Forest Nutrient Cycling: Influence of Acid Precipitation

Acid precipitation is defined as rain or snow having a pH of less than 5.6. The pH parameter is a measurement of the difference in hydrogen ion activity between an unknown solution and a standard buffer of assigned pH value. Upon ionization water yields hydrogen and hydroxyl ions. When the activity of these ions is equal, water is neutral and the pH recorded will be 7. At pH values below 7 water becomes increasingly acid, and above 7 increasingly alkaline. In the absence of air pollutants, the pH of precipitation is presumed to be dominated by carbonic acid formed from ambient carbon dioxide, which produces a pH of approximately 5.6–6.0. The pH of precipitation presently falling in North and Central Europe and in the northeastern United States and adjacent portions of Canada is commonly in the range of 3–5.5. Individual storm events have been recorded with pH values between 2.0 and 3.0.