Weekly periodicities of Aerosol Optical Thickness over Central Europe – evidence of an anthropogenic direct aerosol effect
Statistical analyses of data from ground-based sun photometer stations in Central Europe are presented. All sta- tions are part of the Aerosol Robotic Network (AERONET), and only data of the highest data quality level 2.0 has been applied. The averages by weekday of Aerosol Optical Thick- ness (AOT) at awavelength of 440nm of 12 of the 14 stations in the investigation area show a weekly periodicity with low- est 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. By applying several checks, we exclude that the weekly cycles were caused by a maintenance effect or by different retrieval conditions as a consequence of a weekly cycle in cloud cover. The corresponding weekly cycle of an- thropogenic 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 reduc- tion of the direct sun radiation by the columnar atmospheric aerosol, this result represents strong evidence for an anthro- pogenic direct aerosol effect on shortwave radiation. Further- more, this study makes a first contribution to the understand- ing and explanation of recently observed weekly periodicities in meteorological variables as temperature in Germany.