The oxygen atom reacts with other components of automobile exhaust (e.g. unburnt hydrocarbons) and those of the atmosphere (e.g. oxygen) in a series of complex reactions to produce a variety of lachrymatory and toxic chemicals (e.g. peroxyacetyl nitrate).
The production and destruction of photochemical smog are very complex events that involve hundreds of different chemical reactions.
In the first step of photochemical smog production, sunlight acts on nitrogen dioxide to produce nitric oxide and free oxygen:
NO2 + hv → NO + O
Oxygen atoms formed in this reaction react readily with diatomic oxygen present in the atmosphere to form ozone (O3). The symbol M in the following equation represents some third body that acts as a catalyst, removing energy from the O2/O collision, making it thermodynamically feasible:
O + O2 + M → O3 + M
Levels of ozone produced by this reaction are controlled to some extent by a “scavenging” reaction that takes place with nitric oxide:
O3 + NO → O2 + NO2
The reaction is referred to by this term because nitric oxide forages among ozone molecules, attacking them at random and converting them to oxygen molecules.
These three reactions constitute a cycle that begins with the production of nitrogen dioxide (usually from motor vehicle exhaust) and results in the production of large amounts of atomic oxygen and ozone.
When concentrations of nitrogen dioxide decrease (as during evening hours, when traffic tends to decrease), the scavenging reaction tends to remove excess amounts of ozone from the atmosphere.
Oxygen atoms formed in this cycle of reactions initiate one of two important chain reactions.
In the first, they react with water molecules to generate highly reactive (indicated by the asterisk) hydroxyl radicals:
O* + H2O → 2OH*