US COVID-19 Shutdown Demonstrates Importance of Background NO2 in Inferring NOx Emissions From

Satellite nitrogen dioxide (NO2) measurements are used extensively to infer nitrogen oxide emissions and their trends, but interpretation can be complicated by background contributions to the NO2 column sensed from space. We use the step decrease of US anthropogenic emissions from the COVID-19 shutdown to compare the responses of NO2 concentrations observed at surface network sites and from satellites (Ozone Monitoring Instrument [OMI], Tropospheric Ozone Monitoring Instrument [TROPOMI]). After correcting for differences in meteorology, surface NO2 measurements for 2020 show decreases of 20% in March–April and 10% in May–August compared to 2019. The satellites show much weaker responses in March–June and no decrease in July–August, consistent with a large background contribution to the NO2 column. Inspection of the long-term OMI trend over remote US regions shows a rising summertime NO2 background from 2010 to 2019 potentially attributable to wildfires. Plain Language Summary Nitrogen oxides (NOx) emitted from combustion are a major source of air pollution. Satellite observations of nitrogen dioxide (NO2) have been used to infer NOx emissions, but this inference is complicated by NO2 present in background air. Here, we show that this NO2 background results in a muted response of the satellite observations to the abrupt drop in NOx emissions from the US COVID-19 shutdown. The NO2 background over the US has increased in the past decade, masking the effect of emission decreases. Understanding this background NO2 and its rise is critical for the interpretation of satellite observations.