ON THE PHOTOCHEMICAL OXIDATION OF NATURAL TRACE GASES AND MAN-MADE POLLUTANTS IN THE TROPOSPHERE

Natural trace gases, as well as anthropogenic pollutants, are mostly e mitted as reduced or partly oxidized molecules. In the earth's atmosph ere, which contains approximately 20% oxygen, most of these gases will be oxidized and converted to acidic, and therefore water-soluble, spe cies. These are eventually washed out by precipitation or directly dep osited on the earth's surface and thus removed from the atmosphere. Th e atmospheric cycling of the trace gases is not a simple process. Like oxidation in a flame, atmospheric oxidation is mediated by chains of free radical reactions. By far the most important radical in the tropo sphere is the hydroxyl radical, OH. It is generated photochemically, a nd reacts with most atmospheric trace gases, in many cases as the firs t and rate determining step in the reaction chains leading to their ox idation. Usually, these chains regenerate OH, thus maintaining OH at r elatively high day-time concentrations in the order of 10(6)/cm3. At t hese concentrations OH is by far the most important oxidizing agent in the troposphere. The reactions most important in controlling the loca l OH concentration are presented. They form the framework of a photoch emical reaction system, whose properties are analysed. The OH concentr ations predicted from model calculations are compared with measured on es to test the current status of our understanding of the OH chemistry .