PHOTOCHEMICAL SMOG

The natural dissociation of nitrogen dioxide (by light below 385 nanometers) into atomic oxygen and nitric oxide sets the stage for ozone to be produced. Normally, that ozone would react again with nitric oxide to give back oxygen and nitrogen dioxide (no net reaction). However, the burning of fossil fuels increases the nitric oxide in the atmosphere so that abnormally high levels of ozone are produced. In turn, when the atmosphere is polluted by substances easily oxidized by ozone (such as hydrocarbons), complex reactions take place that result in irritants to eyes and skin as well as show phytotoxicity (plant poisons). The main smog irritants, formaldehyde, acrolein, and peroxyacetylnitrate (PAN), can also act to decrease visibility.