Scientists have discovered that air pollution from the burning of fossil fuels is the major cause of acid rain. The main chemicals in air pollution that create acid rain are sulfur dioxide (SO2) and nitrogen oxides (NOx). Acid rain usually forms high in the clouds where sulfur dioxide and nitrogen oxides react with water, oxygen, and oxidants. This mixture forms a mild solution of sulfuric acid and nitric acid. Sunlight increases the rate of most of these reactions. Rainwater, snow, fog, and other forms of precipitation containing those mild solutions of sulfuric and nitric acids fall to earth as acid rain.
Surprisingly, the effects of acid rain on trees have overshadowed the effects on people. Many toxic metals are held in the ground in compounds. However, acid rain can break down some of these compounds, freeing the metals and washing them into water sources such as rivers. In Sweden, nearly 10,000 lakes now have such high mercury concentrations that people are advised not to eat fish caught in them. As the water becomes more acidic, it can also react with lead and copper water pipes, contaminating drinking water supplies.
The best approach to acid rain is to reduce the amount of NOx and SO2 being released into the atmosphere. Fitting a catalytic converter to a car can reduce the emissions of NOx by up to 90 percent, but they cause more carbon dioxide to be released, which contributes to the greenhouse effect. SO2 emissions from power stations can be reduced before, during, or after combustion. The SO2 created during combustion can be absorbed if an appropriate chemical (such as limestone) is present as the fuel burns. Once the fuel has been burned, the SO2 can be removed from the exhaust gases. Most systems spray a mixture of limestone and water onto the gases. This mixture reacts with the SO2 to form gypsum, a useful building material.
All these methods for reducing acid gases are expensive, and have drawbacks, so laws have been passed to force businesses to use them. The best way to reduce them is not to use as much energy in the first place.
