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			Acid Deposition Ecological Society of America 02.01.01 During the early 1980s and 90s, dramatic visions of acid precipitation corroding buildings and killing trees captured the minds of the public. Since then, the threat of “acid rain” has lost some of its celebrity appeal, due in part to government policies aimed to mitigate the problem and environmental issues such as global warming that have taken centerstage. But acid deposition continues to affect ecological systems and is likely contributing to forest degradation, fish kills, and tainted water quality. But what exactly is acid deposition and where does it come from? What are its short and long-term ecological effects? What is Acid Deposition? Acid deposition, commonly known as acid rain, occurs when emissions from the combustion of fossil fuels and other industrial processes undergo complex chemical reactions in the atmosphere and fall to the earth as wet deposition (rain, snow, cloud, fog) or dry deposition (dry particles, gas). Rain and snow are already naturally acidic, but are only considered problematic when less than a pH of 5 (see ph scale). The main chemical precursors leading to acidic conditions are atmospheric concentrations of sulfur dioxide (SO2) and nitrogen oxides (NOx). When these two compounds react with water, oxygen, carbon dioxide, and sunlight in the atmosphere, the result is sulfuric (H2SO4) and nitric acids (HNO3), the primary agents of acid deposition. Airborne chemicals can travel long distances from their sources and can therefore affect ecosystems over broad regional scales and in locations far from the sources of emissions.


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