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).