Although the combustion of purer coals still results in the emmision of carbon dioxide into the air, the advantage is that purer coals produce fewer byproducts. For instance, impure coal samples contain residues of sulfur and nitrogen. When the coal is burned, oxygen in the air can unite with sulfur to form two potentially poisonous products, sulfur dioxide (SO2) and nitrogen dioxide (NO2). Excess sulfur dioxide in the air is the main cause of abnormal amounts of something called acid rain:

SO2(gas) + H2O(liquid) ---> H2SO3(in solution)

As shown in the above sample equation, when gaseous sulfur dioxide comes in contact with liquid water (rain), something called "sulfurous acid" (H2SO3) is produced. You may have heard of something called "sulfuric acid" (H2SO4), a similar substance. Both are relatively strong acids and dangerous at high concentrations, and both may be found in acid rain. Similarly, nitrogen dioxide can form nitric acid, an extremely dangerous substance present in acid rain. The formation of nitric acid is a little more complicated than that of sulfurous acid in that photochemical reactions are involved.