Plain Language Summary
Atmospheric chemical processes may occur inside an air mass plume as it
moves continuously along the trajectory. This challenges the signal-site
based investigation in understanding the change of aerosol composition.
Here, we organized a campaign using an aircraft in the Northeast China
(NEC) and multiple ground observations in North China Plain (NCP) to
investigate aerosol chemistry during the regional transport from the NCP
to the NEC influenced by a mid-latitude cyclone. High loadings of aged
secondary aerosols observed in the free troposphere in the NEC were
attributed to the regional transport from the NCP. Both sulfate and
nitrate formed strongly in the NCP but behaved differently during the
transport to the north in that sulfate kept constant while nitrate
decreased readily due to evaporation losses.
1 Introduction
Atmospheric aerosols, especially fine particles (PM2.5,
aerodynamic diameter ≤ 2.5μm), have adverse effects on human health
(Wang et al., 2012; Zhang et al., 2015), reduce visibility (Charlson and
Heintzenberg, 1995), and influence climate and ecosystems (Ellison et
al., 2020). Therefore, full characterization of the sources, chemical
compositions, and evolution processes of atmospheric aerosols is crucial
to elucidate the effects they cause. Extensive field studies around the
world have investigated the chemical properties of
PM2.5, and usually found that organics and secondary
inorganic species, i.e., sulfate, nitrate and ammonium (SNA), were the
dominant aerosol components (Jimenez, et al., 2009; Zhang et al., 2007;
Huang et al., 2014; Li et al., 2017b; Zhou et al., 2020). The mass
concentrations and compositions of fine particles show dramatic temporal
and spatial variations due to influences from various emission sources,
atmospheric processes, and meteorological conditions (Li et al., 2017b;
Kim et al., 2017).
The particulate pollution of a specific area is contributed by both its
local sources and regional transport processes, which sometimes have
been identified as a main reason for the occurrence of regional haze
episodes (Sun et al., 2014; Sun et al., 2020; Huang et al., 2020; Wang
et al., 2020; Kim et al., 2018; Dong et al., 2018). Du et al. (2020)
conducted model simulations and illustrated the importance of
transported secondary aerosols and precursors on haze in the North China
Plain (NCP). Zhang et al. (2021) carried out field studies and
investigated compositions and aging of haze aerosols during the
trans-regional transport from NCP to Yangtze River Delta (YRD). Tan et
al. (2021) even found an increasing impacts of the relative
contributions of regional transport on surface air pollution in Beijing
through observational evidence. A Common view has been reached that
coordinated cross-regional emission reduction strategy is required to
further mitigate haze pollution. Though previous studies have emphasized
the importance of regional transport on aerosol loadings and frequent
haze episodes, they were focused on the aerosols within planetary
boundary layer (PBL) based on the ground observations and corresponding
simulations.
The PBL into free troposphere (FT) transport is of great importance for
the understanding of atmospheric chemistry and climate issues (Henne et
al., 2005), but scant research has covered this topic. Partially because
both airborne and ground observations are simultaneously required to
fully capture the composition and evolution of aerosols, which
challenges the observation technology. The frontal systems, especially
the so-called warm conveyor belts (WCBs), associated with cyclones are
the dominant mechanism that could lift or ventilate PBL air into the FT
(Bethan et al., 1998). Air pollutants, including ozone, aerosols, and
their precursors, can substantially change the chemical environment and
radiation property of the troposphere. The longer lifetime of these
pollutants in the FT extends their impact from the regional to the
continental or even global scale because of long-range transport (Henne
et al., 2005; Dickerson et al., 2007; Ding et al., 2009).
North China Plain (NCP), which includes the Beijing and Tianjin city
clusters and many other cities in the flat region of central-eastern
China, experiences severe air pollution owing to high population
density, intense industrial activities, and unfavorable meteorology
conditions in the region (Wang et al., 2006; Hu et al., 2017; Sun et
al., 2013; Li et al., 2017; Sun et al., 2014; Zhang et al., 2016; Wang
et al., 2017). The NCP thus serves as an important pollution source
region, from where polluted air masses are easy to be transported to
other regions through different synoptic systems. For example, air
pollutants transported from NCP to YRD have been fully investigated
(Ding et al., 2013; Sun et al., 2020; Huang et al., 2020; Wang et al.,
2020; Chen et al., 2020b; Bao et al., 2017).
Northeast China (NEC) is easily influenced by the northward transport of
pollutants accumulated in the NCP through synoptic systems (Ma et al.,
2018; Li et al., 2019) and thus becomes an ideal region to study this
PBL to FT transport. Lots of studies have emphasized the important role
that mid-latitude cyclones play in exporting air pollution from the NCP
to the NEC (Eckhardt et al., 2004; Stohl, 2001; Li et al., 2012;
Dickerson et al., 2007; Oshima et al., 2013; Ding et al., 2009; Wu et
al., 2018). However, the understanding of the detailed transport
mechanism and the aerosol chemistry during the transport remains poor.
This mid-latitude cyclone-influenced regional air transport can lift
pollution plumes into FT from PBL (Bethan et al., 1998; Fuelberg, 2006;
Cooper et al., 2004; Stohl et al., 2003). Thus both aircraft and
multiple ground measurements are required to investigate the evolution
of aerosol chemistry along the transport path.
In this study, we organized a campaign to conduct multiple-platform
measurements of chemically-resolved PM2.5. Specifically,
we conducted aircraft measurements in the NEC using a Time-of-Flight
Aerosol Chemical Speciation Monitor (ToF-ACSM) to capture the air plume
over the downwind region, and we also conducted multiple ground
measurements using Monitor for Aerosols and Gases in ambient air (Marga)
in the NCP to measure aerosol chemical compositions in the source
regions and along the transport path. To provide a regional picture of
the air pollution distribution, the monthly average SO2concentrations in July 2018 in eastern China are shown in Fig. 1a. A
flight was specifically designed in late July to investigate the role
that a mid-latitude cyclone plays in air pollution transport. By
combining Lagrangian dispersion modeling and WRF-Chem (the Weather
Research and Forecasting model coupled with Chemistry) numerical
simulations, we present a detailed analysis of the PBL to FT long-range
transport of air pollutants and the evolution of aerosol chemistry
alongside.
2 Materials and Methods
2.1 Aircraft and multiple ground measurements