Characterization of In-Use Heavy-Duty Diesel Truck Pollutant Emission Rates and Emission Control Technology Performance
CHELSEA V. PREBLE, Rebecca A. Sugrue, Thomas W. Kirchstetter,
University of California, Berkeley Abstract Number: 538
Working Group: Urban Aerosols
AbstractDiesel particle filter (DPF) and selective catalytic reduction (SCR) emission control systems are standard equipment on new heavy-duty diesel trucks (HDDTs) to reduce emitted particulate matter (PM) and nitrogen oxides (NO
x = NO + NO
2), respectively. In California, statewide regulations have accelerated fleet turnover and the adoption of these emission controls, such that all HDDTs must now be equipped with 2010 and newer engines that are DPF- and SCR-equipped. Prior work has shown that on-road emissions have decreased over the last decade as DPFs and SCR became widely used (Preble et al.,
ES&T 2015; 2018; 2019). However, that work also showed that a majority of pollution is emitted by a minority of trucks, and that in-use emissions can be higher than corresponding emission standards and mobile source emission model predictions.
To evaluate the impact of DPF and SCR systems on HDDTs under real-world conditions, tailpipe exhaust emissions from thousands of in-use trucks were measured at three California locations between 2019–2023. Each location captured a different driving mode: (1) entering a port terminal entrance, (2) traveling along an urban arterial street, and (3) driving up a 4% gradient on a highway. Fuel-based emission factors (g kg
-1) of black carbon (BC) and key nitrogenous species (NO
x, NO
2, nitrous oxide (N
2O), ammonia (NH
3)) were calculated via a carbon balance method on a truck-by-truck basis, using an exhaust plume capture approach that related pollutant concentrations measured at high time resolution (≥1 Hz) to above-background carbon dioxide (CO
2) concentrations. Emission profiles for each truck were linked to engine model year and installed after-treatment controls by matching recorded license plates to state-managed truck databases.
As the statewide regulations have been phased in over the past decade, a majority of HDDTs are now equipped with DPFs and SCR. This transformation of the on-road fleet has driven down fleet-average BC and NO
x emission factors by ~80% between 2010 (pre-regulation) and 2021. Across all sites, trucks equipped with both SCR and DPF (2010+ engines) had lower BC emissions than those 2007–2009 MY trucks with only a DPF, even though all 2007+ engines must meet the same EPA standard. This difference by technology seems to be impacted by driving mode, increasing with assumed engine load: on average, BC emissions by 2010 and newer engines are 35% lower at the Terminal Entrance, 61% lower at the Arterial Road site, and 90% lower on the highway, compared to 2007–2009 engines with a DPF alone. Notable increases in BC from trucks with pre-2010 engines without SCR indicate reduced DPF performance as systems age. Differences in engine tuning and filter regeneration strategy are likely factors that may contribute to the observed DPF deterioration for 2007–2009 engines compared to 2010 and newer engines that are also equipped with SCR.