Photochemical smog is a brownish haze caused by the action of the sun’s visible and ultraviolet radiation on the primary pollutants from the combustion of fossil fuels – unburnt hydrocarbons, carbon monoxide and nitrogen dioxide. A series of reactions produce secondary pollutants including ground-level ozone, nitric acid, peroxyacetylnitrate and hydrogen peroxide. As you would imagine, it is a serious respiratory health hazard in many cities.
The need for visible and uv radiation (ℎ𝜈) tells us that the first step is essentially an initiation step where bonds are photolysed. Homolytic fission leaves us with exceptionally reactive free radicals in the lower troposphere that catalyse the formation of ozone, peroxyacetylnitrate (PAN) and other secondary pollutants.
Nitrogen(II) oxide (NO•) is produced in when nitrogen from the air burns in the very high temperatures of a car engine. It reacts with dioxygen molecules in the air to form nitrogen(IV) oxide (NO2•).
2NO• + O2 ⇾ 2NO2•
NO2• is readily photolysed to form nitrogen(II) oxide and an oxygen atom.
NO2• + ℎ𝜈 ⇾ NO• + O
The oxygen atoms react with dioxygen molecules to form ozone.
O + O2 ⇾ O3
Under normal conditions nitrogen(II) oxide reacts with the ozone formed to produce dioxygen and nitrogen dioxide is regenerated. Ozone levels do not build substantially at ground level.
NO• + O3 ⇾ O2 + NO2•
However, if there are significant concentrations of unburnt hydrocarbons present in the atmosphere (from the incomplete combustion of fossil fuels or evaporation from fuel tanks or industrial pollution), then we have a problem. These molecules are collectively called volatile organic compounds (VOC). The most reactive VOC contain a C=C bond which allows them to easily add free radicals (you can check out a mechanism for this here).
VOC react with hydroxide radicals present in the atmosphere to create water and a VOC radical (R•).
‘R’ commonly stands for any alkyl group in chemistry shorthand e.g. R-H could be CH3-H, C2H5-H or something considerable more complicated!
R-H + •OH ⇾ R• + H2O
The VOC radical (R•) reacts with dioxygen molecules to form a peroxy radical (R-O-O•), which rapidly reacts with nitrogen(II) oxide. The result is that there is no longer much NO• present to react with ozone. Ozone levels rise rapidly.
R• + O2 ⇾ RO2•
RO2• + NO ⇾ RO• + NO2
Further reactions between VOC and NO• form peroxyacetylnitrate (PAN) which is a carrier of NO2 and causes eye irritation and damages plants in its own right.
R-H + NO• + O2 ⇾ NO2• + PAN
