Figure 9 Map showing the surface spatial distributions of (a)
chemical production of nitrate, (b) average NH3 mixing
ratio, (c) average O3 mixing ratio, (d) average
temperature and Wind field from July 27 to 28.
Previous studies have shown that strong photochemical reactions and high
temperature during summer favored the production of secondary aerosols
in the NCP, especially secondary inorganic aerosols (SNA) (Jiang et al.,
2019; Li et al., 2017; Hu et al., 2017). However, the chemical evolution
of aerosols during regional transport associated with synoptic systems
has been rarely investigated. By combining multiple ground observations
and the WRF-Chem model, we are able to perform an in-depth analysis of
aerosol chemistry during the WCB transport. Note that the aerosol
loadings here were indicated by the total mass concentrations of
sulfate, nitrate, ammonium and chloride.
As shown in Figure 8a, a distinct transition of aerosol composition
across multiple ground sites was observed during the regional transport.
Both the mass concentrations and contributions of particulate nitrate
decreased from south to north (Fig. 8c). Specifically, nitrate
contributed a significant fraction of aerosol mass in Shandong (47%),
while its contribution decreased to 32% in Megacity Beijing and
eventually to 13% in the free troposphere in the NEC. In contrast, the
contribution of particulate sulfate increased from the Shandong (26%)
to Beijing (42%) and dominated NR-PM2.5 (60%) in the
NEC. Accordingly, sulfate-to-nitrate ratio depicted a significant
increase during the WCB transport from Shandong to the NEC (Fig. 8d).
The transition of aerosol chemical compositions suggested the
significant chemical processes occurring inside the air mass plume
during the transport. Sulfate was strongly formed over the NCP,
especially around Beijing (Fig. 8a), which was attributed to the high
local primary emissions of SO2 (Fig. S10) and the strong
atmospheric oxidation capacity indicated by the high concentrations of
O3 (Fig. 9c). Similarly, particulate nitrate was largely
formed over the Shandong due to high mixing ratios of
NO2 and NH3 (Fig. 9b). Sulfate formation
is irreversible and sulfate particles can be transported long-rangely
without significant losses, whereas ammonium nitrate is a semi-volatile
specie and can evaporate easily under high temperature, especially
during summer (Feng and Penner, 2007). As a result, significantly
negative nitrate chemical productions were observed over the north areas
along the transport pathway of the WCB (Fig. 9a). Overall, the WCB
circulations transported pollutants accumulated in PBL in the NCP to the
FT in the NEC, during which an increasing contribution of sulfate to
PM2.5 was observed due to evaporation losses of ammonium
nitrate.
Despite limited number of flight experiments conducted in the NEC, we
have captured 2 sulfate-dominated pollution episodes. Our results
indicate that the pollution accumulated in the NCP are easy to be
transported to the NEC with secondary aerosols being oxidized and aged.
Although particulate nitrate has been identified as the most important
contributor to air pollution in China in recent years due to emission
changes (Li et al., 2018; Sun et al., 2018; Ding et al., 2019), it may
undergo evaporation loss during long-range transport, especially in
summer, and thus sulfate becomes more important and serves as the
driving factor of regional or trans-boundary pollution.
4. Conclusion
A multi-platform based campaign was organized using an aircraft in
Northeast China (NE) and multiple ground observations in North China
Plain (NCP), with the aim to investigate the role of mid-latitude
cyclones in driving air pollutants from the NCP to the NEC, especially
to understand the evolution of aerosol chemistry during the transport.
Aircraft measurements showed relatively high aerosol loadings, dominated
by sulfate in the free troposphere of the NEC despite low loadings of
aerosols dominated by organics within the PBL. Lagrangian dispersion
modeling and WRF-Chem simulation were conducted to understand the
sources and transport characteristics of particulate pollution. Air
pollution in the lower free troposphere was transported directly from
north Hebei Province by warm and moist air masses at 900 hPa after the
warm front. In contrast, pollution in the higher free troposphere was
influenced by the warm conveyor belt, which transported particulate
matters from the NCP and lifted them into the higher free troposphere.
Both sulfate and nitrate formed intensively in the NCP but behaved
differently during the transport to the north, in that sulfate
concentrations stayed relatively constant while nitrate decreased
readily due to evaporation losses. In addition to the well-understood
regional transport processes from the NCP to the YRD (Sun et al., 2020),
our results also identified the “chimney effect” imparted by the NCP,
where aerosols are fast generated and blown by Asian monsoon to the YRD
in winter when nitrate formation is favorable (Wang et al., 2020), and
to the NEC in summer when nitrate formation is restricted.
Acknowledgments
This work was funded by the Ministry of Science and Technology of the
People’s Republic of China (2016YFC0200500), the National Natural
Science Foundation of China (92044301) and High level personel project
of Jiangsu Province (JSSCBS20210033). We thank colleagues at Jilin
Provincial Weather Modification Office for their support on the aircraft
field campaign, and those at Environmental Monitoring Centers in
Shandong Province and Beijing for their contributions on the
ground-based field measurements.
Data availability Statement
The emission data reported in Fig.1 are available at
https://ladsweb.modaps.eosdis.nasa.gov/. All the meteorology data could
be downloaded from the ERA 5 reanalysis website
(https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels).
All the aircraft and multiple ground measurement data used in this study
are available at https://doi.org/10.5281/zenodo.5652522.
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