Technology 39 (5): 25–32.

Dobson, A., 1995. Green Political Thought, Second Edition, London, Routledge.

Dunnett, N. P. & Kingsbury, N., 2004. Planting Green Roofs and Living Walls. Portland (OR): Timber Press.

Dunnett, N. P. & Nolan, A., 2004. The effect of substrate depth and supplementary watering on the growth of nine herbaceous perennials in a semiextensive green roof. Acta Horticulturae 643: 305–309.

Dunnett, N. P. Nagase, A., Booth, R. and Grime, J. P., 2005. Vegetation compositionof green roofs and its influence on runoff and biodiversity. Paper presented at the Third Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show; Washington, DC.

Durhman, A., Rowe, D. B, Ebert-May, D.and Rugh, C. L., 2004. Evaluation of crassulacean species on extensive green roofs. Paper presented at the Second Annual Greening Rooftops for Sustainable Communities.

Earth Pledge., 2005. Green Roofs: Ecological Design and Construction; Schiffer Publishing, Ltd.: Atglen, PA.

Eijk, Henk, V. D., 2010. Historical Gardens. Web.

Emmanuel, M. R., 2005. An Urban Approach to Climate-Sensitive Design; Strategies for the Tropics, London, Spon Press.

Eugene, 2008. Eco-Roof & Roof Garden.Storm water Management Manual. Web.

Farzaneh, R., 2005. Evapotranspiration rates from extensive green roof plant species.Master’s thesis. Pennsylvania State University.

Felson, A. J. and Pickett, S.T. A., 2005. Designed experiments: New approaches to studying urban ecosystems. Frontiers in Ecology and the Environment 3: 549–556.

Foxon, T., 2002.Sustainability Criteria for Decision Support in the UK Water Industry”,Journal of Environmental Planning and Management, 45(2): 285- 301.

Gaffin, S. R., Rosenzweig, C., Parshall, L., Beattie, D., Berghage, R., O’Keeffe, G. &Braman, D., 2005. Energy balance modeling applied to a comparison ofgreen and white roof cooling efficiency. Paper presented at the ThirdAnnual Greening Rooftops for Sustainable Communities Conference,Awards and Trade Show; Washington, DC.

Gaffin, S., Rosenzweig, C, Parshall, L, Hillel, D, Eichenbaum-Pikser, J.,Greenbaum, A., Blake, R., Beattie, R. and Berghage, R., 2006. Quantifying evaporativecooling from green roofs and comparison to other land surfaces. Paperpresented at the Fourth Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show; Boston.

Gedge, D.and Kadas, G., 2004. Bugs, bees, and spiders: Green roof design for rare invertebrates. Paper presented at the Second Annual Greening Rooftops for Sustainable Communities Conference,Awards and Trade Show; Portland, Oregon.

Getter, K. L. and Rowe, D. B., 2006. The role of green roofs in sustainable development.HortScience 41: 1276–1286.

Givoni, B., 1998. Climate Considerations in Building and Urban Design, Ontarion: John Wiley& Sons.

Goward, S.N. Cruickshanks, G.D., andHope, A.S. 1985. Observed Relation between Thermal Emission and Reflected Spectral Radiance of a Complex Vegetated Landscape,RemoteSensing of the Environment, 18: 137–146.

Grant, G., 2006. Extensive green roofs in London. Urban Habitats 4: 51–65.

Grant, G. Engleback, L., Nicholson, B. Gedge, D. Frith, M. and Harvey, P. 2003. Green Roofs: their existing status potential for conserving biodiversity in urban areas English Nature Research Reports. Working today for nature tomorrow. English Nature Research Reports. EcoSchemes Ltd: Northminster House, Peterborough PE1 1UA.

Getter, Kristin L. Rowe, D.B. Robertson, G.P. Cregg, Bert, M. and Andresen, Jeffery, A., 2009. Carbon Sequestration Potential of Extensive Green Roofs. Environmental Science and Technology, ACS publications. Web.

Grime, J. P., 1998. Benefits of plant diversity to ecosystems: Immediate, filter and founder effects. Journal of Ecology 86: 902–910.

Hall, P. and Pfeiffer, U., 2000. Urban Future 21: A Global Agenda for 21 Century Cities,London, Spon Press.

Hanson, P. J. and Lindberg, S. E., 1991. Dry deposition of reactive nitrogen compounds: A review of leaf, canopy, and non-foliar measurements, Atmos. Environ.22A (8), 1615–1634.

Hartig, T.,Mang, M. and Evans, G. W., 1991. Restorative effects of natural Environmentexperience. Environment and Behavior 23: 3–26.

Köhler, M.,and Keeley, M., 2005. The green roof tradition in Germany: The example of Berlin. Pages 108–112 in Hoffman L, McDonough W, eds. GreenRoofs: Ecological Design and Construction.New York: Schiffer.

Kolb, W.and Schwarz, T., 1993. Zum Klimatisierungseffekt von Pflanzenbeständen auf Dächern.Veitshöchheimer Berichte 4: 28–36.

Korhonen, J., 2005. Theory of industrial ecology: The case of the concept of diversity. Progress in Industrial Ecology, an International Journal 2:35–72.

Larson, D.W.Matthes, U.and Kelly, P. E., 2000. Cliff Ecology. Cambridge (United Kingdom): Cambridge University Press.

Mentens, J, Raes, D. andHermy, M., 2005. Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landscapeand Urban Planning 77: 21–226.

Morikawa, H., Takahasi, M. andKawamura, Y., 1998. More than a 600-fold variation in nitrogen dioxide assimilation among 217 plant taxa. Plant Cell Environ. 21, 180–190.

Oberndorfer, E. Lundholm , J. Bass B.Coffman,R.R. Doshi,H. Dunnett,N.Gaffin, S. Köhler, M. Liuk, K.Y. and Rowe, B., 2007. Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services. 57 (10). Photosynthesis: Carbon Reactions., n.d. Chapter 8. Web.

Rosenfeld, A. H. Akbari, H. Bretz, S. Fishman, B. L. Kurn, D. M. Sailor, D. Taha, H., 1995. Mitigation of urban heat islands: materials, utility programs, updates.Energy Build. 22, 255–265.

Saiz, S. Kennedy, C. Bass, B. and Pressnail, K., 2006. Comparative life cycle assessment of standard and green roofs. Environ. Sci.Technol. 40, 4312–4316.

Severinsen, M. andJager, T.,1998.Modelling the influence of terrestrial vegetation on the environmental fate of xenobiotics.Chemosphere37, 41–62.

Taha, H., Akbari, H. and Rosenfeld, A., 1991.Heat Island and oasis effects of vegetative canopies: micrometeorological field measurements. Theor. Appl. Climatol. 44, 123–138.

Takahashi, M. Konaka, D. Sakamoto, A. and Morikawa, H., 2005. Nocturnal uptake and assimilation of nitrogen dioxide by C3 and CAM plants, Z. Naturforsch. 60c, 279–284.

Takebayashi, H.and Moriyama, M., 2007. Surface heat budget on green roof and high reflection roof for mitigation of urban heat island. Build. Environ. 42, 2971–2979.

The Green Roof Code, 2011.Green Roof Code of Best Practice for the UK 2011. Groundwork Sheffield: Sheffield

Theodosiou,.T. G., 2003. Summer period analysis of the performance of a planted roof as a passive cooling technique.Energy Build.35, 909–917.Toronto and Region Conservation. 2006. Evaluation of an Extensive Greenroof. Toronto: York University.

Trumper, K., Bertzky, M., Dickson, B., van der Heijden, G., Jenkins, M., Manning, P.2009. The Natural Fix? The roleof ecosystems in climate mitigation.A UNEP rapid response assessment. Cambridge: United Nations Environment Programme, UNEPWCMC. Web.

U.S. EPA. 1998. Regulatory Impact Analysis for the NOxSIP Call, FIP, and Section 126 Petitions; EPA-452/ R-98–003A; U.S. Environmental Protection Agency: Washington,DC,

VanWoert, N. D. Rowe, B., Andresen, J. A., Rugh, C. L., Fernandez, R. T. & Xiao, L. 2005. Green roof stormwater retention: Effects of roof surface, slope, and media depth, J. Environ. Qual. 34 (3), 1036–1044.

Victoria Transport Policy Institute, 2011.Transportation Cost and Benefit Analysis Air Pollution Costs Web. Available from: www.vtpi.org/tca/tca0510.pdf .

Villarreal, E. L. and Bengtsson, L., 2005. Response of a sedum green-roof to individual rain events.Ecol. Eng. 25 (1), 1–7.

Villarreal, E. L. Semadeni-Davies, A. and Bengtsson, L., 2004. Inner city stormwater control using a combination of best management practices, Ecol. Eng.22, 279–298.

Wark, C. G. and Wark, W. W., 2003. Green Roof Specifications and Standards Establishing an emerging technology.Construction Specifications Institute: Alexandria.Vol. 56, No.8.

Wellburn, A. R.,1990. Why are atmospheric oxides of nitrogen usually phytotoxic and not alternative fertilizers.NewPhytol. 115, 395–429.

Wong, N. H. Chen, Y. Ong, C. L. and Sia, A., 2003. Investigation of thermal benefits of rooftop garden in the tropical environment.Build.Environ.38, 261–270.