The chemistry of the hydroxyl radical in the troposphere

Synopsis Like oxidation in a flame, oxidation in the atmosphere is mediated by free radicals. By far the most important radical in the troposphere is the hydroxyl radical, OH. It is generated photochemically, and reacts with most atmospheric trace gases, in many instances as the first, and rate-determining, step in the reaction chains leading to their oxidation. Usually these chains regenerate OH thus maintaining OH at relatively large day-time concentrations of the order of 10 6 cm −3 . The most important reactions controlling local concentrations of OH form the framework of a photochemical reaction model, the results of which are compared to OH concentrations measured in the field at locations with different concentrations of pollutant attributable to the activities of man. The model-predicted OH concentrations are based on the simultaneously measured concentrations and photolysis rates of those trace gases that dominate OH chemistry at the local level. Predicted and measured OH concentrations correlate reasonably well where concentrations of atmospheric pollutants are small but not where they are large. Explanations are being sought.