d Agrawal (2008), the more precise technical term is 'acid deposition" with two categories, wet deposition and dry deposition. The authors point out that wet deposition refers to acidic rain, fog, and snow, while dry deposition refers to acidic gases and particles.
In terms of the amount of acid in the atmosphere Singh and Agrawal (2008) state that about half of what falls from the atmosphere is dry deposition. In this case, the wind blows acidic particles onto all that is uncovered, e.g., cars, homes, buildings, trees, plants, etc. These particles are then washed down onto other surfaces by rain. If the rain is also acidic, then there is an additive effect.
The United States Environmental Protection Agency (2008) states that acid rain is caused by both natural sources such as volcanoes, lightning, bacterial decomposition, and decaying vegetation, and by man-made sources involving emissions of sulfur dioxide and nitrogen oxides into the air such as the emissions arising from fossil fuel burning. In each country, the amount of acid rain and its sources can vary. In the United States, the EPA states that about two-thirds of sulfur dioxide and one quarter of nitrogen oxides come from the generation of electricity, which relies heavily on fossil burning fuels.
Phamornsuwana (2008) points out that emission levels of sulfur dioxide and nitrogen oxides do not independently determine the level of acidity in the rain; rather, the level is also dependent upon the chemical mixtures in which sulfur dioxide and nitrogen oxides interact in the atmosphere. Of this interaction, Phamornsuwana reports that:
Sulfur dioxide and nitrogen oxides go through several complex steps of chemical reactions before they become the acids found in acid rain. The steps are broken down into two phases, gas phase and aqueous phase. There are various potential reactions that can contribute to the oxidation of sulfur dioxide in the atmos...