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High-Resolution Mapping and Long-Term Trends for Motor Vehicle Emissions

  • Author(s): McDonald, Brian
  • Advisor(s): Harley, Robert A
  • et al.

Motor vehicles are a major source of greenhouse gas and other pollutant emissions that contribute to global climate change and urban and regional air pollution problems. Past efforts to develop motor vehicle emission inventories, needed for air quality planning, have been subject to significant uncertainties related to emission factors and spatial and temporal distributions of vehicle activity. The goal of this dissertation is to develop new inventories for vehicle emissions of greenhouse gases and co-emitted pollutants. A two-step approach was followed. First, motor vehicle emissions of carbon dioxide were mapped spatially and temporally using real-world traffic count data. The mapping was done separately for light- and heavy-duty vehicles so that emission factors specific to each vehicle type could be used to estimate associated air pollutant emissions. Second, long-term trends in emissions of nitrogen oxides, carbon monoxide, volatile organic compounds, and black carbon were analyzed. Emission trends were compared with long-term changes in the measured atmospheric concentrations of related pollutants, to assess the extent to which observed decreases in pollution can be attributed to motor vehicle emission control policies. The resulting motor vehicle emission inventories from this dissertation are more reliable than previous vehicle emission estimates, because spatial and temporal patterns of vehicle activity are explicitly accounted for using real-world traffic count data rather than transportation demand models, and emission factors are derived from real-world on-road studies rather than from laboratory testing.

A fuel-based inventory for vehicle emissions is presented for carbon dioxide (CO2), and mapped at various spatial resolutions (10 km, 4 km, 1 km, and 500 m) using fuel sales and traffic count data. The mapping is done separately for gasoline-powered vehicles and heavy-duty diesel trucks. Emissions estimates from this study are compared with the Emissions Database for Global Atmospheric Research (EDGAR) and VULCAN. All three inventories agree at the national level within 5%. EDGAR uses road density as a surrogate to apportion vehicle emissions, which leads to 20-80% overestimates of on-road CO2 emissions in the largest U.S. cities. High-resolution emission maps are presented for Los Angeles, New York City, San Francisco-San Jose, Houston, and Dallas-Fort Worth. Sharp emission gradients that exist near major highways are not apparent when emissions are mapped at 10 km resolution. High CO2 emission fluxes over highways become apparent at grid resolutions of 1 km and finer. Temporal variations in vehicle emissions are characterized using extensive day- and time-specific traffic count data, and are described over diurnal, day of week, and seasonal time scales. Clear differences are observed when comparing light- and heavy-duty vehicle traffic patterns and comparing urban and rural areas. Decadal emission trends were analyzed from 2000 to 2007 when traffic volumes were increasing, and a more recent period (2007-2010) when traffic volumes declined due to recession. We found large non-uniform changes in on-road CO2 emissions over a period of ~5 years, highlighting the importance of timely updates to motor vehicle emission inventories.

A similar approach is used to estimate nitrogen oxide (NOx = NO + NO2) emissions from gasoline- and diesel-powered motor vehicles. Estimates are made at the national level for the period 1990 to 2010. Vehicle emissions are also estimated at the state level for California, and for the South Coast (Los Angeles) and San Joaquin Valley air basins. Fuel-based emission estimates are compared with predictions from widely used emission inventory models. Changes in diesel NOx emissions vary over time: increasing between 1990 and 1997, stable between 1997 and 2007, and decreasing since 2007. In contrast, gasoline engine-related NOx emissions have decreased steadily, by ~65% overall between 1990 and 2010, except in the San Joaquin Valley where reductions were not as large due to faster population growth. In the San Joaquin Valley, diesel engines were the dominant on-road NOx source in all years considered (reaching ~70% in 2010). In the urbanized South Coast air basin, gasoline engine emissions dominated in the past, and have been comparable to on-road diesel sources since 2007 (down from ~75% in 1990). Other major anthropogenic sources of NOx are added to compare emission trends with trends in surface pollutant observations and satellite-derived data. When all major anthropogenic NOx sources are included, the overall emission trend is downward in all cases (-45% to -60%). Future reductions in motor vehicle NOx will depend on the effectiveness of new exhaust after-treatment controls on heavy-duty trucks, as well as further improvements to durability of emission control systems on light-duty vehicles.

Long-term trends in carbon monoxide (CO) emissions from motor vehicles were also assessed. Non-methane hydrocarbons (NMHC) are estimated based on my CO emission inventory, using ambient NMHC/CO ratios that were adjusted to exclude NMHC contributions from non-vehicular sources. Despite increases in fuel use of ~10-40%, CO running exhaust emissions from on-road vehicles decreased by ~80-90% in Los Angeles, Houston, and New York City, between 1990 and 2010. The ratio of NMHC/CO was found to remain constant at 0.24 ± 0.04 mol C/mol CO over time in Los Angeles, indicating that emissions of both NMHC and CO decreased at a similar rate and were affected by similar emission control policies, whereas on-road data from other cities suggest rates of reduction in NMHC versus CO emissions may differ somewhat. Emission ratios of CO/NOx (nitrogen oxides = NO + NO2) and NMHC/NOx decreased by a factor of ~4 between 1990 and 2007 due to changes in the relative emission rates of passenger cars versus diesel trucks, and slight uptick thereafter, consistent across all urban areas considered here. These pollutant ratios are expected to increase in future years due to (1) slowing rates of decrease in CO and NMHC emissions from gasoline vehicles, and (2) significant advances in control of diesel NOx emissions.

New estimates of particulate matter (PM) and black carbon (BC) emissions from heavy-duty diesel trucks in the Los Angeles area were developed as part of this research. Emission trends are compared with trends in ambient concentrations of particulate black and organic carbon over a 35-year period starting in 1975. On-road heavy-duty diesel emission factors of PM and BC have decreased by a factor of ~4 since 1975. After accounting for rapid growth in diesel fuel sales, on-road diesel BC emissions were found to have decreased by only ~20% between 1975 and 2010. In contrast, ambient measurements of BC concentrations in the Los Angeles basin show a clear downward trend, and have decreased steadily at an average rate of 4.2% per year since 1975. The slopes of best-fit lines in plots of measured OC versus BC concentrations have remained remarkably consistent over time. The stability of this ratio over time implies similar long-term trends in ambient black and organic carbon concentrations. We estimate that ambient OC levels in the Los Angeles basin have decreased by ~3.1% per year since 1975. Ongoing debate about the relative importance of gasoline versus diesel vehicle VOC emission contributions to secondary organic aerosol formation in urban areas is further informed by this research. Between 1995 and 2010, gasoline VOC emissions show a steeper downward trend, decreasing by 75 ± 7% compared to OC which decreased by only 45 ± 22%. The difference in slopes suggests that other sources of particulate organic carbon must also be contributing to the differing trends. When including other primary and secondary sources of organic aerosols from motor vehicles, the ambient and emission trends strongly agree. We conclude that long-term decreases in ambient OC likely resulted from efforts to control on-road gasoline emissions of VOCs. However, as a consequence of these efforts, other sources of organic aerosols have grown in relative importance including emissions from diesel trucks.

Recommendations for future research include development of urban CO2 monitoring networks, modeling effects on air quality of long-term changes in motor vehicle emissions, and projecting future motor vehicle emissions and associated impacts on air quality.

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