Numerical simulations were performed in order to investigate the impact of the direct effect of aerosol particles on radiation and the indirect aerosol effect on meteorological variables and subsequent distributions of near surface ozone and PM10 over Europe. The fully coupled meteorology-chemistry community model WRF/Chem has been applied for June and July 2006 for a baseline case without any aerosol feedback on meteorology, a simulation with the direct effect included, and a simulation including the direct as well as the indirect aerosol effect. The impact of the subsequent changes in temperature, boundary layer height, and clouds that were triggered by the direct effect of aerosol on radiation (“semi-direct effect”) was found to dominate the direct effect of aerosol particles on solar radiation. Over Central Europe the mean reduction of global radiation alone was mostly 3–7 W m?2, but changes in cloud cover due to semi-direct effects resulted in monthly mean changes between ±50 W m?2. The inclusion of the indirect aerosol effect resulted in a pronounced decrease of cloud water content by up to 70% and a significantly higher mean rain water content over the North Atlantic. Although generally plausible, the effect appears to be too strong due to too low simulated aerosol particle numbers in this area. Regional changes in precipitation between ?100% and 100% were simulated over the European continent. For the simulation including only the direct aerosol effect these changes are almost entirely due to semi-direct effects. Mean ozone mixing ratios over Europe in July were modified by up to 4 ppb or 10% over continental Europe, mostly related to changes in cloud cover. For PM10 the inclusion of the direct effect resulted for the considered episode in a mean decrease by 20–50% due to an increased atmospheric boundary layer height except for the regions with high PM10 concentrations. When the indirect aerosol effect was additionally taken into account an increase of the monthly PM10 concentration by 1–3 ?g m?3 was found for July 2006 over large parts of continental Europe.
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