What is acid rain?

The term "acid rain" is used to describe rain, mist or snow that is unusually acidic. A pH value is the measure of acidic or alkaline material. The lower the pH, the higher the acid reading. Rain and snow are naturally slightly acidic due to naturally occurring chemical reactions in the atmosphere. Compared to normal rainwater with a pH readings of 5.6, the Eastern U.S. suffers from some of the most severe acid rain, with levels typically reading at 4.4, though some locations in the west also face severe impacts.

The burning of fossil fuels generates air pollution that scientists have determined is the major cause of acid rain. Power plants, along with factories and vehicles that also burn fossil fuels, all emit sulfur dioxide (SO2) and oxides of nitrogen (NOx). When combined with moisture in the atmosphere, these pollutants are returned to the earth as acids. This process is known as "deposition" and occurs when it rains or snows, but it can also occur when dust settles out of the atmosphere during dry periods.

Acid precursors can be carried in the atmosphere for several days and travel several hundred miles downwind of the power plant stack before being deposited on the earth's surface. Because of prevailing winds, the northeastern United States and Canada receive significant quantities of acid precursors from coal-fired power plants in states stretching from Missouri to the west and Pennsylvania to the east.

What are the consequences of acid rain?

Acid rain is linked to a range of negative impacts on the natural world as well as human environments:

Aquatic impactsScientists
believe that acid rain is responsible for the dramatic disappearance of brook trout and other fish species from pristine lakes and streams. These treasured water bodies receive acid directly from the atmosphere and from runoff from the surrounding watershed. Of the lakes and streams studied in a National Surface Water Survey conducted by the US Environmental Protection Agency, acid rain was determined to cause acidity in 75 percent of the acidic lakes and 50 percent of the acidic streams analyzed. Some lakes are particularly susceptible to acid rain since the underlying soil has limited ability to neutralize, or "buffer," the acids. Lakes suffering from chronic acidity can be found in several regions of the United States and Canada, including the Adirondacks, the mid-Appalachian highlands, the upper Midwest and the high elevation West.

Aquatic species vary in their tolerance to elevated levels of acidity. The acid interferes with reproduction much sooner in some especially sensitive species than with others. Generally speaking, acid rain fosters a shift in fish population from acid-sensitive to acid-tolerant fish and other aquatic plant and animal species.

Forest impacts
Acid rain may render intense impacts on the health of forest ecosystems. According to the National Assessment Precipitation Assessment Program's 1998 Biennial Report to Congress, the current mortality and decline of high elevation red spruce populations in the Northeast, and decline in growth rates for Appalachian red spruce, "are the only cases of significant forest damage for which there is strong scientific evidence that acid deposition is a primary cause." Nonetheless, several recent studies conducted by the United States Geological Survey and others point to acid rain as contributing to long-range damage to forests by depleting calcium, a nutrient vital to plant growth.

Materials
Acid rain affects many types of materials, from objects of particular historical artistic or cultural value -- buildings and monuments -- to more ordinary objects such as cars and trucks. Acid rain, especially in the "dry" form, corrodes metal, and accelerates the deterioration of stone and paint.

Visibility
Sulfur dioxide emissions reduce visibility when they form sulfate particles in the atmosphere. Visibility reductions are most pronounced in the eastern part of the United States, particularly in and around national parks. How does electricity production contribute to acid rain? Electricity generation accounts for the lion's share of air pollutants that spawn acid rain. Every year, the nation's fossil fuel power plants spew roughly 70 percent of SO2 emissions and 30 percent of NOx emissions that are critical ingredients in making acid rain.

Of course, not all power plants generate the same level of air pollutants contributing to acid rain. Emissions rates vary widely depending upon factors as the precise fossil fuel type used, the nature of the combustion process, pre- and post-combustion air emission controls, as well as vintage of the power plant. Older coal plants exempt from modern clean air standards under "grandfathering" provisions of the Clean Air Act (especially those designed to burn high sulfur content coal) are at one extreme and are the most significant source of acid rain pollutants. These power plants are highly concentrated in the Ohio Valley and Midwest. Given the prevailing winds, these older, largely uncontrolled pollution sources exacerbate the acid rain experienced in the Northeast.

On the other end of the spectrum are new natural gas-fired generation fitted with best available control technology. They release a fraction of the SO2 produced by coal-fired power plants. However, the performance of natural gas plants is decidedly more mixed in the area of NOx emissions, the other major precursor of acid rain. Although possible to mitigate NOx emissions using advanced technologies, many gas-fired power plants now in service use older, more polluting technologies.

How can consumer electricity choice address acid rain?

The Energy Project, Land and Water Fund of the Rockies, How the West Can Win: A Blueprint for a Clean and Affordable Energy Future (1996).

ESEERCO, New York State Environmental Externalities Cost Study Vol. 1 (1995).

Pace University Center for Environmental Legal Studies, Environmental Costs of Electricity (1990).

National Science and Technology Council, Committee on Environment and Natural Resources, National Acid Precipitation Assessment Program Biennial Report to Congress: An Integrated Assessment (visited Aug. 12, 1999)

United States Environmental Protection Agency (EPA), Environmental Effects of Acid Rain (last modified April 1999)

United States Geological Survey, USGS Tracks Acid Rain (visited Aug. 12, 1999)

U.S. Environmental Protection Agency: The Effects of Acid Rain http://www.epa.gov/airmarkets/acidrain/effects/index.html

U.S. Environmental Protection Agency: Acid Rain Division
http://www.epa.gov/acidrain/effects/enveffct.html

U.S. Geological Survey -- On-line data and reports (on acid rain, atmospheric deposition and precipitation chemistry) http://bqs.usgs.gov/acidrain/index.htm

Environment Canada's Acid Rain Site http://www.ec.gc.ca/acidrain/