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 CFC substitutes. The hydroxyl radical is formed by the following reactions:

where O* denotes an electronically excited atom.
(a) Explain why the concentration of OH is so small even though the concentrations of O3 and H2O are quite large in the troposphere.
(b) What property makes OH a strong oxidizing agent?
(c) The reaction between OH and NO2 contributes to acid rain. Write an equation for this process.
(d) The hydroxyl radical can oxidize SO2 to H2SO4. The first step is the formation of a neutral HSO3 species, followed by its reaction with O2 and H2O to form H2SO4 and the hydroperoxyl radical (HO2). Write equations for these processes.

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