Global Impact of COVID-19 Restrictions on the Atmospheric Concentrations of Nitrogen Dioxide and Ozone (2008.01127v1)

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 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.