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Acid rain, or acid deposition, is a broad term that includes any form of precipitation with acidic components, such as sulfuric acid or nitric acid that fall to the ground from the atmosphere in wet or dry forms. This can include rain, snow, fog, hail or even dust that is acidic. Acid rain is another environmental problem that affects much of the eastern United States, damaging crops, forests, wildlife populations, and causing respiratory and other illnesses in humans.

Acid rain is formed when oxides of sulfur and oxides of and nitrogen react with water vapor and other chemicals in the presence of sunlight to form various acidic compounds in the atmosphere. Acid rain results when sulfur dioxide (SO₂) and nitrogen oxides (NO_X) are emitted into the atmosphere and transported by wind and air currents. The sulfur dioxide and nitrogen oxides react with water, oxygen and other chemicals to form sulfuric and nitric acids.

These then mix with water and other materials before falling to the ground:

These acids can form particulate matter by reaction with, for example, ammonia in the air or with metals particulates.

While a portion of the sulfur dioxide and oxides of nitrogen that cause acid rain is from natural sources such as volcanoes, another part is due to the combustion of carbonaceous fuels. Wind currents can carry these oxides over long distances and across borders making acid rain a global problem and not just for those who live close to these sources.

Other pollutants that are emitted as a result of the combustion process are hydrocarbon derivatives (including unburned fuel and hydrocarbon products of the process) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which the nitric oxide is converted to nitrogen dioxide (NO₂) – a brown gas and at sufficiently high levels can contribute to urban haze. However, nitrogen dioxide can absorb sunlight and break apart to produce oxygen atoms that combine with the oxygen in the air to produce ozone (O₃), a powerful oxidizing agent, and a toxic gas.

A final comment related to acid rain since the formation of the rain is usually is also attributed to the combustion of fossil fuels and fossil fuel products. Caution is advised when making such a grandiose statement. Any renewable energy source that contains nitrogen and/or sulfur will contribute to the formation of acid rain. In fact, when air is used as the oxidant in a combustor, part of the nitrogen in the air can be converted to nitrogen oxides and thence to acid rain. In all cases, as the fossil fuel users have learned, the gas from combusters such be cleaned and pollutants – such as the oxides of nitrogen and the oxides of sulfur as well as the oxides of carbon – should be removed as soon as possible after they appear from the combustor.

In areas where the biosphere is sensitive to acid rain, there has been ample evidence of the negative effects of acid rain on freshwater ecosystems. Elevated acidity in a lake or river is directly harmful to fish because it corrodes the organic gill material and attacks the calcium carbonate skeleton. In addition, the acidity dissolves metals that (such as aluminum from the sediments) that are toxic to flora and fauna (including humans). There is also evidence that acid rain is harmful to vegetation by a reverse effect insofar as the rain leaches nutrients (such as potassium) from the soil and allows the nutrients to exit the ecosystem by runoff.

Methods of mitigating acid rain formation include (i) reducing the sulfur content of fuels, (ii) using scrubbers such as flue gas desulfurization, (iii) lime injection multi-stage burning, (iv) fluidized bed combustion, or (iv) circulation dry scrubbing of the gas stream. To reduce the formation of nitrogen oxides, methods such as (i) changing air to fuel ratio, (ii) reducing the combustion temperature, which reduces the formation of the oxides of nitrogen that are formed when air is the combustion oxidant, and (iii) a selective catalytic reduction process in which injection of reactive chemicals such as ammonia (NH₃) to react with the nitrogen oxides and convert them into nitrogen and oxygen, represented simply as:

See also: Biogas, Biomass, Waste, Wood.