Acidic deposition: Sources and ecological effects

Acidic atmospheric deposition, popularly referred to as acid rain, is the transfer of strong acids and acid forming substances from the atmosphere to the Earth's surface. Acidic deposition is comprised of sulfuric and nitric acids, and ammonium derived from atmospheric emissions of sulfur dioxide, nitrogen oxides, and ammonia respectively. These compounds are emitted by the burning of fossil fuels and by agricultural activities. Once such compounds enter an ecosystem, they can acidify soil and surface waters and bring about a series of ecological changes. The term acidic deposition encompasses all forms in which these compounds are deposited to the Earth, including gases, particles, rain, snow, clouds, and fog (see Box 3.1). Acidic deposition was first reported in the United Kingdom in the later half of the 19th Century (Gorham 1992). Ecological effects were first documented in Scandinavia in the 1960s with the link between acidic deposition, surface water acidification and loss of fisheries (Gorham 1992). Atmospheric deposition of sulfate, nitrate and ammonium are elevated in eastern North America, Europe and large portions of Asia (Rodhe et al. 1995).