Formation of Acid Rain

The sulfuric and nitric acids formed from gaseous pollutants can easily make their way into the tiny cloud water droplets. These sulfuric acid droplets are one component of the summertime haze in the eastern United States. Some sulfuric acid is formed directly in the water droplets from the reaction of sulfur dioxide and hydrogen peroxide. Some of these sulfuric acid particles drop to the earth as "dry" acid deposition. The tiny water droplets containing sulfuric acid provide a ready surface to attract more molecules of water to form a larger droplet of dilute sulfuric acid.

Water droplets collected from the base of clouds in the Eastern U.S.during the summer have an average pH of 3.6, with some values as low as pH 2.6. The pH in the upper portion of a cloud is much higher. The final rain droplet has an average pH of 4.2 in the Northeast U.S. In Los Angeles, the pH of fog has been measured at 2.0 - about the acidity of lemon juice.