Global impact of COVID-19 restrictions on the surface concentrations of nitrogen dioxide and ozone

Christoph A. Keller*, Mathew J. Evans, K. Emma Knowland, Christa A. Hasenkopf, Sruti Modekurty, Robert A. Lucchesi, Tomohiro Oda, Bruno B. Franca, Felipe C. Mandarino, M. Valeria Díaz Suárez, Robert G. Ryan, Luke H. Fakes, Steven Pawson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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 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 5756 observation sites in 46 countries from January through June 2020. Reductions in NO2 coincide with the 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). On average, NO2 concentrations were 18 (13-23) % lower than business as usual from February 2020 onward. China experienced the earliest and steepest decline, but concentrations since April have mostly recovered and remained within 5 % of the business-as-usual estimate. NO2 reductions in Europe and the US have been more gradual, with a halting recovery starting in late March. We estimate that the global NOx (NO + NO2) emission reduction during the first 6 months of 2020 amounted to 3.1 (2.6-3.6) TgN, equivalent to 5.5 (4.7-6.4) % of the annual anthropogenic total. The response of surface O3 is complicated by competing influences of nonlinear atmospheric chemistry. While surface O3 increased by up to 50 % in some locations, we find the overall net impact on daily average O3 between February-June 2020 to be small. However, our analysis indicates a flattening of the O3 diurnal cycle with an increase in nighttime ozone due to reduced titration and a decrease in daytime ozone, reflecting a reduction in photochemical production. The O3 response is dependent on season, timescale, and environment, with declines in surface O3 forecasted if NOx emission reductions continue.

Original languageEnglish
Pages (from-to)3555-3592
Number of pages38
JournalAtmospheric Chemistry and Physics
Issue number5
Publication statusPublished - 9 Mar 2021

Bibliographical note

Funding Information:
Acknowledgements. Resources supporting the model simulations were provided by the NASA Center for Climate Simulation at the Goddard Space Flight Center ( services/discover, last access: 7 July 2020). We thank Jenny Fisher (University of Wollongong, Australia) for helpful discussions. Mathew J. Evans and Luke H. Fakes are thankful for support from the University of York’s Viking HPC facility.

Funding Information:
Financial support. This research has been supported by the NASA Modeling, Analysis and Prediction (MAP) Program (16-MAP16-0025).

Publisher Copyright:
© 2021 BMJ Publishing Group. All rights reserved.

Copyright 2021 Elsevier B.V., All rights reserved.

Cite this