Acid rain cycle
Acid rain is the result of burning fossil fuels containing sulfur impurities. The burning releases sulfur dioxide (SO2) into the air. In the air, the sulfur dioxide will encounter particulates which catalyze the reaction between the sulfur dioxide and hydroxyl radicals (OH·). The result of this reaction will combine with the condensed water nuclei in our clouds to produce sulfuric and sulfurous acid. These acids decrease the pH of the rain clouds, leading to acid rain.
Acid rain typically has a pH around 4, whereas regular rain has a pH of roughly 5-5.5. Since the pH scale is logarithmic, acid rain is ten times more acidic than regular rain. This difference is why acid rain is so destructive, causing damage to both natural and man-made structures, putting ecosystems off-balance, and poisoning water sources by dropping the pH and thus allowing toxic aluminum compounds to leach into them.
Figure 1: The acid rain cycle. Sulfur dioxide, SO2, is released when burning fossil fuels. Sulfur dioxide reacts with radicals in the atmosphere and further combines with water, resulting in acid rain.