『Abstract
　The study examines the synoptic conditions controlling NOx pollution in the metropolitan area of Tel Aviv, using a semi-objective synoptic classification for the eastern Mediterranean. A day in which NOx 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 variation 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 NOx pollution potential, constituting a basis for alarming against pollution events, based on the predicted synoptic conditions.

Keywords: NOx; Pollution potential; Synoptic classification; Downscaling; Tel Aviv』

1. Introduction
2. Methodology
　2.1. Study region and data base
　2.2. The synoptic classification used
　2.3. Methods
3. Results
　3.1. Distribution of pollution events
　3.2. Atmospheric conditions
　3.3. Pollution potential of the synoptic system
　　3.3.1. The main synoptic systems
　　3.3.2. The specific synoptic types
　3.4. Inter-annual variation of exceeding days
4. The meso-scale processes and their linkage to the synoptic conditions
5. Summary and discussion
Acknowledgements
References