Changes in air quality and human mobility in the U.S. during the COVID-19 pandemic

Abstract

The first goal of this study is to quantify the magnitude and spatial variability of air quality changes in the US during the COVID-19 pandemic. We focus on two federally regulated pollutants, nitrogen dioxide (NO2), and fine particulate matter (PM2.5). Observed concentrations at all available ground monitoring sites (240 and 480 for NO2 and PM2.5, respectively) were compared between April 2020 and April of the prior five years, 2015-2019, as the baseline. Large statistically significant decreases in NO2 concentrations were found at more than 65% of the monitoring sites, with an average drop of 2 ppb when compared to the mean of the previous five years. The same patterns are confirmed by satellite-derived NO2 column totals from NASA OMI. PM2.5 concentrations from the ground monitoring sites, however, were more likely to be higher. The second goal of this study is to explain the different responses of the two pollutants during the COVID-19 pandemic. The hypothesis put forward is that the shelter-in-place measures affected peoples' driving patterns most dramatically, thus passenger vehicle NO2 emissions were reduced. Commercial vehicles and electricity demand for all purposes remained relatively unchanged, thus PM2.5 concentrations did not drop significantly. To establish a correlation between the observed NO2 changes and the extent to which people were sheltering in place, we use a mobility index, which was produced and made public by Descartes Labs. This mobility index aggregates cell phone usage at the county level to capture changes in human movement over time. We found a strong correlation between the observed decreases in NO2 concentrations and decreases in human mobility. By contrast, no discernible pattern was detected between mobility and PM2.5 concentrations changes, suggesting that decreases in personal-vehicle traffic alone may not be effective at reducing PM2.5 pollution.