The oxidation of exhaust gases is one of the primary sources of the world's pollution. The brown haze that is poised over some of the world's largest cities is properly called photochemical smog; it results from chemical reactions that take place in the air, using the energy of sunlight. The production of smog begins when gases are created in the cylinders of vehicle engines. It is there that oxygen and nitrogen gas combine as the fuel borns to form ntric oxide (NO), a colorless gas. The nitric oxide is forced out into the air through the vehicle tailpipe along with other gases.
When the gas reaches the air, it comes into contact with available oxygen from the atmosphere and combines with the oxygen to produce nitrogen dioxide (NO2), which is a gas with a brownish hue. This nitrogen dioxids plays a role in the nitric oxide as it releases an oxygen atom from each molecule; the released oxygen atoms quickly combine with oxygen (O2) molecules to form ozone (O3). The brownish colored nitrogen dioxide is partially responsible for the brown color in smoggy air; the ozone is the toxic substance that causes irritation to eyes.