Atmospheric Pollution and the Ozone Layer

The composition of our atmosphere is complex and varies with altitude. It also varies with time following both short- and long-term variations, from the daily cycles, to processes which cover many years. We are concerned here 

Although air is made up essentially of N2 (78 vol. %) and 02 (21 vol. %) there are many minor components which must be taken into account. Of these C02 (0.03 vol. %) and the rare gases (e.g. Ar, 0.9 vol. %) represent the highest concentrations under normal conditions, but there is also water (of the order of 0.5 p.p.m. depending on the humidity), methane (2 p.p.m.), N20 (0.5 p.p.m.), and hydrocarbons, as well as oxygen compounds of sulfur and of nitrogen. These result from combustion processes, the hydrocarbons being present because of incomplete combustion. 

Ozone absorbs in the near UV region and can therefore be decomposed by sunlight with the production of singlet (excited) oxygen 0 The details of the formation of the hydroxyl radicals, OH , are not yet clear, but these radicals lead to secondary processes with hydrocarbons, RH, and with S02 to produce final products like peroxides and sulfuric acid. 

The oxygen atom O will then react with most organic molecules to form oxidation products peroxides and carbonyl derivatives in particular. With molecular oxygen it leads to formation of ozone 03 

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