The possible emission sources and atmospheric photochemical processes of air pollutants in Tehran, Iran: the role of micrometeorological factors on the air quality

The hourly concentrations of eight air pollutants relevant for human health and climate (PM2.5, PM10, O3, NO, NO2, NOx, SO2, and CO) were investigated for 1 year (January 2018 to December 2018) at an urban location of a megacity in the Middle East (Tehran, Iran). The spatial distributions of air pollutants were detected via inter-site correlations and coefficients of divergence. The most likely predominant atmospheric processes and sources were determined by interpreting the seasonal, weekly, and diel patterns of air pollutants and the lagged correlations among pollutants. The effect of meteorological factors upon the air pollutants was evaluated by investigating the relationships with key weather factors. The locations of the possible local sources were identified by integrating atmospheric circulation and air pollutant data through bivariate polar plots and conditional bivariate probability function. Potential transboundary source areas were detected using potential source contribution function and concentration-weighted trajectory. Results show that emission factors, weather, and photochemical processes mainly shape the diel and weekly cycles of air pollutants. Compared to other pollutants, daily cycles of SO2 are quite different among sites and show both bimodal and unimodal patterns. While the WPSCF map for O3 does not show a remarkable pattern, primary gaseous pollutants presented similar distribution patterns with the most potential source areas with high WPSCF values from the western areas. By providing useful information on air pollutants at local and transboundary scales, the current study finally empowers general considerations upon the atmospheric processes and air quality status over the Tehran metropolitan area.