The acidic deposition can be caused by natural phenomena like volcanoes and decaying vegetation or man-made sources like fossil fuel combustion. Gases like suphur, nitrogen and carbon emitted during these processes react with water molecules in the atmosphere to produce the acids.Wet deposition occurs when the acid is deposited on the earth’s surface through rain, snow, sleet or hail. Dry deposition occurs when particles such as fly ash, sulphates, nitrates and gases are deposited on, or absorbed onto, surfaces.

Human activities are the main cause of acid rain. Over the past few decades, humans have released so many different chemicals into the air that they have changed the mix of gases in the atmosphere. Power plants release the majority of sulfur dioxide and much of the nitrogen oxides when they burn fossil fuels, such as coal, to produce electricity. In addition, the exhaust from cars, trucks, and buses releases nitrogen oxides and sulfur dioxide into the air. These two pollutants are the main man-made sources of acid rain.

Though human causes are primarily responsible for acid rain, natural causes exist as well.  Fires, volcanic eruptions, bacterial decomposition, and lightening also greatly increase the amount of nitrogen oxide on the planet.  However, even the gigantic explosion of Mt. St. Helens released only about what one coal power plant emits in a year.

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