The influence of the global photochemical composition of the troposphere on European summer smog, Part I: Application of a global to mesoscale model chain
Elevated mixing ratios of ozone in the lower troposphere are a major summer time air pollution issue in Europe. Photochemical in-situ production is the most important source of ozone in the planetary boundary layer and has been studied extensively. However, the contributions of background ozone due to stratospheric intrusions, lightning nitrogen oxide followed by ozone production, convective mixing and intercontinental transport are still poorly quantified. We analyze in this paper the influence of the large-scale ozone background on near-surface ozone throughout a summer smog period in July 1994 over Europe. For this purpose a chain of global to mesoscale models is applied with a nesting procedure coupling the individual model simulations. It is found that background ozone as determined by the global model dominates the results of the higher resolution limited area models increasingly with height. But improvements of limited area model results are not only restricted to the free troposphere. Strong convective events like thunderstorms couple free tropospheric and planetary boundary layer air masses so that ozone from above is injected into the planetary boundary layer contributing an amount of 5–10 ppbv to nearsurface ozone in the afternoon hours. A decrease in the same range of 5–10 ppbv in maximum near-surface ozone over Central Europe is found in a model simulation where European anthropogenic emissions are reduced by 25%, an amount equal to the reported emission trends in Germany from 1994 to 2000. We conclude that intercontinental transport of pollution can obscure the results of local efforts to reduce critical exposure levels of ozone.