UC Berkeley

Heavy-duty diesel trucks are a major source of nitrogen oxides (NOx) and the black carbon (BC) fraction of fine particulate matter (PM) in urban environments. These emissions contribute to persistent ozone and PM air quality problems. Recently, diesel particle filter (DPF) and selective catalytic reduction (SCR) emission control systems that target exhaust PM and NOx, respectively, have become standard equipment on new heavy-duty diesel trucks. DPFs can also be installed as a retrofit. Additionally, the California Air Resources Board (CARB) has accelerated the adoption of these advanced emission control systems with two regulations: the Statewide Drayage Truck Regulation and the Truck and Bus Regulation. These rules required universal adoption of DPFs first by drayage trucks operating at ports and rail yards and second by most trucks and buses operating on arterial roadways and highways statewide.

Prior studies, most of which have measured emissions from vehicles or engines operating on dynamometers in the laboratory, have shown DPF and SCR systems to be effective at reducing PM and NOx emission rates. These studies have also identified potential changes to other co-emitted pollutants. There is concern that DPFs may promote the formation of ultrafine particles (UFP) and increase total particle number (PN) emissions while reducing particle mass emissions. The deliberate catalytic oxidation of engine-out nitric oxide (NO) to nitrogen dioxide (NO2) in continuously regenerating DPFs may lead to increased tailpipe emissions of NO2. NO2 is a regulated air pollutant due its toxicity and its role in promoting the formation of other air pollutants such as ozone, nitric acid, and fine PM. While SCR reduces NOx emissions, it may lead to increased emissions of nitrous oxide (N2O), a potent greenhouse gas.

To evaluate the in-use performance of advanced emission control technologies on trucks operating on-road under real-world conditions, exhaust emissions from thousands of heavy-duty diesel trucks were measured over several years at the Port of Oakland and the Caldecott Tunnel in the San Francisco Bay Area. The adoption of DPF and SCR systems was greatly accelerated at these two locations due to new regulations, with phased implementation schedules. Gas- and particle-phase pollutants in the exhaust plumes of individual heavy-duty trucks were measured at high time resolution (≥1 Hz) as trucks were driven under a mobile emissions lab parked on an overpass. Fuel-based emission factors (amount of pollutant emitted per kg of fuel burned) were calculated on a truck-by-truck basis via a carbon balance method. Emission profiles for each truck were linked to vehicle attributes, including engine model year and installed after-treatment controls, by matching recorded license plates to state-managed truck databases. With this information, trucks were categorized by emission control technology: (1) trucks without DPFs, (2) older engines retrofit with DPFs, (3) 2007–2009 model year engines equipped with DPFs at the time of manufacture, and (4) 2010 and newer engines equipped with both DPF and SCR systems at the time of manufacture.

In this dissertation, the impacts of advanced after-treatment control technologies on in-use heavy-duty diesel truck emissions are evaluated. During the phase-in of the Drayage Truck Regulation at the Port of Oakland, the impacts of DPF and SCR systems on drayage truck emissions were quantified by comparing pollutant emission rates for trucks with and without these control technologies. After full implementation of the regulation, changes to the fleet-average emissions and the durability of aging emission control systems were evaluated. The influence of driving mode on the performance of DPF and SCR systems was examined by comparing results for uphill, highway driving conditions at the Caldecott Tunnel versus driving on a flat, arterial roadway approaching the Port of Oakland.

DPF and SCR systems effectively reduced BC and NOx emission rates from drayage trucks operating at the Port of Oakland. Trucks with 2010 and newer model year engines equipped with both DPF and SCR emitted on average 94 ± 32% less BC (average ± 95% confidence interval) than trucks without particle filters. These 2010+ engines also emitted 76 ± 7% less NOx than 1994–2006 engines without SCR. DPFs increased emissions of primary NO2, however, by up to a factor of 6 for trucks with older engines—and higher baseline NOx emissions— that had been retrofitted with DPFs. SCR systems partially mitigated these undesirable DPF-related NO2 emissions, limiting the increase to a factor of 2 compared to trucks without filters. SCR systems can lead to the emission of N2O, although the average emission rate by the drayage trucks at the Port of Oakland was below the California limit of 0.6 g kg-1. Emissions of PN did not increase with use of DPFs. In fact, trucks with filters emitted fewer particles per kg of fuel burned, on average, compared to trucks without DPFs. The newest trucks with both DPF and SCR systems had the lowest PN emission rate, equal to one-fourth that for trucks without filters.

As a result of the Drayage Truck Regulation, the Port of Oakland drayage truck fleet was rapidly modernized to include DPF and SCR emission control systems. Between 2009 and 2015, the fraction of the fleet equipped with DPFs increased from 2 to 99%, SCR use increased from 0 to 25%, and the median engine age decreased from 11 to 7 years. Coincident with these changes, fleet-average emission rates of NOx, BC, and PN decreased by 70 ± 9%, 73 ± 22%, and 74 ± 27%, respectively. These reductions were achieved in two phases. The first phase focused on banning the oldest trucks from the Port, and requiring the universal adoption of DPFs over a three-year period ending in January 2013. The second phase took effect in the following year and replaced older trucks that had just recently been retrofit with DPFs, with newer 2007+ engines. As a result, SCR prevalence increased and this further reduced NOx emissions beyond what was initially achieved in Phase 1. Use of SCR also helped to mitigate DPF-related increases in NO2 emissions, which had doubled in Phase 1 relative to the previously uncontrolled truck fleet. Over time, unfortunately, the BC emission rate for 2007–2009 engines with DPFs increased by 50%. This increase appears to be driven by deteriorating particle filter systems that led to some relatively high-emitting trucks. A small fraction of DPF-equipped trucks was responsible for a majority of the fleet BC emissions in 2015.

At the Caldecott Tunnel, there is similar evidence of deteriorating performance of diesel particle filters systems as they age. The effect on fleet-average BC emissions is smaller, and the overall performance of DPFs is comparable to what was measured at the Port of Oakland. SCR systems were more effective at reducing NOx emissions at the Caldecott Tunnel compared to the Port. This difference is likely due to differences in driving conditions: truck engines are operating with higher power output due to the 4% uphill gradient and higher vehicle speeds. As a result, exhaust temperatures were higher and more likely to exceed the minimum temperature required for SCR operation. However, the elevated exhaust temperature also appears to have led to higher N2O emission rates for SCR-equipped engines. At the Caldecott Tunnel, the N2O emission rate for SCR trucks was more than double the emission rate by SCR-equipped drayage trucks operating at the Port of Oakland, and frequently exceeded the California limit.

PN emission rates also depend on driving mode, with higher exhaust temperatures promoting nucleation of ultrafine particles, and higher observed emissions of PN, by a factor of seven relative to the Port of Oakland in 2015. While DPFs at the Port of Oakland reduced emitted PN regardless of installation type, the effect of filters on PN emission rates at the Caldecott Tunnel depended on the type of DPF installed. Engines equipped with DPFs at the time of manufacture had comparable PN emission rates as observed from trucks without filters, whereas engines retrofitted with DPFs emitted 1.7 times more PN per unit of fuel burned.

This research demonstrates and documents the on-road effectiveness of advanced after-treatment control systems for reducing emission rates of black carbon and nitrogen oxides from diesel trucks. Emission control systems can alter the emission rate of co-emitted pollutants like primary NO2, PN, and N2O, in ways that depend on driving conditions. However, combined use of both DPF and SCR systems appears to offer the greatest air quality benefits: large reductions in both BC and NOx emissions, as well as mitigation of DPF-related increases in tailpipe NO2 emissions.

Future efforts to accelerate reductions in on-road vehicle emissions should focus on engine replacement rather than retrofitting in-use engines with DPFs. In order to maintain the air quality benefits of modern emission control systems over full in-use service lifetimes of on-road vehicles, it would be helpful to better understand why some diesel particle filter systems are failing prematurely, after less than ten years of service. The durability of emission control systems should be improved, and inspection and maintenance/repair programs may be helpful to identify, intervene, and fix the highest emitters that account for a minority of the on-road fleet but emit the majority of pollution.