Figure 6 Map showing the spatial distributions of specific
humidity and wind vector at 900 hPa for (a) the time before the warm
front; (b) the averaged time from 7.28 to 7.30. The purple arrow
symbolizes the air masses transported from the NCP to the NEC. The
meteorology data were from the ERA5 reanalysis website.
Unlike the transport path of the pollution in the LFT, the particles in
the HFT show high residence time of 3 km footprint over the west shallow
belt of the receptor site (Fig. 5e), indicating the different source
regions and the transport path of pollutions in the separate layers. The
surface footprint (under 100 m altitude) showed a high residence time
over the polluted NCP (Fig. 5f). By combining Fig. 5e and Fig. 5f, we
inferred that pollutants in the NCP were transported by the northward
air flow and elevated by the warm front. This transport pathway of the
HFT pollution was close to the warm conveyor belt (WCB) circulation, as
shown in Fig. S1. In addition, the similar LPDM analysis conducted for
pollution episode on August 3 also shows that the air masses arriving at
the free troposphere in the NEC mainly originated from the NCP as shown
in Fig. S11. Our results highlight the important role of regional
transport from NCP in these sulfate-dominate air pollution episodes in
the NEC.
To further investigate the role of the WCB in transporting pollutants
from the NCP to the NEC, we simulated this frontal process using the
WRF-Chem model. The simulated vertical distribution of sulfate mass
concentrations was compared with that from our aircraft observations in
Fig.S5. The observed pollution hot spot D in the HFT pollution was
captured well by the model, while sulfate concentrations in the LFT were
underestimated, possibly due to the the influence of high humidity air
plumes. The uncertainties of uptake coefficient (γ) associated with
relative humidity, aerosol liquid water content, particulate acidity and
other factors in the model usually cause the discrepancy of secondary
inorganic aerosols (Liu et al., 2021).
As shown in Fig. 7a, a high level of sulfate originated from the NCP
(Shandong Province and Beijing) was transported northwardly towards the
warm frontal region. The pollution plume was then lifted over the
convergence belt, leading to high sulfate mass loadings at 2.6 km
altitude (Fig. 7b). After that, this pollution plume was further
transported horizontally to our aircraft site by strong westerly winds
(Fig. S8). A process diagnostic analysis technique, as elaborated in
Text S4, was applied to disentangle each individual contribution from
different physical or chemical processes to sulfate variations over the
study period. To get more insight into the pollution structure of WCB,
the cross-section profiles of the contributions of chemical and
advection processes to sulfate production are shown in Fig. 7c and d.
The chemical formation of sulfate mainly occurred within the PBL from
the ground to nearly 1 km with large values occurring over Beijing.
However, high contribution of advection transport to sulfate
concentrations occurred in the free troposphere caused by strong
vertical wind. When the warm air masses arrived at the frontal region,
pollutants were lifted by the strong updrafts (Fig. 7d). The sulfate
chemically formed within the PBL in the NCP was thus lifted and
transported to the NEC. These vertical structures clearly depicted the
PBL-FT air pollution transport influenced by the WCB circulation. The
diagnostic analysis also suggested that the increase of sulfate mass
concentrations on July 30 in the HFT was attributed to the horizontal
advection transport (Fig. S9).