The global abundance of the hydroxyl radical, OH, is a principal determinant of the amount of acid deposition and the lifetime of literally hundreds of atmospheric trace gases. Local (“in situ” or point) measurements of OH have been made in a number of environments, and technical challenges in this area are being met over time. Typical concentrations near the surface range from (0.5 to 10) × 106 molecules cm−3. These measurements test photochemical theory. Discrepancies between models and measurements are sometimes larger than the apparent uncertainty in either one. There is no way to measure tropospheric OH directly on a global scale. Instead, measurement of species that react with OH are used with models to derive an estimate for the global burden. Through this route it appears that global OH has not significantly increased or decreased over the past decade. The complexities of OH interaction with O3, carbon monoxide, methane, nitrogen oxides, and nonmethane hydrocarbons and uncertainties in models make it difficult to interpret or predict global OH behavior.
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