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Evaluation of Mobile Source Emissions and Trends


Mobile sources contribute significantly to air pollution problems. Relevant pollutants include numerous gaseous and particle-phase species that can affect human health, ecosystems, and climate. Accurate inventories of emissions from these sources are needed to help understand possible adverse impacts, and to develop effective air quality management strategies. Unfortunately large uncertainties persist in the understanding of mobile source emissions, and how these emissions are changing over time. There are more than two hundred million motor vehicles operating in the United States alone, and measurements of emissions from these sources are sparse. Pollutant emission factor distributions are becoming increasingly skewed, and this continually increases the needed vehicle sample size in studies that seek to quantify fleet-average vehicle emission rates. This dissertation aims to evaluate long-term trends in mobile source emissions in the United States, and to make detailed measurements of emissions from present-day fleets of on-road vehicles operating in California. Novel features of this work include studies of the in-use effectiveness of modern control technologies used to reduce diesel engine emissions, and application of advanced instrumentation to measure emissions from large numbers of on-road gasoline and diesel vehicles at high time resolution and with a high level of chemical and physical detail.

Long-term trends in mobile source emissions of nitrogen oxides (NOx) and fine particulate matter (PM2.5) in the United States were investigated through development of a fuel-based emission inventory. Annual emissions from on- and off-road gasoline and diesel engines were quantified for the years 1996-2006. Diesel engines were found to be the dominant mobile source of NOx and PM2.5, and on-road diesel vehicles were identified as the single largest anthropogenic source of NOx emissions in the United States as of 2005. The relative importance of diesel engines as a source of NOx grew over the ten-year time period considered here, while emissions from gasoline engines declined due to increased effectiveness and use of three-way catalytic converters. A comparison with national emission inventory estimates for 2005 found substantial differences in source contributions to overall mobile source emissions, with larger contributions from on-road diesel engines indicated in this study.

The importance of diesel engines as a source of exhaust particulate matter emissions has led to the recent introduction of advanced emission control technologies in the United States, such as diesel particle filters (DPF), which have been required since 2007 for all new on-road heavy-duty (HD) diesel engines. In addition to national requirements for the use of such control devices on new engines, California has mandated accelerated clean-up of statewide emissions from older in-use diesel engines. This goal is to be achieved through filter retrofit and truck/engine replacement programs. This dissertation uses measurements of emissions from in-use HD diesel trucks at the Port of Oakland to evaluate the impacts of a DPF retrofit and truck replacement program. A plume capture method was developed to quantify black carbon (BC) and NOx emission factors for individual trucks and to characterize emission factor distributions. A comparison of emissions measured before and after the implementation of the truck retrofit/replacement rule shows a 54 ± 11% reduction in the fleet-average BC emission factor, accompanied by a shift to a more highly skewed emission factor distribution. Although only particulate matter mass reductions were required in the first phase of the program, a 41 ± 5% reduction in the fleet-average NOx emission factor was observed. These results provide an in-use/real-world assessment of the impact of DPF emission control systems, and a preview of emissions changes that may be expected from the extension of similar control programs to the entire HD truck fleet in California beginning in 2014.

The plume capture method was further applied to measure emissions from a more diverse population of trucks observed at the Caldecott tunnel in summer 2010. Emissions from hundreds of individual trucks were measured, and emission factor distributions were characterized for nitric oxide (NO), nitrogen dioxide (NO2), carbon monoxide (CO), formaldehyde, BC, as well as optical properties of the emitted particles. Emission factor distributions for all species were skewed, with a small fraction of trucks contributing disproportionately to total emissions. For example, half of the total measured NO2 and BC were produced by only 10% of the total measurements. Total NOx and formaldehyde showed less skewed emission factor distributions compared to CO and BC. Emission factors for NO2 were found to be anti-correlated with all other pollutants considered here. Also, the fleet-average NO2 emission factor increased 34 ± 18% relative to the corresponding value measured at the same location in 2006. These findings confirm that the use of catalyzed DPF systems is leading to increased primary NO2 emissions. Absorption and scattering cross-section emission factors were used to calculate the aerosol single scattering albedo (SSA, at 532 nm) for individual truck exhaust plumes, which averaged 0.14 ± 0.03. This value of aerosol SSA is very low compared to typical values (0.90-0.99) observed in ambient air studies. It is indicative of a strongly light-absorbing aerosol, due to the high BC emissions that are a characteristic feature of diesel exhaust PM emissions.

Measurements at the Caldecott tunnel also included efforts to quantify light-duty (LD) gasoline vehicle emission factors, and further investigation of the relative contributions of on-road gasoline and diesel engines to air pollutant emissions. Measurements of CO, NOx, PM2.5, BC, and organic aerosol (OA) were made in a tunnel traffic bore where LD vehicles account for >99% of total traffic. Measured pollutant concentrations were apportioned between LD gasoline vehicles and diesel trucks, and fleet-average emission factors were quantified for LD gasoline vehicles using a carbon balance method. Diesel trucks contributed 18 ± 3, 22 ± 5, 44 ± 8% of measured NOx, OA, and BC concentrations, respectively, despite accounting for <1% of total vehicles. Consequently, methods that do not explicitly account for diesel truck contributions tend to overestimate fleet-average LD gasoline vehicle emission factors for these species. Emission factors and overall fuel consumption for gasoline and diesel engines were used to describe the relative contributions of these sources to overall on-road vehicle emissions. Gasoline engines were found to be the dominant source of CO, an insignificant source of BC, and a relatively minor source of on-road OA emissions at urban, state, and national scales.

Measurements at the Caldecott tunnel also featured use of a new high-resolution time-of-flight aerosol mass spectrometer, which was used to characterize the chemical composition of PM emitted by gasoline and diesel vehicles. Measurements of PM in the exhaust of individual HD trucks show a predominance of cyclyoalkane-derived ion signals relative to saturated alkane ion signals in the truck exhaust OA spectra, indicating that lubricating oil, rather than diesel fuel, was the dominant source of OA emitted by diesel trucks. This conclusion is supported by the presence of lubricant-derived trace elements in truck exhaust, emitted relative to total OA at levels that correspond to their weight fractions in bulk oil. Furthermore, comparison of mass spectra for sampling periods with varying levels of diesel influence found a high degree of similarity in the chemical composition of OA emitted by gasoline and diesel engines, suggesting a common lubricating oil rather than fuel-derived source for OA emissions.

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