Long-term Trends in Nitrogen Oxide Emissions from On-Road Motor Vehicles

Abstract:

Nitrogen oxide (NO_x = NO + NO₂) emissions are a key precursor to tropospheric ozone formation, and thus affect photochemical smog and human health. Fuel combustion in the transportation sector is a major contributor to NO_x emissions worldwide. Over the last several decades, there have been significant advances in emission control technology for gasoline and diesel engines. In 2010, a major step in controlling diesel NO_x emissions for new heavy-duty engines sold in the US took effect with the introduction of selective catalytic reduction (SCR) control systems. The resulting decrease in diesel NO_x emission factors has been especially rapid in California, where accelerated replacement of the legacy fleet of older in-use trucks was mandated. All heavy-duty diesel trucks operating in California are required to meet stringent PM and NO_x standards by 2018 and 2023, respectively.

In this study, we used a fuel-based approach to estimate annual NO_x emissions from on-road vehicles in California and the United States during the period 1990-2019. We combined taxable gasoline and diesel fuel sales data from the Federal Highway Administration with in situ on-road measurements of emission factors to estimate vehicle emissions. The resulting fuel-based inventories are compared to EPA MOVES and California EMFAC emission model estimates. We also compare fuel-based inventory trends with trends in measured ambient NO_x concentrations in Los Angeles, New York City, and Houston. While fuel sales have continued to increase over the last three decades, there has been a steady decline in NO_x emissions due to decreases in both gasoline and diesel emission factors. In California, diesel NO_x emissions have decreased below gasoline NO_x emissions in recent years, reversing an earlier trend in which diesel engines had become the dominant source. The implementation of NO_x control technology has shaped emission trends over time and indicates how further efforts to reduce NO_x emissions should be focused at both state and national levels.