Vertical Distribution and Transport of PM2.5 During Heavy Pollution Events in the Jing-Jin-Ji Region

Abstract

Based on vehicle-borne tethered balloon measurements, the vertical distribution of particulate matter (PM) concentrations were observed in Gaocun in the Wuqing District of Tianjin from December 17 to 19, 2016, during a period of heavy pollution. Using observational data, the transport flux of PM2.5 in the Jing-Jin-Ji region was calculated. The results showed that the mixed layer was low at only 200 m during the heavy pollution period. The vertical distribution of PM2.5 concentrations was closely associated with the heights of mixed layer whereby, below the mixed layer, PM2.5 concentrations were higher. Vertical variation was insignificant, forming a district pollution layer. Above the mixed layer, PM2.5 concentrations rapidly decreased and stabilized at low levels. During the observation period, higher concentrations of PM were found with particle sizes of less than 1.0 μm, and lower concentrations were observed for particle sizes larger than 2.2 μm. The size profiles of PM tallied with relative humidity and the height of the mixed layer. The size distribution was wider during periods of high humidity and with a lower mixed layer height. The greatest PM2.5 transport flux was from the southwest, accounting for 63.3% of the total flux; the highest fluxes occurred at the heights of 46-156 m and 156-296 m. The dominant transport direction was southwest below 300 m, while the dominant transport direction was dispersed over 300 m.