Investigating air quality and the synoptic patterns associated with it during extremely hot days in various local climatic zones of metropolitan Tehran

This study collected daily data on average temperature, maximum temperature, and pollutant concentrations during the year’s warm seasons (2010–2022). The Spearman’s correlation coefficient was calculated between the mentioned parameters. The 95th percentile was used to define the threshold for very hot days. Next, the Local Climate Zones (LCZ) classification was used to assess the vulnerability of different land uses in Tehran to pollutant concentrations on these very hot days. Finally, to determine the atmospheric synoptic patterns of very hot days, the data of geopotential height, temperature, Zonal and meridional wind of 1000 and 700 hectopascals (hPa) were checked on all the days of occurrence. During the statistical period under study, an average of 292 very hot days was recorded. On very hot days, the concentrations of CO, O3, NO2, PM10, and PM2.5 were higher than on normal days. Aerosol pollutants, O3, and SO2 had higher concentrations on hot days, particularly in areas dominated by heavy industries. Similarly, CO and NO2 concentrations were higher in areas with compact mid-rise buildings. The synoptic pattern of Pakistan low pressure, the combined pattern of Pakistan-Arabian Peninsula low pressure, and the combined pattern of Arabian Peninsula low pressure-migratory anticyclone, along with local sources of pollutants, were the main causes of the increase in the concentrations of PM2.5 and PM10. Additionally, the combined pattern of Pakistan, Arabia, and Turkmenistan low-pressure, along with local sources, was the main factor in increasing the concentration of SO2, NO2, O3, and CO.