In the second, the oxygen atoms react with hydrocarbons (VOCs) present in automotive exhaust to produce free radicals (atoms or molecules with a free unpaired electron, indicated by the symbol R• in the following equation) that may or may not contain oxygen:
O + RH → R.
Reactive hydroxyl radicals and ozone may initiate similar changes:
OH* + RH → R. + H2O
O3 + RH → R.
The free radical R• is especially important because it reacts very readily with oxygen to form the peroxyl radical, ROO•, which may itself initiate a series of important reactions:
R. + O2 → ROO.
One of the most important of these reactions occurs between a peroxyl radical and nitrogen dioxide to produce compounds known as peroxyalkyl and peroxyacyl nitrates:
ROO. + NO2 → ROONO2
One product commonly formed in this reaction is peroxyacetyl nitrate (PAN), an important and common component of photochemical smog.
PAN, ozone, and many of the other compounds formed in the series of reactions just described are strong oxidants and are responsible for some of the initial and most aggravating features of smog, including nose, eye, and throat irritation.