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Statistical analyses of data from ground-based sun photometer stations inCentral Europe are presented. All stations are part of the Aerosol RoboticNetwork (AERONET), and only data of the highest data quality level 2.0 hasbeen applied. The averages by weekday of Aerosol Optical Thickness (AOT) ata wavelength of 440 nm of 12 of the 14 stations in the investigation areashow a weekly periodicity with lowest values on Sunday and Monday, butgreatest values from Wednesday until Saturday, that is significant at leaston a 90% level. The stations in Germany and in Greater Paris show weeklycycles with ranges of about 20% on average. In Northern Italy andSwitzerland this range is about 10% on average. By applying severalchecks, we exclude that the weekly cycles were caused by a maintenanceeffect or by different retrieval conditions as a consequence of a weeklycycle in cloud cover. The corresponding weekly cycle of anthropogenicgaseous and particulate emissions leads us to the conclusion of theanthropogenic origin of the weekly AOT cycle. Since these AOT patterns arederived from the reduction of the direct sun radiation by the columnaratmospheric aerosol, this result represents strong evidence for ananthropogenic direct aerosol effect on shortwave radiation. Furthermore,this study makes a first contribution to the understanding and explanationof recently observed weekly periodicities in meteorological variables astemperature in Germany

Weekly periodicities of Aerosol Optical Thickness over Central Europe – evidence of an anthropogenic direct aerosol effect