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First noted in Europe in the 1950s and then in the eastern United States in the 1970s, acid rain was a headline environmental issue for a couple of decades. The acidity was linked to the burning of coal in power plants of the Midwestern U.S. Many coals contain about 1% sulfur content, which is released as SO2 upon combustion. Additionally, the internal combustion engines of automobiles release NOx as they are driven. SO2 and NOx form sulphuric and nitric acids, respectively, in rainfall downwind of the point of emission.
The atmosphere has natural sources of acidity, including carbon dioxide, that together yield a pH for clean rain of about 5.1 – 5.2. However, in areas downwind of fossil fuel combustion, concentrations of SO2 and NOx are much higher than background, routinely yielding a pH below 4.0 in the eastern U.S. in the mid-1980s. (I measured a pH of 3.1 in the fog water at Mount Moosilauke in New Hampshire in 1971). Thus, during the 1970s, the rainfall in regions downwind of the Midwest had 50 to 500 times the acid content that would be found under natural conditions.
While the Clean Air Act was first formulated by Congress in 1970, it didn’t contain any real mechanisms to reduce acidic emissions until it was amended in 1990, when a cap-and-trade program was put into effect by President George H.W. Bush to curtail SO2 emissions from power plants. These have declined steadily since 1990, and the pH of rainfall in the Hudson Valley of New York has steadily increased from 4.1 to 4.7, reflecting a partial return to cleaner conditions.
Regulated only more recently, the emissions of NOx and the resulting nitric acid in rainfall now account for a major fraction of the acidity that remains in rainfall in the eastern U.S. This too is dropping steadily since the regulation of NOx emissions from power plants in 1997 and the more recent replacement of coal by natural gas.
While there are still some that believe that acid rain was a hoax and a trivial addition to the acidity derived naturally in soils, the vast majority of ecologists see evidence for its impact on trees, streams and lakes, largely through exacerbated losses of plant nutrients from the soil and leaching of aluminum from soil to surface waters. Acid rain was implicated in the loss of calcium in the diet of songbirds in the U.S. and Europe—thus in their ability to synthesize eggshells. Some pond amphibians are also known to reproduce poorly in acid waters and many fish are sensitive to elevated aluminum levels which interfere with functioning of their gills.
All this shows that when policy makers are cognizant of careful studies by environmental scientists appropriate regulations can improve the environmental conditions for all of us. The cap-and-trade program achieved greater reductions in SO2 at lower cost than originally estimated by the EPA. Ongoing studies show that reduced acid deposition is allowing forests and lakes to recover, albeit slowly, from the decades of acid deposition they have been exposed to since the 1950s. Significantly, in field experiments in which calcium is added to the soil, it restores the rate of growth of sugar maple growing on acidified soils.
The acidity of rainfall in the eastern U.S. has not yet returned to pre-industrial conditions, and it may take decades for forest soils to recover. The reason we don’t hear much about acid rain in the United States and Europe is that its demise is an environmental success story. We should rejoice.
Graveland, J., R. van der Wal, J.H. van Balen, and A.J. van Noordwijk. 1994. Poor reproduction in forest passerines from decline of snail abundance on acidified soils. Nature 368: 446-448.
Hames, R. S., K.V. Rosenberg, J.D. Lowe, S.E. Barker, and A.A. Dhondt. 2002. Adverse effects of acid rain on the distribution of the Wood Thrush (Hylocichla mustelina) in North America. Proceedings of the National Academy of Sciences 99: 11235-11240.
Juice, S.M., T.J. Fahey, T.G. Siccama, C.T. Driscoll, E.G. Denny, C. Eagar, N.L. Cleavitt, R. Minocha, and A.D. Richardson. 2006. Response of sugar maple to calcium additions to northern hardwood forest. Ecology 87: 1267-1280.
Likens, G.E. 2013. Biogeochemistry of a Forested Ecosystem. 3rd. ed. Springer, New York
Schlesinger, W.H. and E.S. Bernhardt. 2013. Biogeochemistry: An Analysis of Global Change. Academic Press/Elsevier, New York.
 NO and NO2 are so rapidly interconverted in the atmosphere that they are collectively known as NOx
 pH is a measure of the acidity of water. It is defined as the negative of the reciprocal of the logarithm of the hydrogen ion concentration in moles per liter. It is perhaps easiest to remember that pH 7.0 is neutral; low values are acidic and higher values are alkaline. The pH of water equilibrated with CO2 in the atmosphere is about 5.6. The pH of rain collected in pristine remote areas is slightly lower than that due to other trace, acidic substances that dissolve in it.