Global Impact of COVID-19 Restrictions on the Atmospheric Concentrations of Nitrogen Dioxide and Ozone

• Main Authors: Keller, Christoph A, Evans, Mat. J, Knowland, K. Emma, Hasenkopf, Christa A, Modekurty, Sruti, Lucchesi, Robert A, Oda, Tomohiro, Franca, Bruno B, Mandarino, Felipe C, Suárez, M. Valeria Díaz, Ryan, Robert G, Fakes, Luke H, Pawson, Steven
• Format: Journal Article
• Language: English
• Published: 03-08-2020
• Subjects: Physics - Atmospheric and Oceanic Physics; Physics - Physics and Society
• DOI: 10.48550/arxiv.2008.01127

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
(NO$_{2}$) and ozone (O$_{3}$) at 5,756 observation sites in 46 countries from
January through June 2020. Reductions in NO$_{2}$ correlate 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). On
average, NO$_{2}$ concentrations were 18% 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% to the
business as usual estimate. NO$_{2}$ reductions in Europe and the US have been
more gradual with a halting recovery starting in late March. We estimate that
the global NO$_{x}$ (NO+NO$_{2}$) emission reduction during the first 6 months
of 2020 amounted to 2.9 TgN, equivalent to 5.1% of the annual anthropogenic
total. The response of surface O$_{3}$ is complicated by competing influences
of non-linear atmospheric chemistry. While surface O$_{3}$ increased by up to
50% in some locations, we find the overall net impact on daily average O$_{3}$
between February - June 2020 to be small. However, our analysis indicates a
flattening of the O$_{3}$ diurnal cycle with an increase in night time ozone
due to reduced titration and a decrease in daytime ozone, reflecting a
reduction in photochemical production. The O$_{3}$ response is dependent on
season, time scale, and environment, with declines in surface O$_{3}$
forecasted if NO$_{x}$ emission reductions continue.