Clustering of different atmospheric patterns leading to an increase in different types of atmospheric pollutants in the greater Tehran area during the last 17 years (2004-2020)

Introduction: Air pollution is one of the main problems of industrial and large cities like Tehran, due to its harmful effects, especially on human health. Materials and methods: Based on the intensity and continuity of the standard air pollution index during 2004-2020 for 112 cases in Tehran, the principal component analysis method was applied in T-mode and clustering of days for similar 500hPa geopotential height changes were provided. The synoptic patterns leading to these different pollution cases were obtained. Results: The study of the mentioned days shows the activity of five atmospheric patterns with the frequency of 47, 28, 12, 13, and 4%, respectively. Conclusion: The first pattern happens during the cold season, associated with the simultaneous placement of the atmospheric ridge along with Siberian high-pressure system. The second group, which occurs mostly in summer and during late spring, is controlled with the predominant ozone pollutant. The main activity of the third pattern occurrences in summer and late spring, which is associated with the presence of monsoon Indian low-pressure on the southeastern regions in Iran. The fourth pattern, which occurs mostly in spring and autumn, associates with the establishment of dynamic low pressure in the western regions of Iran and Iraq, as well as the establishment of the thermal low-pressure system in the central part of the country. The last group is a local pattern that occurs due to the passage of the gust front and the shear of the wind field dust rise in the Tehran.