Large discrepancy between observed and modeled wintertimetropospheric NO2 variabilities due to COVID-19 controls in China

Recent studies demonstrated the difficulties to explain observed tropospheric nitrogen dioxide (NO₂) variabilities over the United States and Europe, but thorough analysis for the impacts on tropospheric NO₂ in China is still lacking. Here we provide a comparative analysis for the observed and modeled (Goddard Earth Observing System-Chem) tropospheric NO₂ in early 2020 in China. Both ozone monitoring instrument and surface NO₂ measurements show marked decreases in NO₂ abundances due to the 2019 novel coronavirus (COVID-19) controls. However, we find a large discrepancy between observed and modeled NO₂ changes over highly polluted provinces: the observed reductions in tropospheric NO₂ columns are about 40% lower than those in surface NO₂ concentrations. By contrast, the modeled reductions in tropospheric NO₂ columns are about two times higher than those in surface NO₂ concentrations. This discrepancy could be driven by the combined effects from uncertainties in simulations and observations, associated with possible inaccurate simulations of lower tropospheric NO₂, larger uncertainties in the modeled interannual variabilities of NO₂ columns, as well as insufficient consideration of aerosol effects and a priori NO₂ variability in satellite retrievals. In addition, our analysis suggests a small influence from free tropospheric NO₂ backgrounds in E. China in winter. This work demonstrates the challenge to interpret wintertime tropospheric NO₂ changes in China, highlighting the importance of integrating surface NO₂ observations to provide better analysis for NO₂ variabilities.