Using Near-Road Observations to Assess the Impact of COVID-19 Travel Restrictions on Air Pollution Emissions from Mobile Sources

Carbon monoxide (CO), nitrogen dioxide (NO₂), and particulate matter (PM) are criteria air pollutants regulated by the US EPA due to their hazards to human health; CO and NO_x (NO_x = NO₂ + NO) also act as precursors to ozone formation. Mobile sources emit all three pollutants, with vehicles contributing 54% of total national CO, 59% of total NO_x, and 5.4% of total primary PM_2.5 in the 2017 National Emissions Inventory compiled by the US EPA. Black carbon (BC), a carbonaceous component of PM_2.5 often called soot, absorbs solar radiation and is released by vehicles, especially diesel-fueled trucks. Here we use observations of CO, NO_x, BC, CO₂, and traffic counts collected at a near-road (NR) site along Interstate-95, a major highway in Maryland, to infer emissions through use of ratios such as ΔBC/ΔCO, ΔCO/ΔNO_x, ΔCO₂/ΔBC, ΔCO₂/ΔCO, and ΔCO₂/ΔNO_x. The goal is to assess the impact of COVID-19 travel restrictions, implemented in Maryland during March 2020, on the emission of these pollutants. Emission ratios respond to changes in the passing vehicle fleet and are a good indicator of changes in traffic due to COVID-19. We also present evidence for a temperature dependence in vehicular emissions of BC and NO_x and use this to adjust BC and NO_x emissions for variations in temperature from March to summer, when social distancing measures were gradually lifted in Maryland. On average, diesel trucks make up 15% of the vehicles passing the NR site while light-duty gasoline vehicles represent 85% of the fleet. The greatest impact of travel restrictions has been on passenger cars, which decreased by ~50% from 200,000 to 100,000 vehicles/day at the peak of the travel restrictions in early April. Diesel trucks, on the other hand, decreased by at-most 10% by early April. With diesel trucks emitting predominantly NO_x and BC and gasoline cars emitting mainly CO and NO_x, emissions should decline with decreasing vehicle traffic and less congestion. Our study evaluates the impact of reduced traffic of specific vehicle types (gasoline cars and diesel trucks) on mobile emissions of CO, NO_x, and BC at the NR site using emission ratios. Additionally, we use vehicle count data collected at the I-95 NR site to guide reductions of mobile emissions in CMAQ to estimate the regional impacts of the decline in traffic on modeled concentrations of ozone in Maryland.