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Air pollution, due to its harmful effects especially on human health, is one of the major problems of industrial cities and metropolises, including Tehran. Therefore, recognizing the atmospheric conditions that lead to the accumulation of the pollutants can help decision-maker organizations. Materials and methods: In this study, based on the intensity and persistency of the air pollution in the period of 1389-1397 and according to the season of its occurrence, 47 air pollution incidents in Tehran were identified and studied from synoptic perspective. Spatial (T-Mode) principal component analysis was applied to 500-hpa geopotential height data of these events to classify the synoptic patterns which make the city prone to intensification of pollution in different seasons. Results: The results indicate three different synoptic patterns leading to an increase in the potential of pollution of Tehran. In these patterns, the main pollutant is the airborne particulate matter (PM2.5 and PM10). Accordingly, the first pattern with percentage frequency of 62% occurs in the fall and winter. In this pattern, the presence of Siberian high pressure, along with the midtropospheric ridge is obvious. Two other patterns are active in the late spring and summer (related to Indian monsoon in the southeast of Iran) and spring and autumn (related to dynamic low-pressure in Iraq and the west of Iran), respectively. Conclusion: Classifying of the data of polluted days during recent eight years for Tehran results in three synoptic patterns related to different seasons. This information can help better managing of urban activities. A R T I C L E I N F O R M A T I O N Article Chronology: Received 28 April 2019 Revised 30 May 2019 Accepted 17 June 2019 Published 29 June 2019

Seasonal variability of atmospheric patterns leading to air pollution in the metropolis of Tehran