These patches of color represent a range of micromoles per square meter of tropospheric NO2 change over primarily urban regions in the continental United States. Photo credit: Kondragunta et al., 2021

In the spring of 2020, COVID-19-related restrictions in the US resulted in reductions in nitrogen oxide (NOx) emissions due to the sudden drop in road traffic. But even after the restrictions were lifted and traffic flow resumed, NOx emissions from traffic remained around 20% lower in some cities through the end of 2020 than the pre-COVID-19 average.

The continuing decline in air pollution could be explained by an increase in teleworking, according to a new study in the Journal of Geophysical Research: Atmospheres.

In the atmosphere, NOx converts to nitrogen dioxide (NO2), a pollutant that can cause respiratory diseases. Using satellite-collected NO2 data, researchers tracked the decrease and increase in traffic-related air pollution in five US urban areas from February to November 2020: Atlanta, New York City, San Francisco, Los Angeles and the San Joaquin Valley in California. In the latter four regions, air pollution did not return to pre-lockdown levels even after people resumed normal activities in the summer and remained mobile until the fall. Instead, NO2 levels rose to around 80% of their level in February and then stayed on a plateau. Atlanta had a delayed spike in NO2 but returned to pre-lockdown NO2 levels in August 2020.

“Even though the bans were lifted, 25% of the workforce was still working remotely. So if employers continue to offer guidelines for remote working, it could become the new normal in air quality, ”said Shobha Kondragunta, a physicist at the NOAA Center for Satellite Applications and Research, who was the lead author of the study.

When they applied their model to large urban areas in the continental United States, the researchers found a decrease in NO2 concentrations in most metropolitan areas from February to November 2020 compared to the same period in 2019.

The satellites used in the study changed their algorithms in December, so the team could not continue the analysis for December and 2021. But, said Kondragunta, “even in 2021 we will still be below pre-pandemic NOx emissions.”

Kondragunta et al. found a correlation between the national unemployment rate and NO2 pollution in the second fiscal quarter of 2020 (April to June), which they believe could be due to a decline in both passenger and freight traffic.

“Remote working may explain the decline in passenger traffic, but freight is more economically driven. The data shows a bigger drop in passenger traffic than cargo traffic – people still order goods when they work at home, right? ”Said Brian McDonald, an atmospheric scientist at NOAA’s Chemical Sciences Laboratory who co-authored the study.

The changes in air pollution caused by COVID-19 lockdowns varied from region to region and have not yet been fully clarified. The model showed that some cities, such as Seattle and the suburbs of Chicago, had smaller increases in NO2 that exceeded pre-lockdown levels during the study period, as did some rural areas. That increase could be due to the normal background variability of NO2, McDonald said. Understanding regional patterns of air pollution and looking at pollutants other than NO2 “is ultimately the goal. We want to know how air quality has changed in response to lockdowns, and this study is part of that. ”

Air pollution returning to pre-COVID levels

More information:
S. Kondragunta et al., COVID ‐ 19 Induced Fingerprints of a New Normal Urban Air Quality in the United States, Journal of Geophysical Research: Atmospheres (2021). DOI: 10.1029 / 2021JD034797

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