Nitrogen and oxygen molecules dissociate on hot metal surfaces such as those found in car engines creating nitrogen and oxygen atoms which can recombine to form nitrogen monoxide.

Nitrogen monoxide (a primary pollutant) is slowly oxidised to nitrogen dioxide (a secondary pollutant) by oxygen, ozone and HO2 radicals.

Nitrogen dioxide undergoes photolysis in the presence of sunlight creating highly reactive oxygen atoms.

These oxygen atoms react with oxygen molecules forming ozone.

Ozone reacts with hydrocarbons (another primary pollutant) to form a complex mixture including aldehydes (a secondary pollutant).

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