The Emission and Distribution of Ozone Precursors over Europe

The increasing concentration of ozone (03) in the northern hemispheric troposphere is a matter of great concern. The tropospheric content of ozone is partly the result of a downward transport from the stratosphere [1, 2], but ozone is also formed in the free troposphere and in the polluted boundary layer by photo dissociation of nitrogen dioxide (NO2) followed by the recombination of the oxygen atom formed (03P) with an oxygen molecule (O2) [3]. Net production of ozone is only possible if nitrogen monoxide (NO) is converted to NO2 by reacting with species other than ozone itself. In the free troposphere NO reacts with free radicals formed by oxidation of methane or carbon monoxide (CO). In the polluted atmospheric boundary layer free radicals are formed by the oxidation of VOC (volatile organic compounds) [4-7], and the explanation to the post industrial increase of ozone is most likely the enhanced antropogenic emissions of NOx and other precursors [8-10]. Episodic occurrence of elevated ozone is mainly controlled by NMHC (non-methane hydrocarbon) emissions, but NO„ emissions are also important [11-13]). In less polluted areas like Scandinavia NO„ rather than VOC is regarded as the limiting factor in ozone formation [14-16]. The basic mechanisms of the production of tropospheric ozone are thus well known [17-19]. The quantitative role of various compounds on different scales, especially individual hydrocarbons, is however not yet well established. Since the ozone concentrations do not respond linearly to precursor controls [20], the influence of various processes on the production of ozone and other secondary pollutants can best be investigated by a combination of measurements, emission inventories and modelling.