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 U.S. during the COVID-19 pandemic. We focus on two pollutants that are federally regulated, nitrogen dioxide NO2 and fine particulate matter PM2.5. NO2 is emitted during fuel combustion by all motor vehicles and airplanes. PM2.5 is emitted by airplanes and, among motor vehicles, mostly by diesel vehicles, such as commercial heavy-duty diesel trucks. Both are also emitted by fossil-fuel power plants, although PM2.5 almost exclusively by coal power plants. Observed concentrations at all available ground monitoring sites were compared between April 2020, the month during which the majority of U.S. states had introduced some measure of social distancing, 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 parts per billion (ppb) with respect to the baseline. The same patterns are confirmed by satellite-derived NO2 column totals from NASA OMI. PM2.5 concentrations from the ground monitoring sites, however, were not significantly lower in 2020 than in the past five years and were more likely to be higher than lower in April 2020 when compared to the previous five years.

The second goal of this study is to explain the different responses of these two pollutants. The hypothesis put forward is that the shelter-in-place measures affected people's driving patterns most dramatically, thus passenger vehicle NO2 emissions were reduced. Commercial vehicles (generally diesel) 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 actually sheltering in place, we use a mobility index produced and made public by Descartes Labs.