One of the main causes of acid rain is sulfur dioxide. Natural sources which emit this gas are volcanoes, sea spray , rotting vegetation and plankton. However, the burning of fossil fuels, such as coal and oil, are largely to be blamed for approximately half of the emissions of this gas in the world. When sulfur dioxide reaches the atmosphere, it oxidizes to first form a sulfate ion. It then becomes sulfuric acid as it joins with hydrogen atoms in the air and falls back down to earth. Oxidation occurs the most in clouds and especially in heavily polluted air where other compounds such as ammonia and ozone help to catalyze the reaction, converting more sulfur dioxide to sulfuric acid. However, not all of the sulfur dioxide is converted to sulfuric acid. In fact, a substantial amount can float up into the atmosphere, move over to another area and return to earth unconverted.

Sulfur Pie Graph



Nitric oxide and nitric dioxide are also components of acid rain. Its sources are mainly from power stations and exhaust fumes. Like sulfur dioxide, these nitrogen oxides rise into the atmosphere and are oxidized in clouds to form nitric acid. These reactions are also catalyzed in heavily polluted clouds where iron, manganese, ammonia and hydrogen peroxide are present.

The following are the stoichiometric equations for the formation of sulfuric acid: