When pollutants (from cars, factories, etc...) are released into the atmosphere part of their chemical components include nitrogen oxides, hydrocarbons, and sulfur (to name a few). These components react with each other and in the presence of the sun a potent mix of ozone and other chemicals are created. This is caused photochemical smog. Acid raid occurs when rain mixes with these photochemicals and are transported to earth via the rain. When combined with rain the oxides often form nitric or sulfuric acide which is highly toxic and completely corrosive.