Methods of the ecotoxicological evaluation of chemicals : photochemical degradation in the gas phase 6

摘要

More than a thousand chemical compounds are known to be emitted into the atmosphere from both natural and anthropogenic sources /1/. The awareness of environmental problems associated with these emissions has led the European Community and German legislation to establish testing procedures for the atmospheric behaviour of chemicals. The particular aim of such tests should be to determine the lifetime and degradation pathways of environmental chemicals in the troposphere. The earth'S atmosphere is a complex chemical system. The concentration levels of its trace constituents are regulated by the interaction of chemical processes, transport and mass exchange with hydro- and lithosphere. Regarding man-made emissions, transport is only a dilution phenomenon. The only relevant degradation occurs via chemical reactions. Chemical degradation in the atmosphere is a result of direct and indirect photochemical reactions. It is now a well established fact that, among the homogenaus gas phase processes, reactions initiated by the hydroxyl (OH) radical provide the most important degradation routes. This is based on the OH reactivity, as expressed by the rate coefficient k$_{OH}$, and the atmospheric OH concentration level (see section 5).