Acidic deposition is largely comprised of sulfuric and nitric acid derived from sulfur dioxide and nitrogen oxides, respectively, and ammonium resulting from emissions of ammonia. Sulfur dioxide and nitrogen oxides, originating from human activities, are largely emitted into the atmosphere by the burning of fossil fuels, while ammonia is largely the result of agricultural activities. Once these compounds enter an ecosystem, they can acidify soil and surface waters, bringing about a series of ecological changes. The term acidic deposition encompasses all of the forms of these compounds that are transported from the atmosphere to the Earth, including gases, particles, rain, snow, clouds, and fog. Acidic deposition occurs as: (1) wet deposition as rain, snow, sleet or hail; (2) dry deposition as particles or vapor; or (3) cloud or fog deposition, which is more common at high elevations and in coastal areas. Wet deposition is fairly well characterized by monitoring at more than 200 National Atmospheric Deposition Programs (NADP; nadp.sws.uiuc.edu) in the US. In contrast, dry deposition is highly dependent on metrological condition and vegetation characteristics, which can vary markedly over short distances in complex terrains. As a result, dry deposition is poorly quantified and
Effects of Acidic Deposition on Freshwater Aquatic
Ecosystems Further Reading highly uncertain. Dry deposition is characterized through the Clean Air Status and Trends Network (CASTNet; www.epa.gov/castnet), which includes about 100 sites in the US.
Sulfuric and nitric acids lower the pH of rain, snow, soil, lakes, and streams. Areas experiencing elevated acidic deposition include eastern North America (Figure 1), the western US, Europe, and Asia. In 2002-04, wet deposition (i.e., deposition from forms of precipitation such as rain, snow, sleet, and hail) in acid-sensitive regions of the eastern US had average pH values of 4.3-4.5, which is about 3-5 times more acidic than background conditions.
Acidic deposition trends in the eastern US and Europe mirror emission trends in the atmospheric source area or airshed. Long-term data from across the eastern US and Europe show declining concentrations of sulfate in wet deposition since the mid-1970s, coincident with decreases in sulfur dioxide emissions. Based on these long-term data, a strong positive correlation exists between sulfur dioxide emissions in the source area and sulfate concentrations in wet deposition. It is expected that the sulfate concentration of wet deposition will decrease (or increase) in a direct linear response to the decrease (or increase) of sulfur dioxide emissions in the atmospheric source area. These observations strongly suggest a cause and effect relationship between emissions ofsulfur dioxide and deposition of sulfate in sensitive regions. A similar relationship is starting to become evident between emissions of nitrogen oxides and wet deposition of nitrate. This relationship for nitrate is not as strong as the relationship for sulfur because emissions of nitrogen oxides have been relatively constant over the last 20 years. However, it appears that recent decreases in emissions of nitrogen oxides from electric utilities are starting to result in decreases in atmospheric deposition of nitrate.
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