Figure 3 (a) The flight track of July 30, 2018 over the NEC. The top panel is color-coded by the local time and the bottom panel by the mass concentrations of NR-PM2.5. (b) Time series of NR-PM2.5 mass concentrations, Bsp, flight altitude, the mass contribution of different species to total NR-PM2.5, SO2 and O3mixing ratios during the flight on 30 July 2018. The corresponding PM2.5 mass peaks were marked with A B C D in both (a) and (b) panels.
Fig. 4a presents the vertical profile of NR-PM2.5 from 16:10 LT to 16:50 LT. NR-PM2.5 was well-mixed within the PBL with a relative low concentration of ~8 µg m-3 and a composition dominated by organics (65% of NR-PM2.5 mass; Fig. 4c). HOA accounted for 25% of the OA mass within the PBL, indicating the importance of local sources in aerosol loadings in the PBL. A dramatic variation of aerosol mass concentration and composition was observed in the free troposphere, where high NR-PM2.5 mass concentration of up to 74 µg m-3 was observed with sulfate being the dominant component (48% of NR-PM2.5 mass). An extremely low fraction of HOA (2% of NR-PM2.5 mass) in both LFT and HFT suggested that the pollution plumes in the free troposphere were not contributed by local sources, but by regional transport instead. The aerosols in both LFT and HFT had substantially higher value of f44 and SOR, indicative of secondary aerosols undergone extensive atmospheric processing and being aged before transported to the free troposphere. Note that the vertical distribution of aerosol composition observed in this study was different from that acquired by an aircraft study conducted in the NCP, a major pollution source region in China, where high levels of aerosols were trapped inside the PBL and were dominated by organics (Liu et al., 2019).
SO2 can be oxidized into sulfate through multiple chemical pathways, i.e., gas-phase oxidations, aqueous phase reactions, and heterogeneous reactions (Seinfeld and Pandis, 2006). The geographical distribution of SO2 concentrations (Fig. 1a) reveals that the flight reported here was carried out in a regional plume in the NEC due to relatively low emissions of SO2over the flight region. The distribution of the surface PM2.5 mass concentrations during the flight time also denoted the influence of regional transport (Fig. S6) since the hot spots of PM2.5 were mainly located to the south of the flight path.