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 (NO 2 ) emissions. However, a majority of trucks had in-use NO x emission rates that exceeded applicable emission standards. SCR systems increase emissions of nitrous oxide (N 2 O) and ammonia (NH 3 ) 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 NH 3 are skewed; 10% of trucks contribute 95% of the on-road fleet's total NH 3 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 (CO 2 ) emissions and increased N 2 O and NH 3 emissions due to the rapid adoption of DPF and SCR systems by the California truck fleet are: (1) a 65% net decrease in the social cost of statewide exposure to diesel truck emissions (-3.3 billion 2018 US dollars per year), and (2) a 3% net decrease in the global warming potential-weighted emission factor (-27 g CO 2 -eq km -1 ).