Although the hydroxyl radical (OH) is present only in a trace amount in the
troposphere, it plays a central role in its chemistry because it is a strong
oxidizing agent and can react with many pollutants as well as some
\(\mathrm{CFC}\) substitutes. The hydroxyl radical is formed by the following
reactions:
$$
\mathrm{O}_{3} \stackrel{\lambda=320 \mathrm{nm}}{\longrightarrow}
\mathrm{O}^{*}+\mathrm{O}_{2}
$$
where \(\mathrm{O}^{*}\) denotes an electronically excited atom.
(a) Explain why the concentration of \(\mathrm{OH}\) is so small even though the
concentrations of \(\mathrm{O}_{3}\) and \(\mathrm{H}_{2} \mathrm{O}\) are quite
large in the troposphere.
(b) What property makes OH a strong oxidizing agent? (c) The reaction between
\(\mathrm{OH}\) and \(\mathrm{NO}_{2}\) contributes to acid rain. Write an
equation for this process
(d) The hydroxyl radical can oxidize \(\mathrm{SO}_{2}\) to \(\mathrm{H}_{2}
\mathrm{SO}_{4}\). The first step is the formation of a neutral
\(\mathrm{HSO}_{3}\) species, followed by its reaction with \(\mathrm{O}_{2}\) and
\(\mathrm{H}_{2} \mathrm{O}\) to form \(\mathrm{H}_{2} \mathrm{SO}_{4}\) and the
hydroperoxyl radical \(\left(\mathrm{HO}_{2}\right)\). Write equations for these
processes.