A Decade of On-Road Heavy-Duty Diesel Truck Emission Control Performance Characterization during California’s Fleet Modernization
CHELSEA V. PREBLE, Robert Harley, Thomas W. Kirchstetter, University of California, Berkeley
Abstract Number: 528
Working Group: Urban Aerosols
Abstract
Diesel particle filter (DPF) and selective catalytic reduction (SCR) emission controls are standard on new heavy-duty diesel trucks (HDDTs) to reduce particulate matter and nitrogen oxides (NOx = NO + NO2). California’s Truck and Bus Regulation accelerated fleet turnover and adoption of these technologies, such that all HDDTs in the state must now be equipped with 2010 and newer engines that are DPF- and SCR-equipped. To evaluate these controls under real-world conditions over time, tailpipe exhaust emissions from thousands of in-use trucks were measured at the Caldecott Tunnel near San Francisco (2014, 2015, 2018, 2021, 2022, 2023). Emission factors (g kg-1) of black carbon (BC), NOx, NO2, nitrous oxide (N2O), and ammonia (NH3) were calculated on a truck-by-truck basis using a plume-capture, carbon-balance approach. Emission profiles for each truck were linked to engine model year and emission controls by matching license plates to state-managed databases.
DPF-equipped trucks increased from 15% in 2010 (pre-regulation) to >99% of the fleet in 2022, and SCR use increased from 2% to 91%. Fleet-average BC and NOx emission factors over that same period decreased by ~80%. The majority of BC emissions by 2007–2009 engines and NOx emissions by pre-2013 engines measured in 2022 were higher than relevant emission standards. Notable increases in BC from 2007–2009 engines indicate reduced DPF performance with aging, which may be due to frequent active regeneration. Emissions distributions are increasingly skewed, with the highest emitting 10% of the 2022 fleet responsible for 66% of NOx and 91% of BC. The top 10% of emitters do not necessarily overlap between pollutants, however. Removing the top 10% of NOx emitters would have the largest co-benefit by reducing fleet emissions of BC (30%), N2O (25%), and NH3 (18%). These results may reflect changes occurring elsewhere and are important for emission inventory and policy development.