Source contribution analysis of tropospheric NO2 based on two-dimensional MAX-DOAS measurements

Ground-based two-dimensional (2D) Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of nitrogen dioxide (NO2) were collected continuously from June 2015 to May 2016 in Hefei, China. The spatial distribution of NO2 concentrations showed that more air pollution was present in eastern and southeastern direction from the study site, and these areas were located in the center of the city and economic developing zone. The seasonal variations of NO2 concentrations were distinct, and the highest levels were detected in winter, while the lowest levels were detected in summer. The mean concentration of NO2 during the heating season (November–March of the following year) was 6.56 ppbv, while that during the non-heating season (April–October) was 4 ppbv. The highest concentrations of NO2, carbon monoxide (CO), and PM2.5 were all observed during the heating season. The results from the potential source contribution function (PSCF) analysis suggested that NO2 pollution in the summer was caused primarily by long-range transport from the east, while that in winter showed significant local influences. By combining the analyses of meteorological trajectory pressure profiles and aerosol optical depths (AODs), the sources of NO2 during the heating season were found to be mainly local emissions, while those during the non-heating season were mainly from long-range transportation.