Figure 4 (a) vertical profile of mass concentrations of NR-PM2.5 and temperature; (b) sulfate conversion ratio (SOR) and f44 of organic aerosols; (c) averaged contributions of NR-PM2.5 species in the boundary layer (altitude below 900 m), the lower free troposphere (altitude from 900 m to 2000 m), and the higher free troposphere (altitude above 2000 m).
3.2 Regional transport associated with the synoptic processes
To identify the polluted plume sources and investigate their transport pathways, we conducted LPDM for the case on July 30, 2018. The upper panel of Fig. 5 shows that the air masses arriving at the lower free troposphere of all three cities including Baicheng, Daan, and Songyuan mainly originated from north Beijing and Hebei province, corresponding to the areas with high PM2.5 loadings as shown in Fig. S6. As a comparison, air masses on the ground surface (blow 100 m) were mainly from local and transported from nearby northern clean region (Fig. 5d), resulting in the low aerosol loadings within the PBL.
Air stream pattern at 1 km altitude was analyzed using the ERA-5 reanalysis data to address the influence that the synoptic system has on air pollution transport. As shown in Fig. 6a, there were no prominent air flows from north Hebei Province directly to the aircraft flight area before the warm front. However, continuous southwestern winds brought water vapor to the NE China from the NCP after the warm front and led to the relatively high specific humidity over the study region (Fig. 6b). The high relative humidity (close to 100%) observed at the top of PBL (Fig. S7) further confirmed the influence of the water vapor. Warm and moist air masses was frequently observed at 900 hPa during the summer monsoon (Chen et al., 2020a). They were responsible for bringing pollutants of north Hebei directly to the NE China and formed a pollution layer in the lower troposphere, which was captured by our aircraft measurements. In addition, this water channel characterized by high specific humidity may also favor the formation of sulfate through aqueous phase processes (Xue et al., 2019).