Acid rain is precipitation (rain, snow, or fog) that exhibits a higher level of acidity than clean air. Normal rain is slightly acidic, with a pH of approximately 5.6, due to dissolved atmospheric carbon dioxide. Acid rain typically drops below a pH of 4.5, reflecting a significantly greater concentration of hydrogen ions.
The combustion of fossil fuels is the primary human activity responsible for this environmental change. The burning of coal for electricity generation is a major source of the precursor chemicals that lead to acid rain.
Pollutant Release from Coal Combustion
The release of acidic compounds begins with the composition of coal, which is a complex sedimentary rock containing various impurities. Two main elements within the coal structure are responsible for acid rain formation: sulfur and nitrogen. Sulfur is present as an impurity in both organic forms (bound within the hydrocarbon matrix) and inorganic forms, such as iron pyrite.
The sulfur content within coal varies widely depending on the source, ranging from less than 0.2% to more than 5%. When coal is burned in high-temperature industrial boilers, this sulfur is oxidized and released into the atmosphere, predominantly as gaseous sulfur dioxide (SO2).
Nitrogen is also a constituent of coal, mainly in the form of organonitrogen compounds, which are naturally incorporated during the coal formation process. During the combustion process, the high temperatures of the furnace cause the nitrogen atoms present in the fuel to react with oxygen from the air.
This reaction generates various nitrogen oxides, collectively termed NOx, which are released through smokestacks along with the sulfur dioxide. These two gaseous compounds, SO2 and NOx, are the precursor pollutants. They are transported by wind currents far from the power plant source, setting the stage for the chemical transformation that follows.
The Chemical Conversion to Acid
Once sulfur dioxide and nitrogen oxides are emitted into the atmosphere, they undergo complex chemical reactions with atmospheric components to form strong acids. This conversion is driven by the presence of water vapor, oxygen, and powerful atmospheric oxidants. The end products are sulfuric acid (H2SO4) and nitric acid (HNO3), the two main components of acid rain.
Sulfur dioxide first reacts with oxygen and highly reactive atmospheric oxidants. These oxidants drive the transformation of SO2 into sulfur trioxide (SO3). Sulfur trioxide is highly soluble and rapidly combines with water vapor in the air to form droplets of concentrated sulfuric acid.
A similar oxidation process occurs for the nitrogen oxides, though the specific pathway differs slightly. Nitrogen oxides, primarily nitrogen monoxide (NO) and nitrogen dioxide (NO2), react with hydroxyl radicals present in the atmosphere. This reaction results in the formation of gaseous nitric acid (HNO3).
Both the newly formed sulfuric and nitric acids then dissolve into cloud droplets, fog, or snow. The acidic compounds are removed from the atmosphere through acid deposition, which occurs in two forms. Wet deposition is the most recognized form, where the acids fall to the ground mixed with rain, snow, or fog. Dry deposition occurs when acidic particles or gases settle directly onto surfaces like buildings, vegetation, or soil.
Consequences of Acid Deposition
The deposition of these strong acids affects both natural ecosystems and human infrastructure. In aquatic environments, the influx of acidic precipitation into lakes and streams causes a significant reduction in pH levels, a process known as acidification. This change in water chemistry severely impacts aquatic life, particularly acid-sensitive organisms like freshwater shrimp, snails, and mussels.
As the acidity increases, it triggers the leaching of toxic substances, such as aluminum, from the surrounding soil into the water bodies. This mobilized aluminum is toxic to fish, stressing adult populations and often preventing the eggs of species like trout from hatching successfully at pH levels below 5. The loss of sensitive species disrupts the entire food web, causing a reduction in biodiversity.
On land, the sulfuric and nitric acids can damage vegetation directly by scarring leaf tissue and indirectly by altering soil chemistry. The acidic input leaches essential nutrients, such as calcium and magnesium, from the soil, making them unavailable for plant uptake and weakening the overall health of forests and crops.
The corrosive nature of acid deposition extends to man-made structures, accelerating the weathering of materials like marble, limestone, and steel. This corrosion results in the deterioration of historic buildings, statues, and various forms of public infrastructure.