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COATS: Comprehensive observation on the atmospheric boundary layer three-dimensional structure during haze pollution in the North China Plain

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Abstract

The North China Plain (NCP) is troubled by severe haze pollution and the evolution of haze pollution is closely related to the atmospheric boundary layer (ABL). However, experimental and theoretical studies on the physical-chemical processes of the ABL in the NCP are lacking, with many scientific problems to be addressed. To solve these problems, the Comprehensive Observation on the Atmospheric boundary layer Three-dimensional Structure (COATS) during haze pollution was carried out in the NCP from 2016 to 2020. The COATS experiment adopted a “point-line-surface” spatial layout, obtaining both spatial-temporal profiles of the meteorological and environmental elements in the ABL and the turbulent transport data of fine particulate matter (PM2.5) in winter and summer. The research achievements are as follows. The spatial-temporal distribution characteristics of the ABL structure and PM2.5 concentrations in the NCP were determined. The typical thermal structure of persistent heavy haze events and the pollutant removal mechanism by low-level jets were revealed. It was determined that the spatial structure of the ABL adjusted by the Taihang Mountains is responsible for the heterogeneous distribution of haze pollution in the NCP, and that mountain-induced vertical circulations can promote the formation of elevated pollution layers. The restraints of the atmospheric internal boundaries on horizontal diffusion of pollutants were emphasized. The contribution of the ABL to haze pollution in winter and summer was qualitatively compared and quantitatively estimated. The turbulent transport nature behind the relationship between the atmospheric boundary layer height (ABLH) and surface PM2.5 concentrations was analyzed. The concept of “aerosol accumulation layer” was defined, and the applicability of the material method in determining ABLH was clarified. A measurement system for obtaining the turbulent flux of PM2.5 concentrations was developed, and the turbulence characteristics of PM2.5 concentrations were demonstrated. The COATS experiment is of great theoretical significance for thoroughly understanding the physical mechanisms of the ABL during haze pollution and filling the knowledge gap on the impacts of the ABL three-dimensional structure on haze pollution. The results of this study are conducive to the improvement and development of ABL parameterization schemes and serve as a scientific basis for formulating regional pollution prevention and control measures.

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References

  • Banta R M, Newsom R K, Lundquist J K, Pichugina Y L, Coulter R L, Mahrt L. 2002. Nocturnal low-level jet characteristics over Kansas during Cases-99. Bound-Layer Meteor, 105: 221–252

    Article  Google Scholar 

  • Bei N, Zhao L, Xiao B, Meng N, Feng T. 2017. Impacts of local circulations on the wintertime air pollution in the Guanzhong Basin, China. Sci Total Environ, 592: 373–390

    Article  Google Scholar 

  • Bei N, Li X, Tie X, Zhao L, Wu J, Li X, Liu L, Shen Z, Li G. 2020. Impact of synoptic patterns and meteorological elements on the wintertime haze in the Beijing-Tianjin-Hebei region, China from 2013 to 2017. Sci Total Environ, 704: 135210

    Article  Google Scholar 

  • Cai S, Wang Y, Zhao B, Wang S, Chang X, Hao J. 2017. The impact of the “Air Pollution Prevention and Control Action Plan” on PM2.5 concentrations in Jing-Jin-Ji region during 2012–2020. Sci Total Environ, 580: 197–209

    Article  Google Scholar 

  • Cao Z, Sheng L, Liu Q, Yao X, Wang W. 2015. Interannual increase of regional haze-fog in North China Plain in summer by intensified easterly winds and orographic forcing. Atmos Environ, 122: 154–162

    Article  Google Scholar 

  • Chen J, Zhao C S, Ma N, Liu P F, Göbel T, Hallbauer E, Deng Z Z, Ran L, Xu W Y, Liang Z, Liu H J, Yan P, Zhou X J, Wiedensohler A. 2012. A parameterization of low visibilities for hazy days in the North China Plain. Atmos Chem Phys, 12: 4935–4950

    Article  Google Scholar 

  • Collis R T H, Fernald F G, Ligda M G H. 1964. Laser radar echoes from a stratified clear atmosphere. Nature, 203: 1274–1275

    Article  Google Scholar 

  • Collis R T H, Ligda M G H. 1964. Laser radar echoes from the clear atmosphere. Nature, 203: 508

    Article  Google Scholar 

  • Deng Z Z, Zhao C S, Ma N, Ran L, Zhou G Q, Lu D R, Zhou X J. 2013. An examination of parameterizations for the CCN number concentration based on in situ measurements of aerosol activation properties in the North China Plain. Atmos Chem Phys, 13: 6227–6237

    Article  Google Scholar 

  • Fan H, Zhao C, Yang Y. 2020. A comprehensive analysis of the spatio-temporal variation of urban air pollution in China during 2014–2018. Atmos Environ, 220: 117066

    Article  Google Scholar 

  • Guo B, Wang Y, Zhang X, Che H, Zhong J, Chu Y, Cheng L. 2020. Temporal and spatial variations of haze and fog and the characteristics of PM2.5 during heavy pollution episodes in China from 2013 to 2018. Atmos Pollution Res, 11: 1847–1856

    Article  Google Scholar 

  • Guo L J, Guo X L, Fang C G, Zhu S C. 2014. Observation analysis on characteristics of formation, evolution and transition of a long-lasting severe fog and haze episode in North China. Sci China Earth Sci, 58: 329–344

    Article  Google Scholar 

  • Han X, Zhang M, Zhu L, Skorokhod A. 2016. Assessment of the impact of emissions reductions on air quality over North China Plain. Atmos Pollution Res, 7: 249–259

    Article  Google Scholar 

  • Jin X, Cai X, Yu M, Song Y, Wang X, Kang L, Zhang H. 2020. Diagnostic analysis of wintertime PM2.5 pollution in the North China Plain: The impacts of regional transport and atmospheric boundary layer variation. Atmos Environ, 224: 117346

    Article  Google Scholar 

  • Jin X, Cai X, Yu M, Wang X, Song Y, Kang L, Zhang H, Zhu T. 2021. Mesoscale structure of the atmospheric boundary layer and its impact on regional air pollution: A case study. Atmos Environ, 258: 118511

    Article  Google Scholar 

  • Jin X, Cai X, Yu M, Song Y, Wang X, Zhang H, Zhu T. 2022a. Regional PM2.5 pollution confined by atmospheric internal boundaries in the North China Plain: Boundary layer structures and numerical simulation. Atmos Chem Phys, 22: 11409–11427

    Article  Google Scholar 

  • Jin X, Cai X, Yu M, Wang X, Song Y, Wang X, Zhang H, Zhu T. 2022b. Regional PM2.5 pollution confined by atmospheric internal boundaries in the North China Plain: Analysis based on surface observations. Sci Total Environ, 841: 156728

    Article  Google Scholar 

  • Kanawade V P, Srivastava A K, Ram K, Asmi E, Vakkari V, Soni V K, Varaprasad V, Sarangi C. 2020. What caused severe air pollution episode of November 2016 in New Delhi? Atmos Environ, 222: 117125

    Article  Google Scholar 

  • Lee J, Hong J W, Lee K, Hong J, Velasco E, Lim Y J, Lee J B, Nam K, Park J. 2019. Ceilometer monitoring of boundary-layer height and its application in evaluating the dilution effect on air pollution. Bound-Layer Meteorol, 172: 435–455

    Article  Google Scholar 

  • Li G, Su H, Ma N, Tao J, Kuang Y, Wang Q, Hong J, Zhang Y, Kuhn U, Zhang S, Pan X, Lu N, Tang M, Zheng G, Wang Z, Gao Y, Cheng P, Xu W, Zhou G, Zhao C, Yuan B, Shao M, Ding A, Zhang Q, Fu P, Sun Y, Pöschl U, Cheng Y. 2021. Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN): Integrated analysis and intensive winter campaign 2018. Farad Discuss, 226: 207–222

    Article  Google Scholar 

  • Li J, Sun J, Zhou M, Cheng Z, Li Q, Cao X, Zhang J. 2018. Observational analyses of dramatic developments of a severe air pollution event in the Beijing area. Atmos Chem Phys, 18: 3919–3935

    Article  Google Scholar 

  • Li Q H. 2022. Effects of the spatial structure of the atmospheric boundary layer on haze pollution in the North China Plain: An experimental study (in Chinese). Doctoral Dissertation. Beijing: Peking University

    Google Scholar 

  • Li Q H, Wu B G, Liu J L, Zhang H, Cai X H, Song Y. 2020. Characteristics of the atmospheric boundary layer and its relation with PM2.5 during haze episodes in winter in the North China Plain. Atmos Environ, 223: 117265

    Article  Google Scholar 

  • Li Q, Zhang H, Cai X, Song Y, Zhu T. 2021. The impacts of the atmospheric boundary layer on regional haze in North China. NPJ Clim Atmos Sci, 4: 9

    Article  Google Scholar 

  • Li Q, Zhang H, Jin X, Cai X, Song Y. 2022. Mechanism of haze pollution in summer and its difference with winter in the North China Plain. Sci Total Environ, 806: 150625

    Article  Google Scholar 

  • Li R, Li Z, Gao W, Ding W, Xu Q, Song X. 2015. Diurnal, seasonal, and spatial variation of PM2.5 in Beijing. Sci Bull, 60: 387–395

    Article  Google Scholar 

  • Li X, Hu X M, Ma Y, Wang Y, Li L, Zhao Z. 2019. Impact of planetary boundary layer structure on the formation and evolution of air-pollution episodes in Shenyang, Northeast China. Atmos Environ, 214: 116850

    Article  Google Scholar 

  • Liang X, Miao S, Li J, Bornstein R, Zhang X, Gao Y, Chen F, Cao X, Cheng Z, Clements C, Dabberdt W, Ding A, Ding D, Dou J J, Dou J X, Dou Y, Grimmond C S B, González-Cruz J E, He J, Huang M, Huang X, Ju S, Li Q, Niyogi D, Quan J, Sun J, Sun J Z, Yu M, Zhang J, Zhang Y, Zhao X, Zheng Z, Zhou M. 2018. SURF: Understanding and predicting urban convection and haze. Bull Am Meteorol Soc, 99: 1391–1413

    Article  Google Scholar 

  • Liu C, Huang J, Wang Y, Tao X, Hu C, Deng L, Xu J, Xiao H W, Luo L, Xiao H Y, Xiao W. 2020. Vertical distribution of PM2.5 and interactions with the atmospheric boundary layer during the development stage of a heavy haze pollution event. Sci Total Environ, 704: 135329

    Article  Google Scholar 

  • Liu J, Fan S J, Wu D, Wu M, Liao Z H, Li H W. 2015. Boundary layer characteristics of typical haze process in the Pearl River Delta region (in Chinese). China Environ Sci, 35: 1664–1674

    Google Scholar 

  • Liu Q, Quan J, Jia X, Sun Z, Li X, Gao Y, Liu Y. 2019. Vertical profiles of aerosol composition over Beijing, China: Analysis of in situ aircraft measurements. J Atmos Sci, 76: 231–245

    Article  Google Scholar 

  • Liu X, Zhang Y, Jung J, Gu J, Li Y, Guo S, Chang S Y, Yue D, Lin P, Kim Y J, Hu M, Zeng L, Zhu T. 2009. Research on the hygroscopic properties of aerosols by measurement and modeling during CAREBeijing-2006. J Geophys Res, 114: D00G16

    Article  Google Scholar 

  • Ma N, Zhao C S, Nowak A, Müller T, Pfeifer S, Cheng Y F, Deng Z Z, Liu P F, Xu W Y, Ran L, Yan P, Göbel T, Hallbauer E, Mildenberger K, Henning S, Yu J, Chen L L, Zhou X J, Stratmann F, Wiedensohler A. 2011. Aerosol optical properties in the North China Plain during HaChi campaign: An in-situ optical closure study. Atmos Chem Phys, 11: 5959–5973

    Article  Google Scholar 

  • Miao Y, Liu S, Sheng L, Huang S, Li J. 2019. Influence of boundary layer structure and low-level jet on PM2.5 pollution in Beijing: A case study. Int J Environ Res Public Health, 16: 616

    Article  Google Scholar 

  • Pan L, Xu J, Tie X, Mao X, Gao W, Chang L. 2019. Long-term measurements of planetary boundary layer height and interactions with PM2.5 in Shanghai, China. Atmos Pollut Res, 10: 989–996

    Article  Google Scholar 

  • Peng H Q, Liu D Y, Zhao B, Su Y, Wu J M, Shen H, Wei J S, Cao L. 2016. Boundary-layer characteristics of persistent regional haze events and heavy haze days in eastern China. Adv Meteorol, 2016: 6950154, doi: https://doi.org/10.1155/2016/6950154

    Google Scholar 

  • Qu Y, An J, Li J, Chen Y, Li Y, Liu X, Hu M. 2014. Effects of NOx and VOCs from five emission sources on summer surface O3 over the Beijing-Tianjin-Hebei region. Adv Atmos Sci, 31: 787–800

    Article  Google Scholar 

  • Quan J, Dou Y, Zhao X, Liu Q, Sun Z, Pan Y, Jia X, Cheng Z, Ma P, Su J, Xin J, Liu Y. 2020. Regional atmospheric pollutant transport mechanisms over the North China Plain driven by topography and planetary boundary layer processes. Atmos Environ, 221: 117098

    Article  Google Scholar 

  • Ran L, Zhao C S, Xu W Y, Lu X Q, Han M, Lin W L, Yan P, Xu X B, Deng Z Z, Ma N, Liu P F, Yu J, Liang W D, Chen L L. 2011. VOC reactivity and its effect on ozone production during the HaChi summer campaign. Atmos Chem Phys, 11: 4657–4667

    Article  Google Scholar 

  • Ren Y, Zhang H, Wei W, Wu B, Liu J, Cai X, Song Y. 2019a. Comparison of the turbulence structure during light and heavy haze pollution episodes. Atmos Res, 230: 104645

    Article  Google Scholar 

  • Ren Y, Zhang H, Wei W, Wu B, Cai X, Song Y. 2019b. Effects of turbulence structure and urbanization on the heavy haze pollution process. Atmos Chem Phys, 19: 1041–1057

    Article  Google Scholar 

  • Ren Y, Zhang H, Wei W, Cai X, Song Y. 2020. Determining the fluctuation of PM2.5 mass concentration and its applicability to Monin-Obukhov similarity. Sci Total Environ, 710: 136398

    Article  Google Scholar 

  • Ren Y, Zhang H, Zhang X, Li Q, Cai X, Song Y, Kang L, Zhu T. 2021. Temporal and spatial characteristics of turbulent transfer and diffusion coefficient of PM2.5. Sci Total Environ, 782: 146804

    Article  Google Scholar 

  • Shi Y, Hu F, Fan G, Zhang Z. 2019. Multiple technical observations of the atmospheric boundary layer structure of a red-alert haze episode in Beijing. Atmos Meas Tech, 12: 4887–4901

    Article  Google Scholar 

  • Shi Y, Hu F, Xiao Z, Fan G, Zhang Z. 2020. Comparison of four different types of planetary boundary layer heights during a haze episode in Beijing. Sci Total Environ, 711: 134928

    Article  Google Scholar 

  • Su T, Li Z, Kahn R. 2020. A new method to retrieve the diurnal variability of planetary boundary layer height from lidar under different thermodynamic stability conditions. Remote Sens Environ, 237: 111519

    Article  Google Scholar 

  • Silcox G D, Kelly K E, Crosman E T, Whiteman C D, Allen B L. 2011. Wintertime PM2.5 concentrations during persistent, multi-day cold-air pools in a mountain valley. Atmos Environ, 46: 17–24

    Google Scholar 

  • Sun H, Shi Y, Liu L, Ding W, Zhang Z, Hu F. 2021. Impacts of atmospheric boundary layer vertical structure on haze pollution observed by tethered balloon and lidar. J Meteorol Res, 35: 209–223

    Article  Google Scholar 

  • van Pinxteren D, Brüggemann E, Gnauk T, Iinuma Y, Müller K, Nowak A, Achtert P, Wiedensohler A, Herrmann H. 2009. Size- and time-resolved chemical particle characterization during CAREBeijing-2006: Different pollution regimes and diurnal profiles. J Geophys Res, 114: D00G09

    Article  Google Scholar 

  • Wang L, Gao Z, Miao S, Guo X, Sun T, Liu M, Li D. 2015. Contrasting characteristics of the surface energy balance between the urban and rural areas of Beijing. Adv Atmos Sci, 32: 505–514

    Article  Google Scholar 

  • Wang Z, Cao X, Zhang L, Notholt J, Zhou B, Liu R, Zhang B. 2012. Lidar measurement of planetary boundary layer height and comparison with microwave profiling radiometer observation. Atmos Meas Tech, 5: 1965–1972

    Article  Google Scholar 

  • Wang Z F, Li J, Wang Z, Yang W Y, Tang X, Ge B Z, Yan P Z, Zhu L L, Chen X S, Chen H S, Wand W, Li J J, Liu B, Wang X Y, Wand W, Zhao Y L, Lu N, Su D B. 2014. Modeling study of regional severe hazes over mid-eastern China in January 2013 and its implications on pollution prevention and control. Sci China Earth Sci, 57: 3–13

    Article  Google Scholar 

  • Wei W, Zhang H, Wu B, Huang Y, Cai X, Song Y, Li J. 2018. Intermittent turbulence contributes to vertical dispersion of PM2.5 in the North China Plain: Cases from Tianjin. Atmos Chem Phys, 18: 12953–12967

    Article  Google Scholar 

  • Wei W, Zhang H, Cai X, Song Y, Bian Y, Xiao K, Zhang H. 2020. Influence of intermittent turbulence on air pollution and its dispersion in winter 2016/2017 over Beijing, China. J Meteorol Res, 34: 176–188

    Article  Google Scholar 

  • Whiteman C D, Bian X, Zhong S. 1999. Wintertime evolution of the temperature inversion in the Colorado Plateau Basin. J Appl Meteor, 38: 1103–1117

    Article  Google Scholar 

  • Wu Z, Wang Y, Tan T, Zhu Y, Li M, Shang D, Wang H, Lu K, Guo S, Zeng L, Zhang Y. 2018. Aerosol liquid water driven by anthropogenic inorganic salts: Implying its key role in haze formation in the North China Plain. Environ Sci Technol Lett, 5: 160–166

    Article  Google Scholar 

  • Xu T, Song Y, Zhang M, Liu M, Cai X, Zhang H, Tao Z, Pan Y, Zhu T. 2020. Investigation of the atmospheric boundary layer during an unexpected summertime persistent severe haze pollution period in Beijing. Meteorol Atmos Phys, 132: 71–84

    Article  Google Scholar 

  • Xu X D, Ding G A, Bian L G. 2006. Beijing city air pollution observation experiment (in Chinese). J Appl Meteorol Sci, 17: 815–828

    Google Scholar 

  • Xu X D, Ding G A, Bian L G, Xie L A. 2004. Characteristics of atmospheric environment of boundary layer structure of city community in BECAPEX and integrate influence (in Chinese). Acta Meteorol Sin, 62: 663–672

    Google Scholar 

  • Yang Y, Liu X, Qu Y, Wang J, An J, Zhang Y, Zhang F. 2015. Formation mechanism of continuous extreme haze episodes in the megacity Beijing, China, in January 2013. Atmos Res, 155: 192–203

    Article  Google Scholar 

  • Ye X, Song Y, Cai X, Zhang H. 2016. Study on the synoptic flow patterns and boundary layer process of the severe haze events over the North China Plain in January 2013. Atmos Environ, 124: 129–145

    Article  Google Scholar 

  • Yu M, Cai X, Xu C, Song Y. 2019. A climatological study of air pollution potential in China. Theor Appl Climatol, 136: 627–638

    Article  Google Scholar 

  • Zhang Q, Quan J, Tie X, Li X, Liu Q, Gao Y, Zhao D. 2015. Effects of meteorology and secondary particle formation on visibility during heavy haze events in Beijing, China. Sci Total Environ, 502: 578–584

    Article  Google Scholar 

  • Zhong J, Zhang X, Wang Y, Sun J, Zhang Y, Wang J, Tan K, Shen X, Che H, Zhang L, Zhang Z, Qi X, Zhao H, Ren S, Li Y. 2017. Relative contributions of boundary-layer meteorological factors to the explosive growth of PM2.5 during the red-alert heavy pollution episodes in Beijing in December 2016. J Meteorol Res, 31: 809–819

    Article  Google Scholar 

  • Zhong J, Zhang X, Dong Y, Wang Y, Liu C, Wang J, Zhang Y, Che H. 2018a. Feedback effects of boundary-layer meteorological factors on cumulative explosive growth of PM2.5 during winter heavy pollution episodes in Beijing from 2013 to 2016. Atmos Chem Phys, 18: 247–258

    Article  Google Scholar 

  • Zhong J, Zhang X, Wang Y, Liu C, Dong Y. 2018b. Heavy aerosol pollution episodes in winter Beijing enhanced by radiative cooling effects of aerosols. Atmos Res, 209: 59–64

    Article  Google Scholar 

  • Zou Q Q, Cai X H, Guo M T, Song Y, Zhang X L. 2018. Long-term mean footprint and its relationship to heavy air pollution episodes in Beijing (in Chinese). Acta Sci Nat Univ Pekin, 54: 341–349

    Google Scholar 

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Acknowledgements

We sincerely thank the three anonymous reviewers for their constructive comments on the early version of the manuscript. This work was supported by the National Natural Science Foundation of China (Grant Nos. 42090031, 42175092, 92044301 & 91544216), the National Key R&D Program of China (Grant Nos. 2016YFC0203306 & 2017YFC0209600) and the National Research Program for Key Issues in Air Pollution Control (Grant Nos. DQGG0104 & DQGG0106).

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Authors and Affiliations

  1. Tianjin Key Laboratory for Oceanic Meteorology, Tianjin Meteorological Service Center, Tianjin Meteorological Bureau, Tianjin, 300074, China

    Qianhui Li

  2. Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, 100871, China

    Qianhui Li, Hongsheng Zhang & Lu Zhang

  3. State Key Laboratory of Severe Weather and Key Laboratory of Atmospheric Chemistry of CMA, Chinese Academy of Meteorological Sciences, China Meteorological Administration (CMA), Beijing, 100081, China

    Xiaoye Zhang

  4. State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

    Xuhui Cai, Xipeng Jin, Yu Song, Ling Kang & Tong Zhu

  5. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China

    Fei Hu

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  1. Qianhui Li
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  2. Hongsheng Zhang
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Li, Q., Zhang, H., Zhang, X. et al. COATS: Comprehensive observation on the atmospheric boundary layer three-dimensional structure during haze pollution in the North China Plain. Sci. China Earth Sci. 66, 939–958 (2023). https://doi.org/10.1007/s11430-022-1092-y

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