Acid rain is precipitation with a pH lower than 7. Acid rain occurs when emissions of sulfur dioxide (SO2) and oxides of nitrogen (Nox) react in the atmosphere with water, oxygen, and oxidants to form various acidic compounds. These compounds then fall to the earth in either dry form (such as gas and particles) or wet (such as rain, snow, and fog).
Acid rain causes acidification of lakes and streams and contributes to damage of trees at high elevations (for example, red spruce trees above 2000 feet in elevation). In addition, acid rain accelerates the decay of building materials and paints, including irreplaceable buildings, statues, and sculptures that are part of our nation's cultural heritage. Prior to falling to the earth, SO2 and Nox gases and their particulate matter derivatives, sulfates and nitrates, contribute to visibility degradation and impact public health.
SURFACE WATERSAcid rain primarily affects sensitive bodies of water, that is, those that rest atop soil with a limited ability to neutralize acidic compounds (called "buffering capacity"). Many lakes and streams examined in the National surface water survey (NSWS) suffer from chronic acidity, a condition in which water has a constant low pH level. The survey investigated the effects of acidic depostition over 1000 lakes larger than 10 acres and in thousands of miles of streams believed to be sensitive to acidification.
In some sensitive lakes and streams, acidification has completely eradicated fish species, such as brook trout, leaving these bodies of water barren. In fact, hundreds of lakes in the Adrondacks surveyed in NSWS have acidity levels indicative of chemical conditions unsuitable for survival of sensitive fish species.
Streams flowing over soil with low buffering capacity are equally as susceptible to damage from acid rain as lakes are. Approximately 580 of the streams in the Mid-Atlantic Coastal Plain are acidic primarily due to acidic deposition.
The impact of nitrogen on surface waters is also critical. Nitrogen plays a significant role in episodic acidification and new research recognizes the importance of nitrogen in long-term chronic acidification as well. Furthermore, the adverse impact of atomospheric nitrogen deposition an estuaries and other large water bodies may be significant. For example, 30 to 40 percent of nitrogen in Chesapeake Bay comes from atmospheric deposition Nitrogen is an important factor in causing eutrophication (oxygen deplention) of water bodies. FORESTS
Acid rain has been implicated in contributing to forest degradation, especially in high- elevation spruce trees that populate the ridges of the Appalachian Mountains from Maine to Georgia, including national park areas as the Shenandouh and Great Smokey Mountain national parks. Acidic deposition seem to impair the trees growth in several ways; for example, acidic cloud water at high elevations may increase the susceptibility of the red spruce to winter injury.
There also is a concern about the impact of acid rain on forest soils. There is good reason to believe that long-term changes in the chemistry of some sensitive soils may have already occurred as a result of acid rain. As acid rain moves through the soils it can strip away vital plant nutrients through chemical reactions, thus posing a potential threat to future forest productivity. Acid is more destructive in some areas than others because, a type of rock called limestone (calcium carbonate) makes up much of the land and also lies below many lakes. Limestone neutralizes acids. In other areas, however, such as on the Canadien sheild in much of Ontario, the rock is largely granite. Unlike limestone, granite does not neutralize the effects of acid rain. As a result, lakes in the region have become increasingly acidic.