Introduction
Acid rain refers to rain containing strong inorganic acids in solution. The acids include sulfuric acid, ammonium and nitric acid. The acids originate from acid forming substances emitted into the atmosphere from combustion of hydrocarbon fuels and farming activities (Driscoll et al. 2005, p. 27). These chemicals interact with ozone and atmospheric moisture to form the acids which are incorporated into rain droplets and transferred to the earth’s surface.
This report explains the processes leading to acidic atmospheric deposition. It begins with a brief description of how acid rain is formed from acid forming substances emitted from the earth’s surface. A discussion of the effects of rain on ecosystems and man-made structures then follows. Acidic atmospheric deposition alters the chemistry of soil, strains vegetations, acidifies water bodies, dissolves limestone used in buildings, and corrodes metallic structures.
Formation of acid rain
Sulfuric acid is the main component of acid rain. The other components are nitric acid and ammonium. Sulfuric acid is formed by a chemical reaction involving sulfur and hydroxyl radicals (OH). Combustion of sulfur containing substances is a major source of atmospheric sulfur. Major contributors of atmospheric sulfur are power plants, automobile engines, and decomposing fertilizers.
It is emitted in various forms but the most commonly occurring forms are sulfur dioxide (SO2) and sulfur trioxide (SO3). Following emission, sulfur dioxide reacts with OH radicals from ozone photolysis to form SO3 (Goodarzi, Solimannejad & Vahedpour 2012, p. 1610). Photolysis of hydroxide is a complex processes involving break down of Ozone to form highly reactive oxide radicals which then react with water to from OH radicals. SO3 then attacks two water molecules to form sulfuric acid.
SO3 + 2H2O H2SO4
Figure 1. Formation of sulfuric acid from two molecules of water and sulfur trioxide (Goodarzi, Solimannejad & Vahedpour 2012, p. 1610).
Nitric acid and ammonium are formed from gaseous compounds containing nitrogen molecules. Nitrogen dioxide is also emitted during combustion. Another source of nitrogen dioxide and nitric oxide is ammonium containing fertilizer. Ammonium fertilizers decay giving rise to the gases. The gases then undergo chemical changes in the atmosphere producing nitric acid.
Effects of acid rain
Acid rain causes a variety of changes in the ecosystems and man-made structures. It alters the chemical composition of soil, causes erratic growth of plants, and acidifies aquatic ecosystems. The effects are either direct or indirect. Direct effects include those that cause changes by themselves like alteration of soil composition. Indirect effects include depletion of fish food stocks in the aquatic ecosystems.
Effects of acid rain on soil
Acid deposition causes serious alteration in the chemical composition of soil. The alteration has serious consequences on plant growth especially in forests. Acid rain causes depletion of soil cations like calcium, magnesium, potassium and sodium. Acidification of soil by acid deposition causes the cations to be released and washed away by surface water.
At the same time, inorganic aluminum accumulates in the soil causing aluminum overload. Normally, cations originate from weathering and atmospheric deposition. The cations are then used by plants. Acid rain alters this cycle causing a situation in which the cations are washed away faster than they can be replenished through natural processes. Depletion of cations also makes it difficult to rid soil of added acids. Fertilizers like ammonium nitrate contain acids.
Other fertilizers may be modified in soil to form strong acids. Acid deposition also causes accumulation of sulfur and nitrogen. Excess sulfur and nitrogen creates conditions that do not support proper growth of plants. Excess sulfur and nitrogen can also be washed into water bodies where they undergo chemical reactions to produce acids. Sulfur reacts with water molecules to form sulfuric acid. The end result is the acidification of aquatic environment.
Effect of acid rain on vegetation
Acid deposition affects vegetation in two ways. First, it alters the chemical composition of soil thus depriving plants of essential nutrients. Acid rain causes leaching of cations which are important nutrients for plants. Therefore, acid rain causes growth retardation of some plants.
Second, acid rain dissolves cations in the leaves. This causes a significant strain on the plants. Growth of some plants becomes erratic. Acid deposition may also cause decreased decomposition of dead plants. Decomposition is an important means of nutrient recycling. Formation of humus is slowed down by acid rain. Decomposition appears to depend on cation-containing enzymes.
Effects of acid deposition on aquatic ecosystems
Acid rain causes acidification of water bodies like streams, rivers and lakes. Acidification of water bodies has profound effects on aquatic life. Most fish species cannot thrive in acidic environments (Mason 1994, p. 1250). In extreme cases, acidification of water causes sudden death of fish.
An indirect cause of the deaths is depletion of food. Algae and planktons which form a great percentage of fish food do not survive in acidic environments. This may cause complete elimination of all forms of aquatic life. Accumulation of leached aluminum in the water is also responsible for fish poisoning. Acidification of water bodies occurs over a long period of time. In some parts of the world it is thought that this process has been going on for over two hundred years. This implies that de-acidification will take a number of years.
Effects of acid deposition on man-made structures
Acid deposition corrodes metallic structures. The most affected metallic structures are those made of iron. Corrosion causes structural weaknesses thus endangering life. Structures made of limestone are also affected by acid deposition. Acid rain causes dissolution of calcium carbonate. Calcium carbonate is a constituent of limestone.
Over an extended period of time, it compromises the structural strength of buildings. This problem is particularly serious in some areas. This may be attributed to the fact that emission of acid forming substances is greater in some regions. In some regions building materials do not contain a lot of calcium carbonate.
Figure 2. The mean number of fish species in lakes across Eastern United States of America. The pH scale is specified. N is the number of lakes. The mean number of fish species per lake decline with decreasing pH (Driscoll et al. 2005, p. 43).
Conclusion
This report discussed the formation and effects of acid rain. Acid rain is also known as acidic atmospheric deposition. Acid rain is formed when acid forming substances interact with ozone and water forming strong acids which are then transferred to the earth’s surface. Sulfuric acid is a major component of acid rain.
Sulfuric acid is formed following emission of sulfur from combustion. The gaseous form of sulfur exists in a variety of species. The most common being sulfur dioxide and sulfur trioxide. Sulfuric acid is formed by a chemical reaction between sulfur trioxide and hydroxyl radicals.
Acid rain acidifies water, alters soil composition, corrodes buildings, and strains plants. In addition to draining the soil of cations, it causes build up of soil sulfur and nitrogen. It also bleaches the cations from plant leaves. Acidification of water bodies alters aquatic ecosystems. This may cause deaths of all forms of aquatic life. Some fish species cannot thrive in acidic environments. Planktons and algae are a major source of food for small fish.
References
Driscoll, C et al. 2005, Acidic deposition: Sources and Effects, John Wiley & Sons Ltd, Chichester.
Goodarzi, M, Solimannejad, M & Vahedpour, M 2012, ‘Investigation of the formation of acid rain based on the sulfur tetroxide (SO4(C2v)) and OH radical reaction’, Struct Chem, vol. 23, pp.1609–1615.doi: 10.1007/s11224-012-9966-5.
Mason, J 1994, ‘Acid Rain: Its Effects on Lakes, Streams and Fish’, Renewable Energy, Vol.5, no. 2, pp. 1247-1253.