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