Vertical Distribution of PM2.5 Transport Flux in Summer and Autumn in Beijing

Temporal and spatial distribution characteristics of PM2.5 transport fluxes between Beijing and its adjacent cities in July and October 2016 were estimated quantitatively using the cross-boundary transport flux method based on the WRF-CMAQ model in this study. Vertical distribution of PM2.5 net fluxes was revealed, and hourly evolution characteristics of PM2.5 transport fluxes in typical heavy pollution processes were illustrated. We verified significant seasonal differences in PM2.5 transport fluxes between Beijing and its adjacent cities during the study period. PM2.5 total inflow and outflow fluxes in autumn were 3187 t d(-1) and -2721 t d(-1), which were significantly higher than that of 2134 t d(-1) and -2172 t d(-1) in summer. In autumn, Beijing received more PM2.5 from adjacent cities, while the results in summer were opposite. Maximum net fluxes appeared at 600-800 m and 1000-1260 m above the ground in summer and autumn, respectively. The vertical distribution characteristics of pollution days and clean days were consistent, both of which show a net inflow from Baoding and Langfang to Beijing, while Beijing was in a state of net outflow to Chengde. During the whole heavy pollution process, the evolution characteristics of PM2.5 flux at low and high altitudes were consistent, and the intensity of the latter was 2.15-5.30 times of the former. Meanwhile, local discharging was more likely to cause extreme PM2.5 heavy pollution, and a better peak clipping effect can be achieved when emission reduction measures are initiated 1 to 2 days before heavy pollution. The results can provide scientific support to put forward effective joint control measures and obtain insights into the evolutionary mechanism of haze episodes in Beijing.