Acid rain is precipitation that carries abnormally high levels of hydrogen ions, resulting from atmospheric pollution. Normal rain is slightly acidic, typically having a pH of about 5.6 due to dissolved carbon dioxide. Precipitation with a pH lower than 5.6 is defined as acid rain. This global environmental problem affects both natural ecosystems and man-made infrastructure. The resulting damage is primarily caused by sulfuric and nitric acids formed from industrial and vehicular emissions.
The Mechanism of Acid Deposition
Acid deposition occurs through two distinct processes: wet deposition and dry deposition. Wet deposition is the form most commonly recognized as acid rain, involving acidic compounds dissolved in atmospheric moisture that fall as rain, snow, fog, or hail. These acidic compounds are primarily sulfuric acid and nitric acid, which are formed in the atmosphere.
These acids are created when primary pollutants, specifically sulfur dioxide (SO2) and nitrogen oxides (NOx), are released from sources like power plants and vehicles. Once airborne, these gases react with water, oxygen, and other chemicals, undergoing complex oxidation processes often accelerated by sunlight. The resulting sulfuric and nitric acids then mix with water vapor before falling to the Earth’s surface.
Dry deposition involves acidic gases and fine particulate matter settling directly onto surfaces without precipitation. Approximately half of the total acidity in the atmosphere can be deposited in this dry form. These acidic particles and gases are later washed off surfaces by subsequent rainfall, which adds to the overall acid load in the runoff water.
Impact on Aquatic Ecosystems
The ecological effects of acid deposition are most pronounced in sensitive aquatic environments. These water bodies become acidified when the surrounding soil and water lack sufficient buffering capacity to neutralize the incoming acid. This decrease in acidity causes a cascade of biological and chemical problems that disrupt the entire food web.
One of the most damaging effects is the mobilization of aluminum from soil particles into the water. In its ionic form, aluminum is highly toxic to fish and other aquatic organisms. The toxic aluminum damages fish gills by interfering with their ability to regulate salt and water balance, essentially leading to suffocation.
The reproductive cycle of aquatic life is particularly sensitive to changes in acidity. At a pH level of 5, the eggs of most fish species are unable to hatch, causing rapid population decline. Even adult fish that survive chronic low pH experience chronic stress, leading to lower body weight and smaller size, making them less able to compete for resources. A sudden, intense pulse of acidity, known as “acid shock,” often occurs during the spring snowmelt, releasing a large concentration of accumulated acid and aluminum into the waterways, which can wipe out large numbers of aquatic organisms.
Impact on Terrestrial Ecosystems
Acid rain causes long-term harm to terrestrial environments, primarily through profound changes to soil chemistry. The hydrogen ions present in the acidic deposition displace vital plant nutrients, such as calcium and magnesium, from the soil particles. This process, known as leaching, systematically strips the soil of necessary cations, making them unavailable for plant uptake and weakening the overall health of the vegetation.
Concurrently, the increased soil acidity mobilizes aluminum that was previously bound within clay particles in a harmless form. The resulting soluble aluminum ions are toxic to plant root systems, hindering the absorption of water and nutrients. This combination of nutrient depletion and aluminum toxicity weakens trees and other plants, making them susceptible to secondary stressors.
Trees do not usually die directly from acid rain, but the chronic stress makes them vulnerable to disease, insect infestation, and freezing temperatures. High-elevation forests are especially affected because they are frequently bathed in highly acidic fog and clouds. This acid fog can directly strip nutrients from tree foliage and needles, causing visible damage and impairing the tree’s ability to photosynthesize.
Damage to Built Structures and Materials
Beyond the natural environment, acid deposition aggressively attacks human infrastructure and cultural heritage sites. This damage is a result of chemical weathering, where the acids react directly with construction materials. Structures made of stone, particularly limestone and marble, are highly vulnerable because they contain calcium carbonate.
The sulfuric and nitric acids in the deposition react with the calcium carbonate, dissolving the stone and causing surface erosion and pitting. This reaction leads to the loss of fine details on statues and monuments and the weakening of building facades over decades of exposure. The deterioration of historical objects and buildings requires costly preservation and restoration efforts.
Acid rain also accelerates the corrosion of metal structures. Exposed metals like steel and bronze experience an increased rate of oxidation due to the acidity. This increased corrosion is a significant concern for infrastructure such as bridges, railings, and vehicles, leading to increased maintenance costs and potential structural integrity issues.