Global Impact of COVID-19 Restrictions on the Surface Concentrations of Nitrogen Dioxide and Ozone Christoph A. Keller1,2, Mat. J. Evans3,4, K. Emma Knowland1,2, Christa A. Hasenkopf5, Sruti Modekurty5, Robert A. Lucchesi1,6, Tomohiro Oda1,2, Bruno B. Franca7, Felipe C. Mandarino7, M. 5 Valeria Díaz Suárez8, Robert G. Ryan9, Luke H. Fakes3,4, Steven Pawson1 1NASA Global Modeling and Assimilation Office, Goddard Space Flight Center, Greenbelt, MD, USA 2Universities Space Research Association, Columbia, MD, USA 3Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, 10 YO10 5DD, UK 4National Centre for Atmospheric Science, University of York, York, YO10 5DD, UK 5OpenAQ, Washington, DC, USA 6Science Systems and Applications, Inc., Lanham, MD, USA 7Municipal Government of Rio de Janeiro, Rio de Janeiro, Brazil 15 8Secretaria de Ambiente, Quito, Ecuador 9School of Earth Sciences, The University of Melbourne, Australia Correspondence to: Christoph A. Keller (
[email protected]) Abstract. Social-distancing to combat the COVID-19 pandemic has led to widespread reductions in air pollutant emissions. Quantifying these changes requires a business-as-usual counterfactual that accounts 20 for the synoptic and seasonal variability of air pollutants. We use a machine learning algorithm driven by information from the NASA GEOS-CF model to assess changes in nitrogen dioxide (NO2) and ozone (O3) at 5,756 observation sites in 46 countries from January through June 2020. Reductions in NO2 coincide with timing and intensity of COVID-19 restrictions, ranging from 60% in severely affected cities (e.g., Wuhan, Milan) to little change (e.g., Rio de Janeiro, Taipei).