Nitrogen Oxide occurs naturally as a result of lightning. However, higher levels can result from incomplete burning of fossil fuels in vehicles and power plants. Nitrogen Oxide are key contributors to the formation of ozone pollution and acid rain. Hydrocarbons are produced when fossil fuels are burned. They are also released naturally from vegetation.
Hydrocarbons react with other chemicals to form smog.
Ozone is formed when hydrocarbons and nitrogen oxide are mixed in the air. There are two types of ozone. Good ozone is the upper atmosphere, high above the Earth. Bad ozone forms the smog found in the lower atmosphere where people live and breathe. The two kinds of ozone, generally, never mix.