One issue related to the general effects of air pollution is the physical and chemical quality of rainfall. Air pollution can cause the pH of rainfall to decrease, while the suspended dissolved solids and total solids in rainfall increase. Nitrogen and phosphorus concentrations in rainfall can also increase as a result of the atmospheric releases of pollutants containing these nutrients. Finally, increases in lead and cadmium in rainfall are also a result of air pollutant emissions.
An important issue related to air pollution effects is acid rainfall and the resultant effects on aquatic ecosystems. Acid rainfall is any rainfall with a pH less than 5.7. The natural pH of rainfall is 5.7 and reflects the presence of weak carbonic acid (H2CO3) resulting from the reaction of water and carbon dioxide from green plants. Rainfall becomes more acid as a result of acidic mists such as H2SO4 and HNO3. Atmospheric emissions of carbon monoxide also add to the carbonic acid mist in the atmosphere and cause the pH of rainfall to be less than 5.7. Numerous locations in the United States have rainfall with the pH values around 4.0. Some of the lowest recorded pH values of rainfall are 2.0 to 3.0.
The chief concerns related to acid rainfall are the potential adverse effects. For example, the pH of the soil can be changed and this change can have unfavorable implications. Changes in the pH in soil can cause changes in adsorption and desorption patterns and lead to differences in nonpoint source water pollution as well as changes in nutrients in both surface runoff as well as from infiltration to groundwater. Acid rain can decrease plant growth, crop growth, and growth in forested areas. Acid rainfall can accelerate the weathering and erosion of metals, stone buildings, and monuments. One concern is related to changes in the quality of surface water and the resultant potential toxicity to aquatic species.
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