Acid Rain: The Once and Future Issue

Most Americans are unaware that the serious and widespread environmental problem of acid rain continues and, in fact, may be much more severe than it was thought to have been some 19 years ago when the 1990 Amendments to the Clean Air Act were enacted.

Following passage of the 1990 Amendments to the Clean Air Act, most Americans, politicians and funders of acid rain research assumed that the problem of acid rain had been "solved," and this environmental issue dropped off the table of awareness rather quickly. Some five years were required to implement the provisions of the 1990 Clean Air Act Amendments; emissions of sulfur dioxide declined markedly in 1995 and have continued to decline. The Act had less impact on reducing emissions of NOx, the co-producer of acid rain in the atmosphere.

Continued deposition of both nitric and sulfuric acids has ongoing impacts on sensitive ecosystems, in eastern North America robbing them of elements like calcium, magnesium, sodium and potassium -- nutrients that the ecosystems need to thrive. Nevertheless, no one expected that acid rain would have significant impacts on forest soils, but it did. It is now known that acid rain has depleted significant amounts of these elements, particularly calcium. Consequently, the rains, though overall less corrosive than at their peak, are falling on forests and soils that are more fragile than ever before. The loss of these elements from the soil, carried away primarily in drainage water, as well as the increase in dissolved aluminum in soil and drainage waters have severely impacted forest vegetation. For example, studies since 1965 at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire have shown remarkably that this forest is no longer accumulating biomass or sequestering carbon dioxide from the atmosphere as a result of acid rain impacts. Though it is still standing, compared to a healthy forest it is essentially dying.

With most of the focus of attention in the 1990 Clean Air Act Amendments on reducing emissions of sulfur dioxide, NOx emissions are now becoming increasingly important in contributing to the mix of acids falling on landscapes in eastern North America. Long-term studies at the Hubbard Brook Experimental Forest suggest that within the next decade or two nitric acid will be the dominant acid in rain and snow, surpassing sulfuric acid. The ecological impact of nitric acid falling on terrestrial and aquatic ecosystems will be quite different than that of sulfuric acid in the past.

Acid rain is not only a problem of eastern North America -- it also has been a severe problem in northwestern and central Europe for many decades, and is a rising menace in the rapidly-growing, industrialized portions of southeast Asia. A host of recent research has shown that the large amount of sulfur being emitted to the atmosphere in China (primarily from coal combustion), is currently being prevented from producing an acid-rain problem by large emissions of neutralizing, dust particles. Once these particles are removed to protect human health or for other reasons, it is likely that acid rain will become a major environmental problem in China. It is important to note that the root cause of both acid rain and climate change, and indeed for a majority of the mercury emissions to the atmosphere, is the combustion of fossil fuels, primarily coal.

These environmental issues are connected, and their solution will require a massive, coordinated effort to make the atmosphere a cleaner and more suitable life support system for this planet.

Article posted on May 25, 2009

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