Investigation of the chemical nature of precipitation and source apportionment of its constituents in Tehran metropolis, Iran

Abstract

Precipitation is a key process for purifying the atmosphere of pollutants. However, precipitation chemistry is also a significant environmental catastrophe on a global scale. Tehran Metropolitan Area, Iran's capital, is one of the world's most polluted cities. Nonetheless, little effort has been paid to determining the chemical composition of precipitation in this polluted metropolis. The chemical components and likely sources of trace metals and water-soluble ions in precipitation samples collected from 2021 to 2022 at an urban location in Tehran, Iran, were investigated in this study. The pH of the rainwater samples varied from 6.330 to 7.940 (mean 7.313, volume weighted mean (VWM) 7.523). The following is the order of the VWM concentration of main ions: Ca2+ > HCO3- > Na+ >SO42- > NH4+ > Cl− > NO3- > Mg2+> K+> F−. Furthermore, we discovered that VWM concentrations for trace elements are modest, with the exception of Sr (39.104 eq L−1). The primary neutralizing species for precipitation acidity were Ca2+ and NH4+. Vertical feature mask (VFM) diagrams derived from cloud-aerosol lidar and infrared pathfinder satellite observation (CALIPSO) track data indicated that polluted dust was the most common pollutant in the Tehran sky that might contribute significantly to the neutralization of precipitation. A study of species concentration ratios in seawater and the earth's crust indicated that virtually all Se, Sr, Zn, Mg2+, NO3-, and SO42- were anthropogenic. While Cl− was largely obtained from sea salt, K+ was obtained from both the earth's crust and the sea, with the earth's crust playing a larger role in K+. The earth's crust, aged sea salt, industry, and combustion processes were all verified as sources of trace metals and water-soluble ions by positive matrix factorization analysis.