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Acid deposition is formed from SO2 and NOx emitted to the atmosphere, largely because of fossil-fuel combustion. The most important sources are energy production, especially coal-and oil-fired power plants, and transportation sources, such as vehicles and ships. The air pollutants are transformed in the atmosphere to H2SO4 and HNO3, transported across distances potentially as far as hundreds of kilometers, and deposited as precipitation (wet deposition) and as gas and particles (dry deposition). Alkaline dust and NH3 are other important components. These compounds act to neutralize the acids. The main source of NH3 to the atmosphere is agriculture. Although NH3 neutralizes acidity in precipitation, the resulting NH4+ contributes to acidification of soil and surface water through chemical processes in the soil. Alkaline dust in the atmosphere can, for instance, be particles of limestone (CaCO3) or CaO. The …