This study examines the synoptic conditions controlling NO x pollution in the metropolitan area of Tel Aviv, using a semi-objective synoptic classification for the eastern Mediterranean. A day in which NO x concentration exceeded the Israeli standard in >1 of the seven monitoring stations was defined an "exceeding day" and in >5 as an "extensive exceeding day". For 1998-2004, 19% and 3% of the days were found exceeding and extensive exceeding days, respectively, over 85% of them in the winter months, November-March. The inter-annual variation in the occurrence of the synoptic types was found to explain 72% of the variations in the number of exceeding days. A significant negative trend in the occurrence of types with high pollution potential explained the decrease of 10% per year in the number of exceeding days during 1998-2004. The Red Sea Trough, though being cyclonic system, contributed 51% of the exceeding days, while highs, though being more frequent, contributed only 35%. The "pollution potential" of a synoptic type was defined as the percentage of exceeding days belonging to this type. The majority of synoptic types with the highest pollution potential were cyclonic, most being the Red Sea Trough with western axis, with 82% potential. Our findings indicate that the identity of the synoptic system as cyclonic or anticyclonic is not the key factor for the pollution potential in the study region, but rather, the ambient atmospheric conditions they induce, i.e., high temperatures, static stability, and weak easterly offshore flow. Local processes are the direct cause of the pollution and that the role of the synoptic conditions is to enable, or even to reinforce, the supportive meso-scale processes. This study is a first step in downscaling synoptic features to local NO x pollution potential, constituting a basis for alarming against pollution events, based on the predicted synoptic conditions.
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Acknowledgments This study was supported by the Ministry of Environmental Protection (grant 6-801) and the Israeli Science Foundation (ISF, grant 764/06). Special thanks are due to Orna Zafrir-Reuven for drawing Fig. 1.