Investigating Emissions of CO, NOy, and CO2 Inferred from Near-Road Observations in the Baltimore-Washington Region: Impact of Ambient Temperature

Carbon monoxide (CO) and nitrogen dioxide (NO₂), pollutants emitted by mobile sources, as well as other combustion sources, are regulated by the U.S. Environmental Protection Agency (U.S. EPA) due to their adverse impacts on human health and the environment. Emissions of CO and NO_x (NO + NO₂) from mobile sources have steadily declined over the past few decades due to the implementation of emission control technologies, however these emissions still remain major precursors of ozone (O₃) pollution. Recent studies have shown that the representation of NO_x emissions within the EPA's National Emissions Inventory (NEI) may be overestimated compared to observations, especially in the warm summer months. Our research aims to better understand this discrepancy by analyzing observations of ambient CO, NO_x, and CO₂ abundances and meteorological variables collected at two near-road (NR) monitoring sites within the Baltimore-Washington region. Emissions of NO_x inferred from NR observations reveal a modest sensitivity to absolute humidity and a strong dependence on ambient temperature. The default configuration of MOtor Vehicle Emissions Simulator (MOVES), used to calculate mobile emission factors for air quality models, shows a modest humidity effect but no temperature dependence, other than a small response in emissions of CO, NO_x, and CO₂ above ambient temperatures of 68°F from air conditioner use. MOVES has the capability to adjust running exhaust emissions directly through a multiplicative temperature adjustment factor, which is routinely set to unity, while air conditioner use is modified through an indirect adjustment. Setting the multiplicative adjustment factor within MOVES to a value needed to fit the observed temperature dependence will allow for more realistic evaluations of the impact of vehicle emissions on surface O₃ simulated by air quality models. Including this sensitivity of emissions on ambient temperature in MOVES may help in part to resolve the discrepancy in mobile NO_x reported in recent studies.