Impact of thermal structure of planetary boundary layer on aerosol pollution over urban regions in Northeast China

This study investigated the characteristics of aerosol pollution in three provincial capitals (Shenyang, Chang-chun, and Harbin) in Northeast China and the impact of the thermal structure of the planetary boundary layer (PBL) using surface air quality monitoring data and meteorological observations and sounding data from January 1 to January 20, 2020. The number of pollution days in Shenyang, Changchun, and Harbin were 13, 17, and 19, respectively, and the number of heavy pollution days was 4, 3, and 13, respectively. Harbin suffered the heaviest and longest pollution with the smallest diurnal variation between daytime low and nighttime high PM2.5. The atmosphere in Harbin was more stable than that in Shenyang and Changchun, and PM2.5 concentration in the three cities tended to increase with decreasing planetary boundary layer height (PBLH). The lower winter air temperature in Northeast China led to a shallower PBL and weaker ventilation coefficient, which in turn led to an increase in PM2.5 concentration, indicating that air temperature is the key meteorological factor affecting local pollution in Northeast China. The analysis of two typical pollution events showed that a warm advection aloft suppressed the development of the PBL in Shenyang. In contrast, the suppressed effect of warm advection on the PBL was weakened in Harbin, which originally had a low PBLH due to cold weather. Moreover, the daytime convective boundary layer in Harbin developed 1-2 h later than that in Shenyang; the delayed development of PBL during the daytime in Harbin was also conducive to poor air quality.