US COVID‐19 Shutdown Demonstrates Importance of Background NO 2 in Inferring NO x Emissions From Satellite NO 2 Observations

Satellite nitrogen dioxide (NO 2 ) measurements are used extensively to infer nitrogen oxide emissions and their trends, but interpretation can be complicated by background contributions to the NO 2 column sensed from space. We use the step decrease of US anthropogenic emissions from the COVID‐19 shutdown to compare the responses of NO 2 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 NO 2 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 NO 2 column. Inspection of the long‐term OMI trend over remote US regions shows a rising summertime NO 2 background from 2010 to 2019 potentially attributable to wildfires.