Acid rain is precipitation that is more acidic than would be expected from the natural background chemistry of the atmosphere, particularly because atmospheric sulfur and nitrogen compounds are transformed into acids and deposited at the Earth's surface. The term is commonly used broadly for acid deposition, which includes acidic rain, snow, fog and other forms of wet deposition as well as dry deposition of acidic gases and particles.
Natural rain is already mildly acidic because atmospheric carbon dioxide dissolves in water and forms carbonic acid, so acid rain does not mean that all rain is strongly acidic or corrosive. The key distinction is additional acidity associated especially with sulfuric and nitric acids produced from sulfur dioxide and nitrogen oxides. Acid deposition can alter soils and freshwater chemistry, affect vegetation, mobilise metals and accelerate deterioration of some materials. Its effects depend on atmospheric chemistry, precipitation, geology, soil buffering capacity and the sensitivity of ecosystems receiving the deposition.
The acidity of a solution is commonly described using pH, a logarithmic measure related to hydrogen ion activity. Lower pH indicates greater acidity, while higher pH indicates greater alkalinity. Because the pH scale is logarithmic, a change of one pH unit represents a tenfold change in hydrogen ion activity under the simplified concentration interpretation commonly used for basic explanation. Natural precipitation varies in acidity because atmospheric gases and aerosols dissolve into water and because local atmospheric composition differs.
Carbonic acid formed from carbon dioxide normally gives uncontaminated rain a mildly acidic character. Acid deposition becomes environmentally important when sulfuric and nitric acids substantially increase hydrogen ion inputs beyond the natural background and when ecosystems cannot neutralise those inputs. Consequently, acid rain is not defined merely by one universal pH value in every scientific context. Assessment considers both precipitation chemistry and the environmental acid loading delivered to a location.
Sulfur dioxide is an important precursor of acid deposition. It can be emitted by combustion of sulfur containing fossil fuels, particularly coal and certain heavy oils, as well as by metal smelting and volcanic activity. In the atmosphere, sulfur dioxide can be oxidised through gas phase and aqueous phase reactions to form sulfuric acid and sulfate compounds. Sulfate particles can travel considerable distances before being deposited.
Nitrogen oxides, including nitric oxide and nitrogen dioxide, are produced by high temperature combustion in engines, power stations and industrial processes and also arise from natural sources such as lightning and soils. Atmospheric oxidation transforms nitrogen oxides into nitric acid and nitrate compounds. These sulfur and nitrogen products can be incorporated into cloud droplets, precipitation or particles and then deposited on land and water. The chemistry is complex and depends on oxidants, sunlight, cloud water, humidity, temperature and atmospheric transport.
Acid deposition can affect freshwater ecosystems by lowering the pH of lakes, streams and wetlands when acidic inputs exceed the system's natural buffering capacity. Buffering is the ability of a chemical system to resist changes in pH. Watersheds underlain by carbonate rich rocks often have greater capacity to neutralise acidity because minerals can react with acids, whereas areas dominated by certain granitic or other poorly buffering rocks can be more sensitive.
Acidification can increase the solubility and mobility of metals such as aluminium in soils and waters. Elevated dissolved aluminium can be harmful to aquatic organisms under suitable conditions. Fish eggs, juvenile fish and invertebrates can be particularly sensitive to changes in water chemistry. Species composition may change as sensitive organisms decline and more acid tolerant species persist. Recovery can occur when acid inputs are reduced, but chemical and biological recovery may occur at different rates and can be limited by stored acidity or depleted soil nutrients.
Terrestrial ecosystems can also be affected by acid deposition. Hydrogen ions entering soils can displace nutrient cations such as calcium and magnesium from exchange sites, potentially contributing to nutrient depletion when losses exceed replenishment. Acidification can alter microbial processes and influence decomposition and nutrient cycling. Mobilisation of aluminium and other metals can affect plant roots.
Direct damage to vegetation can occur under particular circumstances, especially where acidic fog or cloud water interacts with leaves and where other pollutants are present, although ecosystem effects are often mediated through soil chemistry and nutrient changes rather than through rain physically burning leaves. Acid deposition can also alter forest species composition and make ecosystems more sensitive to other stresses. The severity of impacts depends strongly on soil mineralogy, climate, vegetation, atmospheric inputs and land management. Some ecosystems show substantial resilience because their soils contain minerals capable of neutralising acidity, while others can be highly sensitive because their buffering reserves are limited.
Acid rain can affect buildings, monuments and infrastructure through chemical reactions with materials. Acidic deposition can accelerate the weathering of carbonate based stone such as limestone and marble because acids react with carbonate minerals. Sulfur containing pollutants can contribute to the formation of compounds that alter stone surfaces, while particles can soil buildings and deposition can interact with moisture to accelerate corrosion of certain metals.
The rate of material deterioration depends on pollutant concentrations, humidity, material composition, surface condition and environmental exposure. Acid deposition can therefore have economic and cultural consequences as well as ecological effects. Historic monuments in polluted regions have provided visible evidence of atmospheric pollution for decades. However, ordinary rain, wind, freeze thaw cycles and other environmental processes also weather materials, so not every form of building deterioration should be attributed to acid rain. Environmental chemistry is needed to distinguish the contributions of different processes.
The history of acid rain demonstrates the importance of long range atmospheric transport and international environmental management. During industrialisation, emissions of sulfur dioxide and nitrogen oxides increased substantially in many industrial regions. Scientific monitoring subsequently showed that emissions in one area could contribute to acid deposition far away because atmospheric transport carries gases and particles across administrative and national boundaries.
This created a policy problem in which the source of emissions and the location of environmental damage could be geographically separated. Emission controls, especially reductions in sulfur dioxide from power generation and industry, produced major decreases in acid deposition in many regions. Technologies such as flue gas desulfurisation, fuel switching, low sulfur fuels and improved combustion controls contributed to these reductions. Nitrogen oxide controls have also reduced acid forming emissions, although nitrogen compounds have multiple environmental effects and remain important pollutants.
Acid rain should not be confused with ordinary acidic precipitation caused by natural carbon dioxide, nor with every environmental effect associated with sulfur and nitrogen pollution. Volcanic emissions, lightning, sea spray, biological processes and natural atmospheric chemistry all influence background deposition. Acid deposition also differs from ozone pollution, although both can arise from atmospheric chemical reactions and human emissions.
Modern understanding treats acid deposition as part of a larger atmospheric deposition system involving sulfur, nitrogen, base cations, organic compounds and particles. Reducing acid deposition can produce ecological recovery, but recovery is influenced by soil chemistry, accumulated pollutants and continuing nutrient inputs. Some regions remain sensitive even after substantial emission reductions because ecosystems have lost buffering capacity. Acid rain is therefore both an atmospheric chemistry phenomenon and an ecosystem process, linking combustion emissions, long range transport, precipitation chemistry, soil reactions, freshwater ecology and environmental policy.
