Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 4 2007 10.5194/acpd-7-11545-2007 http://www.atmos-chem-phys-discuss.net/7/11545/2007/ http://www.atmos-chem-phys-discuss.net/7/11545/2007/acpd-7-11545-2007.html http://www.atmos-chem-phys-discuss.net/7/11545/2007/acpd-7-11545-2007.pdf 11545 11560 2007-08-06 Weekly periodicities of aerosol optical thickness over Central Europe – evidence of an anthropogenic direct aerosol effect D. Bäumer dominique.baeumer@imk.fzk.de R. Rinke B. Vogel Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universität Karlsruhe, Karlsruhe, Germany Statistical analyses of data from 14 ground-based sun photometer stations all over Central Europe are presented. All stations are part of the Aerosol Robotic Network (AERONET), and only data of the highest data quality level 2.0 had been applied. The averages by weekday of aerosol optical thickness (AOT) at a wavelength of 440 nm of 12 of the 14 stations show a weekly periodicity with lowest values on Sunday and Monday, but greatest values from Wednesday until Saturday, that is significant at least on a 90% level. The stations in Germany and in Greater Paris show weekly cycles with ranges of about 20% on average. In Northern Italy and Switzerland this range is about 10% on average. The corresponding weekly cycle of anthropogenic gaseous and particulate emissions leads us to the conclusion of the anthropogenic origin of the weekly AOT cycle. Since these AOT patterns are derived from the reduction of the direct sun radiation by the columnar atmospheric aerosol, this result represents strong evidence for an anthropogenic direct aerosol effect on shortwave radiation. 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