Three-Dimensional Distribution of PM2.5 over the Yangtze River Delta as Cold Fronts Moving Through

Long-range transport of PM2.5 from the North China Plain (NCP) by cold frontal passage is considered to be an important haze formation mechanism in the Yangtze River Delta (YRD). However, detailed 3D distributions of PM2.5 and meteorological fields during the front moving through are not fully understood. Herein, in situ observations showed that most haze events in autumn and winter in the YRD were associated with cold frontal passages. A typical cold front episode on November 2-3, 2017, was investigated with unmanned aerial vehicle sounding and model simulations, which both revealed that PM2.5 in the upper air (similar to 1.0 km) reached the YRD earlier than that moving at the ground level. When the cold front reached the NCP, air pollutants ahead of the front was lifted from the ground level to the upper air. Subsequently, increased PM2.5 concentrations and higher wind speed in the upper air resulted in stronger PM2.5 outflow from the NCP to YRD compared to that at ground level. The maximum PM2.5 mass contribution from the NCP to YRD was 15.6 mu g m(-3) (accounted for 31.5% of the total PM2.5 in the YRD), which appeared at similar to 1.0 km altitude. Compared to the noncold front period, the PM2.5 mass transported from the NCP was 10-15 times higher during the cold front episode. Results indicate that the combination of multisite sounding and elaborate simulation can effectively explain the impact of synoptic processes on the transport, distribution, and evolution of air pollution and provide precise pollution control directives. Key Points Most haze events in autumn and winter in the Yangtze River Delta were associated with cold frontal passages Both unmanned aerial vehicle sounding and model simulation revealed a stronger and faster upper-level transport of PM2.5 than at ground level The PM2.5 mass transported from the North China Plain was 10-15 times higher during a cold front episode