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.

References

Bethan, S., G. Vaughan, C. Gerbig, A. Volz-Thomas, H. Richer, and D. A. Tiddeman (1998), Chemical air mass differences near fronts, J Geophys Res-Atmos, 103(D11), 13413-13434.
Bao, M., et al. (2017), Characteristics and origins of air pollutants and carbonaceous aerosols during wintertime haze episodes at a rural site in the Yangtze River Delta, China, Atmospheric Pollution Research, 8(5), 900-911, doi:10.1016/j.apr.2017.03.001.
Chen, G. (2020a), Diurnal Cycle of the Asian Summer Monsoon: Air Pump of the Second Kind, J Climate, 33(5), 1747-1775, doi:10.1175/jcli-d-19-0210.1.
Chen, X., et al. (2020b), Common source areas of air pollution vary with haze intensity in the Yangtze River Delta, China, Environ Chem Lett, 18(3), 957-965.
Dickerson, R. R., et al. (2007), Aircraft observations of dust and pollutants over northeast China: Insight into the meteorological mechanisms of transport, J Geophys Res-Atmos, 112(D24).
Ding, A., T. Wang, and C. Fu (2013), Transport characteristics and origins of carbon monoxide and ozone in Hong Kong, South China, Journal of Geophysical Research: Atmospheres, 118(16), 9475-9488.
Ding, A., et al. (2009), Transport of north China air pollution by midlatitude cyclones: Case study of aircraft measurements in summer 2007, Journal of Geophysical Research, 114(D8).
Ding, A., et al. (2019), Significant reduction of PM2.5 in eastern China due to regional-scale emission control: evidence from SORPES in 2011–2018, Atmos Chem Phys, 19(18), 11791-11801.
Dong, X. Y., et al. (2018), Long-range transport impacts on surface aerosol concentrations and the contributions to haze events in China: an HTAP2 multi-model study, Atmos Chem Phys, 18(21), 15581-15600.
Du, H., et al. (2020), Effects of Regional Transport on Haze in the North China Plain: Transport of Precursors or Secondary Inorganic Aerosols, Geophys Res Lett, 47(14).
Ellison, E., L. Baker, and A. Wilson (2020), IPCC Special Report Meeting: Climate Change Around the Globe, Weather, 75(9), 293-294.
Feng, Y., and J. E. Penner (2007), Global modeling of nitrate and ammonium: Interaction of aerosols and tropospheric chemistry, J Geophys Res-Atmos, 112(D1).
Fröhlich, R., et al. (2013), The ToF-ACSM: a portable aerosol chemical speciation monitor with TOFMS detection, Atmospheric Measurement Techniques, 6(11), 3225-3241.
Grell, G., S. R. Freitas, M. Stuefer, and J. Fast (2011), Inclusion of biomass burning in WRF-Chem: impact of wildfires on weather forecasts, Atmos Chem Phys, 11(11), 5289-5303.
Henne, S., J. Dommen, B. Neininger, S. Reimann, J. Staehelin, and A. S. H. Prévôt (2005), Influence of mountain venting in the Alps on the ozone chemistry of the lower free troposphere and the European pollution export, Journal of Geophysical Research, 110(D22), doi:10.1029/2005jd005936.
Hu, W., M. Hu, W.-W. Hu, J. Zheng, C. Chen, Y. Wu, and S. Guo (2017), Seasonal variations in high time-resolved chemical compositions, sources, and evolution of atmospheric submicron aerosols in the megacity Beijing, Atmospheric Chemistry and Physics, 17(16), 9979-10000.
Huang, R. J., et al. (2014), High secondary aerosol contribution to particulate pollution during haze events in China, Nature, 514(7521), 218-222.
Huang, X., Z. L. Wang, and A. J. Ding (2018), Impact of Aerosol-PBL Interaction on Haze Pollution: Multiyear Observational Evidences in North China, Geophys Res Lett, 45(16), 8596-8603.
Huang, X., A. Ding, Z. Wang, K. Ding, J. Gao, F. Chai, and C. Fu (2020), Amplified transboundary transport of haze by aerosol–boundary layer interaction in China, Nat Geosci, 13(6), 428-434.
Jiang, Q., F. Wang, and Y. Sun (2019), Analysis of Chemical Composition, Source and Processing Characteristics of Submicron Aerosol during the Summer in Beijing, China, Aerosol and Air Quality Research, 19(6), 1450-1462.
Jimenez, J. L., et al. (2009), Evolution of Organic Aerosols in the Atmosphere, Science, 326(5959), 1525-1529.
Kiley, C. M., and H. E. Fuelberg (2006), An examination of summertime cyclone transport processes during intercontinental chemical transport experiment (INTEX-A), J Geophys Res-Atmos, 111(D24).
Kim, H., Q. Zhang, G. N. Bae, J. Y. Kim, and S. B. Lee (2017), Sources and atmospheric processing of winter aerosols in Seoul, Korea: insights from real-time measurements using a high-resolution aerosol mass spectrometer, Atmos Chem Phys, 17(3), 2009-2033.
Kim, H., Q. Zhang, and J. Heo (2018), Influence of intense secondary aerosol formation and long-range transport on aerosol chemistry and properties in the Seoul Metropolitan Area during spring time: results from KORUS-AQ, Atmos Chem Phys, 18(10), 7149-7168.
Li, C., J. W. Stehr, L. T. Marufu, Z. Q. Li, and R. R. Dickerson (2012), Aircraft measurements of SO2 and aerosols over northeastern China: Vertical profiles and the influence of weather on air quality, Atmospheric Environment, 62, 492-501
Li, H., et al. (2017), Wintertime aerosol chemistry and haze evolution in an extremely polluted city of the North China Plain: significant contribution from coal and biomass combustion, Atmospheric Chemistry and Physics, 17(7), 4751-4768.
Li, H., Q. Zhang, B. Zheng, C. Chen, N. Wu, H. Guo, Y. Zhang, Y. Zheng, X. Li, and K. He (2018), Nitrate-driven urban haze pollution during summertime over the North China Plain, Atmospheric Chemistry and Physics, 18(8), 5293-5306, doi:10.5194/acp-18-5293-2018.
Li, X. L., X. M. Hu, Y. J. Ma, Y. F. Wang, L. G. Li, and Z. Q. Zhao (2019), Impact of planetary boundary layer structure on the formation and evolution of air-pollution episodes in Shenyang, Northeast China, Atmospheric Environment, 214.
Li, Y. J., Y. Sun, Q. Zhang, X. Li, M. Li, Z. Zhou, and C. K. Chan (2017b), Real-time chemical characterization of atmospheric particulate matter in China: A review, Atmospheric Environment, 158, 270-304.
Liu, X., M. Chang, J. Zhang, J. Wang, H. Gao, Y. Gao, and X. Yao (2021), Rethinking the causes of extreme heavy winter PM2.5 pollution events in northern China, Science of The Total Environment, 794, doi:10.1016/j.scitotenv.2021.148637.
Paatero, P., and U. Tapper (1994), Positive Matrix Factorization - a Nonnegative Factor Model with Optimal Utilization of Error-Estimates of Data Values, Environmetrics, 5(2), 111-126.
Sarangi, C., S. N. Tripathi, A. K. Mishra, A. Goel, and E. J. Welton (2016), Elevated aerosol layers and their radiative impact over Kanpur during monsoon onset period, Journal of Geophysical Research: Atmospheres, 121(13), 7936-7957, doi:10.1002/2015jd024711.
Seinfeld, J. H., and Pandis, S. N. (2006), Atmospheric Chemistry and Physics: From Air Pollution to Climate 591 Change, John Wiley & Sons, New York, 2nd edition, 1232 pp., 13: 978-0-471-72018-8.
Stein, A. F., R. R. Draxler, G. D. Rolph, B. J. B. Stunder, M. D. Cohen, and F. Ngan (2015), Noaa’s Hysplit Atmospheric Transport and Dispersion Modeling System, B Am Meteorol Soc, 96(12), 2059-2077.
Sun, P., et al. (2020), Impact of air transport and secondary formation on haze pollution in the Yangtze River Delta: In situ online observations in Shanghai and Nanjing, Atmospheric Environment, 225, 117350.
Sun, P., et al. (2018), Two years of online measurement of fine particulate nitrate in the western Yangtze River Delta: influences of thermodynamics and N2O5 hydrolysis, Atmos Chem Phys, 18(23), 17177-17190.
Sun, Y. L., Q. Jiang, Z. F. Wang, P. Q. Fu, J. Li, T. Yang, and Y. Yin (2014), Investigation of the sources and evolution processes of severe haze pollution in Beijing in January 2013, J Geophys Res-Atmos, 119(7), 4380-4398.
Sun, Y. L., Z. F. Wang, P. Q. Fu, T. Yang, Q. Jiang, H. B. Dong, J. Li, and J. J. Jia (2013), Aerosol composition, sources and processes during wintertime in Beijing, China, Atmospheric Chemistry and Physics, 13(9), 4577-4592.
ten Brink, H., R. Otjes, P. Jongejan, and S. Slanina (2007), An instrument for semi-continuous monitoring of the size-distribution of nitrate, ammonium, sulphate and chloride in aerosol, Atmospheric Environment, 41(13), 2768-2779.
Qixin Tan, B. G., Xiaobin Xu, Lu Gan, Wenyi Yang, Xueshun Chen, Xiaole Pan, Wei Wang, Jie Li, Zifa Wang, (2021), Increasing impacts of the relative contributions of regional transport on air pollution in Beijing: Observational evidence, Environmental Pollution, doi:org/10.1016/j.envpol.2021.118407.
Ulbrich, I. M., M. R. Canagaratna, Q. Zhang, D. R. Worsnop, and J. L. Jimenez (2009), Interpretation of organic components from Positive Matrix Factorization of aerosol mass spectrometric data, Atmos Chem Phys, 9(9), 2891-2918.
Wang, J. D., et al. (2017), Particulate matter pollution over China and the effects of control policies, Sci Total Environ, 584, 426-447.
Wang, T., A. J. Ding, J. Gao, and W. S. Wu (2006), Strong ozone production in urban plumes from Beijing, China, Geophys Res Lett, 33(21).
Wang, T., X. Huang, Z. Wang, Y. Liu, D. Zhou, K. Ding, H. Wang, X. Qi, and A. Ding (2020), Secondary aerosol formation and its linkage with synoptic conditions during winter haze pollution over eastern China, The Science of the total environment, 730, 138888.
Wang, X., X. Ding, X. Fu, Q. He, S. Wang, F. Bernard, X. Zhao, and D. Wu (2012), Aerosol scattering coefficients and major chemical compositions of fine particles observed at a rural site in the central Pearl River Delta, South China, Journal of Environmental Sciences, 24(1), 72-77.
Wu, J. R., N. F. Bei, X. Li, J. J. Cao, T. Feng, Y. C. Wang, X. X. Tie, and G. H. Li (2018), Widespread air pollutants of the North China Plain during the Asian summer monsoon season: a case study, Atmos Chem Phys, 18(12), 8491-8504.
Xia, L., B. Zhu, H. Wang, H. Kang, and J. An (2020), Characterization and Source Apportionment of Fine Particles during a Heavy Pollution Episode over the Yangtze River Delta, China, Atmosphere, 11(7), doi:10.3390/atmos11070720.
Xue, J., X. Yu, Z. Yuan, S. M. Griffith, A. K. H. Lau, J. H. Seinfeld, and J. Z. Yu (2019), Efficient control of atmospheric sulfate production based on three formation regimes, Nat Geosci, 12(12), 977-982.
Xue, L., A. Ding, J. Gao, T. Wang, W. Wang, X. Wang, H. Lei, D. Jin, and Y. Qi (2010), Aircraft measurements of the vertical distribution of sulfur dioxide and aerosol scattering coefficient in China, Atmospheric Environment, 44(2), 278-282.
Zhang, J., et al. (2021), Trans-Regional Transport of Haze Particles From the North China Plain to Yangtze River Delta During Winter, J Geophys Res-Atmos, 126(8).
Zhang, Q., J. L. Jimenez, M. R. Canagaratna, I. M. Ulbrich, N. L. Ng, D. R. Worsnop, and Y. Sun (2011), Understanding atmospheric organic aerosols via factor analysis of aerosol mass spectrometry: a review, Analytical and bioanalytical chemistry, 401(10), 3045-3067.
Zhang, Q., et al. (2007), Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes, Geophys Res Lett, 34(13), n/a-n/a.
Zhang, R. Y., G. H. Wang, S. Guo, M. L. Zarnora, Q. Ying, Y. Lin, W. G. Wang, M. Hu, and Y. Wang (2015), Formation of Urban Fine Particulate Matter, Chem Rev, 115(10), 3803-3855.
Zhang, Y., A. J. Ding, H. T. Mao, W. Nie, D. R. Zhou, L. X. Liu, X. Huang, and C. B. Fu (2016), Impact of synoptic weather patterns and inter-decadal climate variability on air quality in the North China Plain during 1980-2013, Atmos Environ, 124, 119-128.
Zhou, W., W. Xu, H. Kim, Q. Zhang, P. Fu, D. R. Worsnop, and Y. Sun (2020), A review of aerosol chemistry in Asia: insights from aerosol mass spectrometer measurements, Environmental science. Processes & impacts, 22(8), 1616-1653.

References from the Supporting Information

Huang, X., A. Ding, Z. Wang, K. Ding, J. Gao, F. Chai, and C. Fu (2020), Amplified transboundary transport of haze by aerosol–boundary layer interaction in China, Nat Geosci, 13(6), 428-434.
Ng, N. L., M. R. Canagaratna, J. L. Jimenez, Q. Zhang, I. M. Ulbrich, and D. R. Worsnop (2011), Real-Time Methods for Estimating Organic Component Mass Concentrations from Aerosol Mass Spectrometer Data, Environ Sci Technol, 45(3), 910-916.
Sun, P., et al. (2020), Impact of air transport and secondary formation on haze pollution in the Yangtze River Delta: In situ online observations in Shanghai and Nanjing, Atmospheric Environment, 225, 117350.
Wang, T., X. Huang, Z. Wang, Y. Liu, D. Zhou, K. Ding, H. Wang, X. Qi, and A. Ding (2020), Secondary aerosol formation and its linkage with synoptic conditions during winter haze pollution over eastern China, The Science of the total environment, 730, 138888.
Xu, W., P. Croteau, L. Williams, M. Canagaratna, T. Onasch, E. Cross, X. Zhang, W. Robinson, D. Worsnop, and J. Jayne (2016), Laboratory characterization of an aerosol chemical speciation monitor with PM2.5 measurement capability, Aerosol Science and Technology, 51(1), 69-83.