Changes in criteria air pollution levels in the US before, during, and after Covid-19 stay-at-home orders: evidence from regulatory monitors

The widespread and rapid social and economic changes from Covid-19 response might be expected to dramatically improve air quality. However, national monitoring data from the US Environmental Protection Agency for criteria pollutants (PM2.5, ozone, NO2, CO, PM10) provide inconsistent support for that expectation. Specifically, during stay-at-home orders, average PM2.5 levels were slightly higher (~10% of its multi-year interquartile range [IQR]) than expected; average ozone, NO2, CO, and PM10 levels were slightly lower (~30%, ~20%, ~27%, and ~1% of their IQR, respectively) than expected. The timing of peak anomaly, relative to the stay-at-home orders, varied by pollutant (ozone: 2 weeks before; NO2, CO: 3 weeks after; PM10: 2 weeks after); but, by 5–6 weeks after stay-at-home orders, the concentration anomalies appear to have ended. For PM2.5, ozone, CO, and PM10, no US state had lower-than-expected pollution levels for all weeks during stay-at-home-orders; for NO2, only Arizona had lower-than-expected levels for all weeks during stay-at-home orders. Our findings show that the enormous changes from the Covid-19 response have not lowered PM2.5 levels across the US beyond their normal range of variability; for ozone, NO2, CO, and PM10 concentrations were lowered but the reduction was modest and transient.
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Highlights • Impacts of stay-at-home orders on air pollution were evaluated using EPA monitoring data from 100s of stations across the US. • During stay-at-home orders, ozone, NO2, CO and PM10 were lower and PM2.5 were higher than expected levels by 1%-30% of their IQR. • Concentration anomalies ended only 5-6 weeks after stay-at-home orders were issued. • Ozone, NO2, and CO concentrations returned to expected levels and PM2.5 and PM10 levels were higher than expected. • Reductions in ozone, NO2, and CO levels were modest and short-lived. PM10 levels did not change and PM2.5 levels increased.