Evaluation of heavy-duty vehicle emission controls with a decade of California real-world observations
Creators: name: Ruehl, Chris
name: Misra, Chandan
name: Yoon, Seungju
name: Smith, Jeremy D.
name: Burnitzki, Mark
name: Hu, Shaohua
name: Collins, John
name: Tan, Yi
name: Huai, Tao
name: Herner, Jorn
Publisher: name: Taylor & Francis
Year: 2021
Subjects: subject: Biochemistry
subject: Neuroscience
subject: Physiology
subject: FOS: Biological sciences
subject: Pharmacology
subject: 59999 Environmental Sciences not elsewhere classified
subject: FOS: Earth and related environmental sciences
subject: 39999 Chemical Sciences not elsewhere classified
subject: FOS: Chemical sciences
subject: Ecology
subject: Immunology
subject: FOS: Clinical medicine
subject: Developmental Biology
subject: 111714 Mental Health
subject: FOS: Health sciences

description: Over the past decade, efforts to reduce emissions of particulate matter (PM) and oxides of nitrogen (NO + NO<sub>2</sub>, or NO<i><sub>x</sub></i>) from heavy-duty diesel vehicles (HDDVs) have led to the widespread adoption of both Diesel Particulate Filters (DPFs) to control PM and Selective Catalytic Reduction (SCR) to control NO<i><sub>x</sub></i>. We evaluated the performance of DPFs and SCR with 13,327 real-world fuel-based Black Carbon (BC) and NO<i><sub>x</sub></i> emission factors from 9,167 unique heavy-duty vehicles (primarily HDDVs) measured at four sites in California (two ports, two highways) from 2011 to 2018. BC emission factors have decreased by 90% during the past decade. At the same time, BC distributions have become increasingly skewed toward “high-emitters” – e.g., the portion of the HDDV fleet responsible for half of all BC emissions has decreased from ~16% to ~3%. NO<i><sub>x</sub></i> emission factors have also decreased over the past decade, but by only 31%. They remain roughly five times greater than in-use thresholds.We examined changes in BC and NO<i><sub>x</sub></i> emissions with engine age. BC emissions from DPF-only trucks decreased slightly but insignificantly, by 6 ± 15 mg/kg fuel per year, while for DPF+SCR trucks they increased by 5 ± 3. These changes are less than 5% of in-use thresholds. The annual increase in NO<i><sub>x</sub></i> emissions with age was much greater: 1.44 ± 0.28 g/kg for older SCR trucks without on-board diagnostic (OBD) capabilities and 0.48 ± 0.35 for newer trucks with OBD, roughly 20– 50% of in-use thresholds. Paired <i>t</i>-tests on the over 600 vehicles that were observed in multiple campaigns were consistent with these results. Observed changes in BC emissions with age were best fit with a “gross emitter” model assuming an annual DPF failure rate of 0.83 ± 0.01% for DPF-only trucks and 0.56 ± 0.01% for DPF+SCR trucks.<i>Implications</i>: These observations of real-world HDV emission factors have several major implications for regulatory efforts to reduce them. The increasing importance of a relatively small number of high BC emitters suggests that widespread sampling of the on-road fleet will be necessary to identify these vehicles. On the other hand, the much more ubiquitous deterioration in NO<i><sub>x</sub></i> control measures may be better addressed by incorporating on-board diagnostic systems, with telematic data transfer when possible, into inspection and maintenance programs. These NO<i><sub>x</sub></i> observations also highlight the need for strengthening heavy-duty SCR durability demonstration requirements.