A Three-Year Perspective of Air Quality During COVID-19 Lockdown in The United States: Changes in Nitrogen Oxides, Particulate Matter and Ozone

Abstract:

Air quality policy minimizes the effects of air pollution. Continuous air quality monitors help quantify how air pollution changes over time. Events such as the COVID-19 pandemic offer a view of the air quality changes over a very short time during which major polluting activities (i.e., vehicle traffic, industry, etc.) were curtailed. In this study, we focused on four air pollutants (nitrogen dioxide (NO₂), nitrogen oxide (NO), particulate matter with diameter smaller than 2.5 μm (PM_2.5) and ozone (O₃)) to study air quality changes during the COVID-lockdown in three major cities in the United States (Seattle, Houston and New York) over a three-year period from March to May (2018-2020). We used daily averages and 1-h averages from one or two monitoring sites in each city to quantify changes in mixing ratios and concentrations of each pollutant. We found that the 3-month daily NO_x (except for NO in New York) decreased from March to May in 2020 compared to 2018-2019, on average by 40-55% (NO) and 26-29% (NO₂). Daily O₃ increased by 2% (New York) or did not change significantly (Seattle and Houston), while daily PM_2.5 decreased by 26-29% (Seattle and New York) or showed no change (Houston). Over the 3-month span, diurnal patterns of 1-h averages show that daytime (7 am to 6 pm) NO_x decreased in all cities by 34-44% (NO) and 25-34% (NO₂), daytime PM_2.5 also decreased by 29-32% (Seattle and New York) or did not change (Houston), while daytime O₃ did not change significantly. Decreases of nighttime (7 pm to 6 am) NO_x and PM_2.5 were also observed in all cities (except for NO in New York and PM_2.5 in Houston), with ranges of 46-55% (NO), 24-26% (NO₂) and 18-23% (PM_2.5). Nighttime O₃ increased in New York by 7%, while it did not change significantly in Seattle and Houston. Overall, decreases of primary pollutants (NO and NO₂) and secondary pollutants (PM_2.5) are consistent with the expected air quality responses to COVID-lockdown driven reductions in pollutant emissions.