"Acid rain" is the common term for a complex process more appropriately referred to as acid deposition. It includes the deposition of acidic compounds onto the ground and onto surface waters when it rains (wet deposition), and at other times as well (dry deposition). The acid compounds include both acidic gases, such as sulfur dioxide (SO2) and nitrogen dioxide (NO2), and acidic particles, such as sulfate and nitrate compounds. Acid deposition is believed to have adversely affected lakes and forests in the northeastern United States, Canada, and Europe, and to have caused material damage as well.
The primary anthropogenic source of airborne acidity is the burning of fossil fuels. Coal-and oil-fired electric utilities and industries emit gaseous SO2 and nitrogen oxides (NO and NO2) into the atmosphere. Automobiles and other mobile sources also contribute significant amounts of nitrogen oxides.
As these primary pollutants are transported by the wind, sometimes over long distances, they are slowly transformed through a variety of atmospheric reactions to secondary pollutants, such as nitric acid vapor and sulfuric acid droplets, which are strongly acidic. With further transport and reactions with ammonia gas (NH3) from biological decay processes at the ground level, they are transformed to less strongly acidic sulfate and nitrate particles. These atmospheric reaction products
The area affected by the emission sources is determined to a large extent by the time that pollutants stay in the atmosphere before removal through deposition.
Sulfur and nitrogen deposition have caused adverse impacts on highly sensitive forest ecosystems in the United States and northern Europe, such as high-elevation spruce and fir forests in the eastern United States. On the other hand, most U.S. forest ecosystems are not currently known to be adversely impacted. The gradual leaching of soil nutrients from sustained acid deposition can impede forest nutrition and growth. Potential risk depends on numerous factors, including rate of cation (positively charged ion) deposition, soil cation reserves, age of forest, weathering rates, species composition, and disturbance history. Dry deposition is now considered to be more damaging to stone than wet deposition.
Since sulfate significantly contributes to visibility-reducing particles in the eastern United States, reduced SO2 emissions will reduce sulfate concentrations and, in turn, their contribution to haze. In the 1990 U.S. Clean Air Act Amendments, Congress mandated reductions in annual emissions of SO2 by 1995 and nitrogen oxides from utilities burning fossil fuels starting in 1995.
As a result, statistically significant reductions in the acidity (represented by hydrogen ion content) and sulfate concentrations in precipitation were reported at deposition-monitoring sites in the Midwest, Mid-Atlantic, and northeast United States. Although utilities have significantly reduced their emissions, observable responses will lag due to inherent time lags between changes in emissions and responses by sensitive receptors, especially within ecosystems.
It is still too early to determine whether changes in aquatic ecosystems have resulted from emission reductions. Over the last fifteen years, lakes and streams throughout many areas of the United States have experienced decreases in sulfate concentrations in response to decreased emissions and deposition of sulfur, and there is evidence of recovery from acidification in New England lakes. In contrast, the acidity levels of the majority of Adirondack lakes have remained fairly constant, while the most sensitive Adirondack lakes have continued to acidify.
The kind of damages seen in forests and lakes in the northeastern United States have also been witnessed in Scandinavia and other parts of northern Europe.
National Acid Precipitation Assessment Program (1998). National Acid Precipitation Assessment Program Biennial Report to Congress: An Integrated Assessment. Silver Spring, MD: Author.