Acid rain is a global problem due to its adverse impact on plants, aquatic animals, infrastructures, and human
health. It is primarily produced by the reaction of water in the atmosphere with SO2 and NOx
released from natural occurrences including volcanic eruptions and lightning strikes. Some human activities, such as burning coal in
power plants and exhaust from motor vehicles can also contribute to the contamination of SO and NOx in the air. Because of widespread deterioration of the environment caused by acid rain, many countries have regulations to enforce the reduction of SO2 and NOx
released to the air. In the U.S., Congress passed a series of
amendments to the Clean Air Act in 1989, establishing the Acid Rain Program to control emissions of SO2 and NOx. The monitoring of air and rain has become an important task worldwide for investigating the effects of pollutants on global ecology and assessing the progress of pollution control measures.
United States
