L05 Photochemical smog

Ozone, O₃ , a reactive and irritant substance, is formed by the action of sunlight on nitrogen dioxide:

Ozone itself reacts with NO•, a less reactive radical, producing the starting materials, NO₂ and O₂ again.

These reactions are thus in a state of equilibrium. This process produces a steady state concentration of O₃ that is a function of the initial concentrations of NO and NO₂.
Unless other factors interfere, the O₃ concentration cannot exceed the irritation threshold.

Under the influence of sunlight, hydroxyl radicals are formed in the atmosphere, for example by the following reaction:

These hydroxyl radicals are fairly active. They react with volatile organic compounds such as methane, petrol fumes, etc. In the presence of NO• this yields NO₂, water, aldehydes and many other products without consuming a molecule of O₃.
The following is an example of such a reaction sequence:

The natural volatile organic compounds emitted from vegetation also react rapidly with the ozone and whether they are a net source of ozone or a net consumer of ozone depends on the natural volatile organic compound to NO_x ratio and the sunlight intensity.

At high ratios of volatile organic compounds to NO_x, the NO₂ concentration rises and the NO concentration falls.

The increased NO₂ concentration accelerates the formation of ozone (reaction 1).
Reduced concentrations of NO• make the removal of ozone more difficult (reaction 2).

To summarise, oxides of nitrogen, volatile organic compounds and sunlight, are necessary to form photochemical smog.

Oxides of nitrogen are emitted by industry and traffic.
Volatile organic compounds include solvents, unburned petrol, etc. Since sunlight is also needed, this type of airpollution is most common on windless summer days.
One important consequence of an increased ozone concentration is its adverse effect on the respiratory system: difficult breathing, irritation of the bronchial passages, headaches ,etc. An increased ozone concentration also has an adverse effect on plant life. There is not only visible damage, but also reduced yield and slower growth.
Many materials such as elastomers, e.g. rubber tyres, textile-fibres, paint, etc. are also oxidised faster by ozone.

The natural ozone concentration is about 60 µg/m³. The maximum acceptable concentration over an 8-hour period has been established by the World Health Organisation to be 110 µg/m³. This value is, however, regularly exceeded on windless summer days.