Tropospheric photochemistry and ozone formation

The only significant process forming O3 in the lower atmosphere is the photolysis of NOj (reaction with sunlight), followed by the rapid reactions of the oxygen atoms formed with 

This reaction cycle results in a photostationary state for O3, where concentrations only depend on the amount of sunlight available, dictated by the NO2 photolysis rate (ki) and the [N02]/[N0] concentration ratio. 

Because of this photostationary state, ozone levels generally rise and fall with the sun, behavior that is referred to as diurnal.  

If the above NO cycle were the only chemical process at work, the steady-state concentrations of ozone would be relatively low. However, when VOCs such as organic solvent compounds are present, they react to form radicals that may either (1) consume NO or (2) convert NO to NO2. This additional reaction cycle combined with the above photostationary state relationship causes O3 to increase. 

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Low-molecular mass carbonyls are among the most abundant and ubiquitous volatile organic compounds in the atmosphere. They are produced from industrial activity and incomplete combustion of fossil fuels and biomass. Many aldehydes are also emitted indoors (plastic, foam insulation, lacquers, etc.). As a source of free radicals, aldehydes play an important role in the ozone formation, in urban smog events, as well as in the photochemistry of the unpolluted troposphere. Aldehydes are recognized irritants of the eye and respiratory tract, and often, carcinogenic and mutagenic characteristics are also attributed to them. 

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