Control Technology-Driven Changes to In-Use Heavy-Duty Diesel Truck Emissions of Nitrogenous Species and Related Environmental Impacts

Emissions from thousands of in-use heavy-duty diesel trucks were sampled at a highway and an arterial street location in the San Francisco Bay Area, spanning a time period when use of diesel particle filters (DPFs) and selective catalytic reduction (SCR) increased rapidly. At the highway site where a diverse mix of trucks is observed, SCR systems on 2010 and newer engines reduce emitted nitrogen oxides (NO_x) by 87 ± 5% relative to pre-2004 engines. SCR also mitigates DPF-related increases in nitrogen dioxide (NO₂) emissions. However, a majority of trucks at both locations had in-use NO_x emission rates that exceeded applicable emission standards. SCR systems increase emissions of nitrous oxide (N₂O) and ammonia (NH₃), from near-zero levels to 0.93 ± 0.13 and 0.18 ± 0.07 g kg^-1, respectively. Emissions of all nitrogenous species and especially NH₃ are skewed; 10% of trucks contribute 95% of the on-road fleet’s total NH₃ emissions. Similar emission changes are observed at the arterial street site where exclusively drayage trucks operate. The environmental effects of decreased black carbon, NO_x, and carbon dioxide (CO₂) emissions and increased N₂O and NH₃ emissions due to rapid adoption of DPF and SCR systems by the California truck fleet are: (1) a net decrease in the social cost of statewide exposure to diesel truck emissions (–3.3 billion 2018 US dollars per year), and (2) a net decrease in the global warming potential-weighted emission factor (–27 g CO₂-eq km^-1).