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.