Spatial homogeneity and heterogeneity of ambient air pollutants in Tehran.

To investigate spatial inequality of ambient air pollutants and comparison of their heterogeneity and homogeneity across Tehran, the following quantitative indicators were utilized: coefficient of divergence (COD), the 90 percentile of the absolute differences between ambient air pollutant concentrations and coefficient of variation (CV). Real-time hourly concentrations of particulate matter (PM) and gaseous air pollutants (GAPs) of twenty-two air quality monitoring stations (AQMSs) were obtained from Tehran Air Quality Control Company (TAQCC) in 2017. Annual mean concentrations of PM, PM, and PM (PM) ranged from 21.7 to 40.5, 37.3 to 75.0 and 58.0 to 110.4 μg m, respectively. Annual mean PM and PM concentrations were higher than the World Health Organization air quality guideline (WHO AQG) and national standard levels. NO, O, SO and CO annual mean concentrations ranged from 27.0 to 76.8, 15.5 to 25.1, 4.6 to 12.2 ppb, and 1.9 to 3.8 ppm over AQMSs, respectively. Our generated spatial maps exhibited that ambient PM concentrations increased from the north into south and south-western areas as the hotspots of ambient PM in Tehran. O hotspots were observed in the north and south-west, while NO hotspots were in the west and south. COD values of PM demonstrated more results lower than the 0.2 cut off compared to GAPs; indicating high to moderate spatial homogeneity for PM and moderate to high spatial heterogeneity for GAPs. Regarding CV approach, the spatial variabilities of air pollutants followed in the order of O (87.3%) > SO (65.2%) > CO (61.8%) > PM (52.5%) > PM (48.9%) > NO (48.1%) > PM (42.9%), which were mainly in agreement with COD results, except for NO. COD values observed a statistically (P < 0.05) positive correlation with the values of the 90 percentile across AQMSs. Our study, for the first time, highlights spatial inequality of ambient PM and GAPs in Tehran in detail to better facilitate establishing new intra-urban control policies.