Analysis and Attribution of the Decrease in Tropospheric Column NO2 as Observed by TROPOMI Over the Mid-Atlantic During the COVID-19 Pandemic

Abstract:

Nitrogen Dioxide (NO₂) is an air pollutant that is an important precursor to ozone (O₃) formation. Both species are regulated by the United States Environmental Protection Agency through the National Ambient Air Quality Standards (NAAQS). During the COVID-19 pandemic, the Mid-Atlantic region experienced decreases in tropospheric column NO₂, as observed by TROPOMI. From March - July, high population centers and urban corridors experienced larger decreases (40-50%) than more rural areas (30-40%) compared to the same time period in 2019. It is believed that government actions taken to slow the spread of the virus account for at least some of this decline in column NO₂. Quantifying the root cause of this change requires a multifaceted approach that can account for long-term changes and meteorology. In this study, we use satellite observations, traffic counts, measurements of air pollutants from continuous emission monitoring systems installed on electrical generating units (EGUs), and air quality modeling to determine the overall decreases in NO₂ emissions from these important source sectors. These empirical results are used as a basis for modifications of the anthropogenic NO_x emissions used in the Community Multiscale Air Quality model (CMAQ). CMAQ output is convolved with the TROPOMI averaging kernels and sampled using the TROPOMI quality flag to facilitate model/measurement comparison. The CMAQ model simulations allow us to quantify the relative contribution of mobile and EGU emissions sources on tropospheric column NO₂ as well as the impact from changes in meteorology. The implications for air quality policy will also be discussed.