The primary cause of 'acid rain,' more accurately called acid deposition, is air pollution from burning fossil fuels. Fossil fuel use does not directly emit acids into the atmosphere. Instead, it releases large amounts of acid precursors, primarily sulfur oxides (SOx) and nitrogen oxides (NOx). When exposed to the atmosphere, these react with water to form sulfuric acid and nitric acid, components of acid deposition.

Sulfur dioxide (SO2) is emitted through combustion of fossil fuels containing sulfur as an impurity. Coal combustion is by far the major source of sulfur dioxide emitted into the atmosphere. During combustion, sulfur is oxidized to form sulfur dioxide (SO2). Sulfur dioxide rises into the atmosphere and is oxidized once again in the presence of atmospheric hydroxyl radicals to form sulfur trioxide (SO3). Sulfur trioxide reacts with atmospheric water droplets to form sulfuric acid (H2SO4). Sulfur dioxide emission is the most common contributor to acid deposition, responsible for about 70% of the total. The greatest source of sulfur dioxide is electrical utility plants, which pump approximately 15 million tons of SO2 into the atmosphere each year, out of the total 22 million tons generated annually by human activities. Other contributors of sulfur dioxide include industrial processes and automobiles and other motor vehicles.

Nitrogen oxides (NOx) are also formed through fossil fuel use. In contrast to sulfur, nitrogen is not an impurity but rather an integral part of the organic material making up fossil fuels. Fossil fuel combustion releases nitrogen into the atmosphere, usually in the form of nitric oxide (NO). Nitric oxide (NO) is oxidized by atmospheric molecules, such as ozone (O3) or hydrogen dioxide (HO2), to form nitrogen dioxide (NO2). Nitrogen dioxide (NO2) reacts with OH in the atmosphere to form nitric acid (HNO3). Nitric acid can also form when nitrogen dioxide (NO2) reacts with the nitrate radical (NO3) in the presence of atmospheric water or aldehydes. Nitrogen oxides account for approximately 30% of all acid deposition. Major sources of nitrogen oxide emissions are automobiles and fossil fuel burning power stations.

Nitric acid and sulfuric acid eventually fall back to the Earth's surface as acid deposition. This precipitation can be wet (rain, snow, or fog) or dry (gases or acidic salts).