Acid Rain

Abstract

Abstract Acid rain is a popular term that can include all forms of precipitation, as well as fog and cloudwater, that is more acidic than expected from natural causes. Natural background levels of precipitation acidity tend to be around pH of 5.1–5.2. This compares with present annual average values of pH 4.3–4.4 for parts of eastern North America, and pH values less than 4.5 for large areas of Europe and eastern Asia. As pH decreases, H + concentration or acidity increases exponentially. Individual storms in affected areas, especially in the summer, can often produce pH values below 3.5, and cloudwater in some areas can be below pH 3.0. A more appropriate term than acid rain is “acidic deposition,” which includes both wet and dry deposition (gases and particles) of acidic compounds on the earth's surface from the atmosphere. Accurate measurements of dry deposition are unfortunately much more difficult to obtain than wet deposition. However, dry deposition can account for 10–70% of the total amount of acidic deposition.