Science Inventory

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

Citation:

Qu, Z., D. Jacob, R. Silvern, V. Shah, P. Campbell, L. Valin, AND L. Murray. US COVID-19 Shutdown Demonstrates Importance of Background NO2 in Inferring NOx Emissions From Satellite NO2 Observations. GEOPHYSICAL RESEARCH LETTERS. American Geophysical Union, Washington, DC, 48(10):e2021GL092783, (2021). https://doi.org/10.1029/2021GL092783

Impact/Purpose:

Satellite measurements of tropospheric NO2 columns are used extensively to infer nitrogen oxide (NOx) emissions and trends, but interpretation can be complicated by the contribution of background sources to the NO2 column detected from space. We confirm the large impact of background NO2 through the different responses of the satellite and surface NO2 measurements to the emission reductions during the COVID-19 shutdown. These background sources should be appropriately accounted for before applying satellite-derived NOx emissions for predictions of changes in secondary pollutants (O3 and PM2.5) during the COVID-19 shutdown and for long-term NO2 trend analysis over high background regions. Understanding background NO2 may also have implications for the observed rise in global tropospheric ozone over the past decade.

Description:

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.