Photooxidation of trace gases in the troposphere Plenary Lecture

In the earth's atmosphere, which contains 21% oxygen, natural and man-made trace gases are broken down by oxidation. This oxidation is a complex process and generally mediated by chains of free radical reactions. By far the most important oxidizing agent in the troposphere is the hydroxyl radical, OH. It is generated primarily through the photolysis of ozone, and reacts with most atmospheric trace gases, in many cases as the first and rate determining step in the reaction chains leading to their oxidation. Usually, these chains regenerate OH, thus maintaining OH at daytime concentrations of up to 107 cm-3. At these concentrations OH determines the destruction rate and tropospheric lifetime of most trace gases. In turn, all these gases influence the concentration of OH in the troposphere. There is thus the possibility of a regional, even global, impact on OH by man-made pollutants. The basic photochemical reaction system controlling OH is developed from simple examples of the various types of reactions that produce or destroy OH. The salient properties of the system are analyzed. The system is tested by comparing the OH concentrations predicted from the numerical simulation of the OH chemistry in clean surface air with those measured in these conditions.