Puzzling Haze Events in China During the Coronavirus (COVID‐19) Shutdown

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  • Yunhua Chang
    KLME & CIC‐FEMD, Yale‐NUIST Center on Atmospheric Environment Nanjing University of Information Science and Technology Nanjing China
  • Ru‐Jin Huang
    State Key Laboratory of Loess and Quaternary Geology, Center for Excellence in Quaternary Science and Global Change, and Key Laboratory of Aerosol Chemistry and Physics Institute of Earth and Environment, Chinese Academy of Sciences Xi'an China
  • Xinlei Ge
    Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering Nanjing University of Information Science and Technology Nanjing China
  • Xiangpeng Huang
    Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering Nanjing University of Information Science and Technology Nanjing China
  • Jianlin Hu
    Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering Nanjing University of Information Science and Technology Nanjing China
  • Yusen Duan
    Shanghai Environmental Monitoring Center Shanghai China
  • Zhong Zou
    Department of Environmental Science and Engineering Fudan University Shanghai China
  • Xuejun Liu
    College of Resources and Environmental Sciences China Agricultural University Beijing China
  • Moritz F. Lehmann
    Department of Environmental Sciences University of Basel Basel Switzerland

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<jats:title>Abstract</jats:title><jats:p>It is a puzzle as to why more severe haze formed during the New Year Holiday in 2020 (NYH‐20), when China was in an unprecedented state of shutdown to contain the coronavirus (COVID‐19) outbreak, than in 2019 (NYH‐19). We performed a comprehensive measurement and modeling analysis of the aerosol chemistry and physics at multiple sites in China (mainly in Shanghai) before, during, and after NYH‐19 and NYH‐20. Much higher secondary aerosol fraction in PM<jats:sub>2.5</jats:sub> were observed during NYH‐20 (73%) than during NYH‐19 (59%). During NYH‐20, PM<jats:sub>2.5</jats:sub> levels correlated significantly with the oxidation ratio of nitrogen (<jats:italic>r</jats:italic><jats:sup>2</jats:sup> = 0.77, <jats:italic>p</jats:italic> < 0.01), and aged particles from northern China were found to impede atmospheric new particle formation and growth in Shanghai. A markedly enhanced efficiency of nitrate aerosol formation was observed along the transport pathways during NYH‐20, despite the overall low atmospheric NO<jats:sub>2</jats:sub> levels.</jats:p>

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