Acid rain, or acidic precipitation, first attracted scientific attention in the mid 1950s in Scandinavia, with studies focused on acidity and surface waters. Concern in the United States, Europe, and Canada began to rise in the 1960s when researchers noticed that fish populations in remote wilderness lakes were declining for unknown reasons and forests were showing significant leaf damage. Scientists speculated that sulfuric and nitric acid falling to the earth as acid precipitation might be the cause. These acids form high in the clouds when sulfur dioxide (SO₂) and nitrogen oxides (NO_x)-acid rain precursors emitted by coal-fired electric utilities and other fossil fuel burning sources-react with water, oxygen, and sunlight. Sulfur dioxide (SO₂), the main source of acid rain, is a pungent toxic gas produced when sulfur laden coal is burned. The acids formed in the clouds are brought to the earth through rain...