Long-Term Trends in Nitrogen Oxides Emissions from Heavy-Duty Diesel Vehicles in California

Reducing nitrogen oxides (NO x ) in California is critical to improving air quality and meeting National Ambient Air Quality Standards for ozone and fine particulate matter. On-road heavy-duty diesel vehicles (HDDVs) are a major source of NO x . In this study, the impact of on-board diagnostic (OBD) HD regulations on tailpipe NO x emissions during running hot in-use operations (exhaust temperature >200 °C) was quantified; cold-start and low-load operations which can significantly contribute to in-use NO x emissions are not considered. Onboard NO x emissions were measured between 2013 and 2024 from class 7 and 8 HDDVs. Vehicles were classified as pre-OBD engines equipped with an Engine Manufacturers Diagnostic (EMD) system, HD OBD phase-in engines, and HD OBD engines. HD OBD engines showed significant reductions in distance-based tailpipe NO x emission factors (0.2 ± 0.3 g·mi –1 ) compared to EMD engines (3.2 ± 1.9 g·mi –1 ). HD OBD engines exhibited significantly lower tailpipe NO x emissions even when compared to vehicles of the same age, likely reflecting improved SCR control efficiencies to meet increasingly stringent OBD control thresholds. However, the data suggests evidence of SCR deterioration in the in-use fleet. This study highlights the maturing of SCR technology and the efficacy of the OBD program in reducing HDDV NO x emissions.