Quantifying urban, industrial, and background changes in NO 2 during the COVID-19 lockdown period based on TROPOMI satellite observations

Abstract. The COVID-19 lockdown had a large impact on anthropogenic emissions of air pollutants and particularly on nitrogen dioxide (NO2). While the overall NO2 decline over some large cities is well-established, its quantification remains a challenge because of a variety of sources of NO2. In this study, a new method of isolation of three components: background NO2, NO2 from urban sources, and from industrial point sources is applied to estimate the COVID-19 lockdown impact on each of them. The approach is based on fitting satellite data by a statistical model with empirical plume dispersion functions driven by the observed winds. Population density and surface elevation data as well as coordinates of industrial sources were used in the analysis. The NO2 vertical column density (VCD) values measured by Tropospheric Monitoring Instrument (TROPOMI) on board Sentinel‐5 Precursor over 263 urban areas for the period from March 16 to June 15, 2020, were compared with the average VCD values for the same period in 2018 and 2019. While background NO2 component remained almost unchanged, the urban NO2 component declined by 18–28 % over most regions. India, South America, and a part of Europe (particularly, Italy, France, and Spain) demonstrated a 40–50 % urban emissions decline. In contrast, decline over urban area in China, where the lockdown was over during the analyzed period, was only 3 % except for Wuhan, where more than 60 % decline was observed. Emissions from large industrial sources in the analyzed urban areas varies largely from region to region from +5 % for China to −40 % for India. Changes in urban emissions are correlated with changes in Google mobility data (the correlation coefficient is 0.66) confirming that changes in traffic was one of the key elements in decline of urban NO2 emissions. No correlation was found between changes in background NO2 and Google mobility data.