Atmospheric Photooxidants

Earth’s atmosphere is made up of 78% nitrogen and 21% oxygen. It is therefore highly oxidising but the oxygen only acts as the main source of the more reactive molecules and free radicals that provide the atmosphere’s real oxidising power. These are formed almost entirely by photochemistry and many involve coupling between ozone and water vapour, the influence of which on the overall oxidising power is often underestimated. The oxidants to be dealt with here therefore include ozone (O3), hydroxyl radicals (OH), peroxy radicals (both inorganic (HO2) and organic (RO2)), and peroxides (H2O2 and RO2H). Other oxidants include nitrate radicals (NO3) and halogen atoms; however, these play a subsidiary role and probably are unimportant over large parts of the atmosphere.