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In-use measurements of NOx and particulates trends for heavy-duty vehicles in California

A team at the University of Denver has measured the emissions from two in-use California heavy-duty truck fleets—at the Port of Los Angeles and the Cottonwood weigh station in Northern California—using the On-Road Heavy-Duty Measurement System in 2013, 2015 and 2017. A report on their findings is published in the ACS journal Environmental Science & Technology.


Credit: ACS, Haugen and Bishop (2018)

The California Air Resources Board issued the California Diesel Risk Reduction Plan in 2000 with the goal of reducing diesel PM emissions 85% statewide by 2020. Numerous rules and regulations have encouraged the retirement of older trucks and accelerated the penetration of lower-emitting newer or retrofitted vehicles.

The San Pedro Bay Ports Clean Air Action Plan required all vehicles operating within the Ports of Los Angeles and Long Beach to have model year 2007 and newer engines equipped with diesel particulate filters (DPFs), resulting in a complete turnover of the Port fleet in 2010.

Statewide, the California Truck and Bus Rule established a time schedule for requiring all HDVs to meet PM and NOx standards. This rule required older model year vehicles operating in California to either be replaced or comply with PM filter requirements as of 1 January 2015. In addition, all trucks operating in California by 2023 must comply with both the 0.01 g/bhp-hr PM standard and 0.2 g/bhp-hr NOx standard.

To assess the progress made toward reaching the 85% PM reduction foal by 2020, emissions measurements have been made at the Port of Los Angeles (POLA) and the Cottonwood weigh station.

  • The POLA fleet generally comprises local trucks with first-generation DPFs that are involved in primarily short-haul activities. The fleet has been slow to introduce SCR-equipped vehicles.

  • The Cottonwood weigh station fleet consists of interstate trucks that are predominantly involved in long-haul operations.

These contrasting fleets and retirement schedules have established different dynamics in the emission trends observed. This research adds a third in-use emissions data set to the measurements previously collected in 2013 and 2015 to form one of the largest in-use emissions database for HDVs. We have used this data set to compare and contrast these California fleet emission trends and examine the in-use effectiveness of the new after-treatment systems.

—Haugen and Bishop

Among their findings:

  • The Port of Los Angeles drayage fleet has increased in age by 3.3 model years (4.2−7.5 years old) since 2013, with little fleet turnover.

  • Large increases in fuel-specific particle emissions (PM) observed in 2015 in the POLA fleet were reversed in 2017, returning to near 2013 levels, suggesting repairs and or removal of high emitting vehicles.

  • Fuel-specific NOx emissions of the POLA fleet have increased; NOx after-treatment systems do not appear to perform ideally in this setting.

  • The Cottonwood fleet age has declined (7.8 to 6 years old) since 2013 due to fleet turnover, significantly lowering the average fuel-specific emissions for PM (−87%), black carbon (−76%), and particle number (−64%). Installations of retrofit-diesel particulate filters in model year 2007 and older vehicles have further decreased particle emissions.

  • Cottonwood fleet fuel-specific NOx emissions have decreased slightly (−8%) during this period; however, newer technology vehicles with selective catalytic reduction systems (SCR) promise an additional factor of 4−5 further reductions in the long-haul fleet emissions as California transitions to an all SCR-equipped fleet.

  • As the particle emissions at Cottonwood have steadily decreased, the fleet averages are now dominated by a few high-emitting vehicles.


  • Molly J. Haugen and Gary A. Bishop (2018) “Long-Term Fuel-Specific NOx and Particle Emission Trends for In-Use Heavy-Duty Vehicles in California” doi: 10.1021/acs.est.8b00621


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