Acid rain occurs when the pH of precipitation is lowered below a pH of 5. This means that the precipitation is more acidic than normal. You should remember that the pH scale ranges from 0-14, with 7 being neutral, less than 7 being acidic, and more than 7 being basic. Natural precipitation has a pH range of 5 to 6.

Typically, sulfuric and nitric acids are responsible for increasing the acidity of the precipitation, leading to acid rain. Sulfur dioxides (SO₂) and nitrogen oxides (NO_x), when released into the atmosphere, combine with water vapor to create sulfuric and nitric acids. Where do sulfur dioxide and nitrogen oxides come from? Primarily from the combustion of fossil fuels (coal, oil, and natural gas). VOCs (volatile organic compounds) also contribute to acid rain. VOCs come from man-made chemicals and the evaporation of gasoline at gas stations; they also are naturally emitted from plants.



Acid rain is a concern because it damages sensitive ecosystems. For example, it causes streams, rivers, and lakes to become more acidic, which in turn affects the diversity of the organisms that are able to live in that aquatic ecosystem. It is also damaging to soils and vegetation. In forests, acid rain can harm trees by damaging their leaves, reducing the amount of nutrients available to them, and exposing them to toxic chemicals slowly released from the soil. It alters the chemical makeup of the soil as well, which changes the amount of nutrients in the soil and affects the health and growth of tree species.

Source: Effects of Acid Rain - Forests. Retrieved from http://www.epa.gov/acidrain/effects/forests.html on September 30, 2010.