Emissions of nitrogen dioxide in the northeast U.S. during the 2020 COVID-19 lockdown

We have quantified the emissions of Nitrogen dioxide (NO(2)) in the Northeast megalopolis of the United States during the COVID-19 lockdown. The measurement of NO(2) emission serves as the indicator for the emission of the group of nitrogen oxides (NO(x)). Approximately 56% of NO(2) emissions in the US are from mobile sources, and the remainder is from stationary sources. Since 2002, clean air regulations have resulted in approximately 5% compound annual reduction of NO(x) emissions in the US (8.2% in the study area). Therefore, when studying the impact of sporadic events like an epidemic on emissions, it is necessary to account for the persistent reduction of emissions due to policy driven emission reduction measures. Using spaceborne sensors, ground monitors, National Emission Inventory data, and the US Motor Vehicle Emission Simulator, we quantified the reduction of total NO(x) emissions, distinguishing stationary sources from on-road mobile sources (trucks and automobiles). When considering total NO(x) emissions (stationary and mobile combined), we find that the pandemic restrictions resulted in 3.4% reduction of total NO(x) emissions in the study area in 2020. This is compared to (and in addition to) the expected 8.2% policy driven reduction of NO(x) emissions in 2020. This somewhat low reduction of emissions is because most stationary sources (factories, power plants, etc.) were operational during the pandemic. Truck traffic, a significant source of mobile emissions, also did not decline significantly (average 4.8% monthly truck traffic reduction in the study area between March and August 2020), as they were delivering goods during the lockdown. On the other hand, automobile traffic, responsible for 24% of total NO(x) emissions, dropped significantly, 52% in April, returning to near normal after 5 months. While the reduction of automobile traffic was significant, especially in the early months of the pandemic, its effect on emissions was relatively insignificant.