Trends in the Reduction of Nitrogen Dioxide Emissions: Effect of 2020 SARS-CoV-2 Pandemic Versus Emission Reduction Regulations

Nitrogen Dioxide is primarily emitted to the atmosphere through the combustion of fossil fuels, and the measurement of its emission serves as the indicator for the emission of the group of nitrogen oxides (NOx), which have adverse effects on health, environment, and climate. Over the past few decades, countries around the world have enacted policies to reduce the emission of air pollutants and greenhouse gasses. In 2019, NOx concentration in the United States was 62% lower than the 2002 levels. Therefore, when studying the impact of sporadic events such as COVID-19 restrictions on emissions, it is necessary to account for such ongoing reductions. Using Sentinel-5P satellite and twenty-six roadside ground monitors, we studied the change of NO2 concentration in the US Northeast megapolis. Using the National Emission Inventory (NEI) report on total NOx emissions (stationary and mobile) and the US EPA Motor Vehicle Emission Simulator (MOVES), we quantified the reduction of total NOx emissions and those from mobile sources for the study region. We find that the pandemic resulted in a 3.4% reduction of total NOx emissions. This is compared to (and in addition to) the expected 8.2% reduction, which has been persistently occurring over the past decade as the result of the Clean Air Act, and which would have taken place even in the absence of the pandemic. When considering NOx emissions from mobile sources only, the pandemic resulted in a 5% reduction of NOx emissions, compared to (and in addition to) the expected 10.9% policy-driven reduction. Using mobility data from the Apple Corporation and roadside vehicle count sensors, we find that the volume of freight traffic, the principal source of mobile emissions, reduced considerably less than automobile traffic volume during the pandemic restrictions. We believe this is the primary contributing factor to the relatively low pandemic-related reduction of NO2 emissions.