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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2014)

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.

    Cover page: High-Resolution Mapping and Long-Term Trends for Motor Vehicle Emissions
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2016)
    The majority of anthropogenic CO2 emissions are attributable to urban areas. While the emissions from urban electricity generation often occur in locations remote from consumption, many of the other emissions occur within the city limits. Evaluating the effectiveness of strategies for controlling these emissions depends on our ability to observe urban CO2 emissions and attribute them to specific activities. Cost-effective strategies for doing so have yet to be described. Here we characterize the ability of a prototype measurement network, modeled after the Berkeley Atmospheric CO2 Observation Network (BEACO2N) in California's Bay Area, in combination with an inverse model based on the coupled Weather Research and Forecasting/Stochastic Time-Inverted Lagrangian Transport (WRF-STILT) to improve our understanding of urban emissions. The pseudo-measurement network includes 34 sites at roughly 2 km spacing covering an area of roughly 400 km2. The model uses an hourly 1 × 1 km2 emission inventory and 1 × 1 km2 meteorological calculations. We perform an ensemble of Bayesian atmospheric inversions to sample the combined effects of uncertainties of the pseudo-measurements and the model. We vary the estimates of the combined uncertainty of the pseudo-observations and model over a range of 20 to 0.005 ppm and vary the number of sites from 1 to 34. We use these inversions to develop statistical models that estimate the efficacy of the combined model-observing system in reducing uncertainty in CO2 emissions. We examine uncertainty in estimated CO2 fluxes on the urban scale, as well as for sources embedded within the city such as a line source (e.g., a highway) or a point source (e.g., emissions from the stacks of small industrial facilities). Using our inversion framework, we find that a dense network with moderate precision is the preferred setup for estimating area, line, and point sources from a combined uncertainty and cost perspective. The dense network considered here (modeled after the BEACO2N network with an assumed mismatch error of 1 ppm at an hourly temporal resolution) could estimate weekly CO2 emissions from an urban region with less than 5 % error, given our characterization of the combined observation and model uncertainty.
      Cover page: Network design for quantifying urban CO2 emissions: assessing trade-offs between precision and network density
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2023)
      The extent to which emission control technologies and policies have reduced anthropogenic NOx emissions from motor vehicles is large but uncertain. We evaluate a fuel-based emission inventory for southern California during the June 2021 period, coinciding with the Re-Evaluating the Chemistry of Air Pollutants in CAlifornia (RECAP-CA) field campaign. A modified version of the Fuel-based Inventory of Vehicle Emissions (FIVE) is presented, incorporating 1.3 km resolution gridding and a new light-/medium-duty diesel vehicle category. NOx concentrations and weekday-weekend differences were predicted using the WRF-Chem model and evaluated using satellite and aircraft observations. Model performance was similar on weekdays and weekends, indicating appropriate day-of-week scaling of NOx emissions and a reasonable distribution of emissions by sector. Large observed weekend decreases in NOx are mainly due to changes in on-road vehicle emissions. The inventory presented in this study suggests that on-road vehicles were responsible for 55-72% of the NOx emissions in the South Coast Air Basin, compared to the corresponding fraction (43%) in the planning inventory from the South Coast Air Quality Management District. This fuel-based inventory suggests on-road NOx emissions that are 1.5 ± 0.4, 2.8 ± 0.6, and 1.3 ± 0.7 times the reference EMFAC model estimates for on-road gasoline, light- and medium-duty diesel, and heavy-duty diesel, respectively.
        Cover page: Improved Spatial Resolution in Modeling of Nitrogen Oxide Concentrations in the Los Angeles Basin.
        • Article
        • Peer Reviewed
        LBL Publications (2023)
        Los Angeles is a major hotspot for ozone and particulate matter air pollution in the United States. Ozone and PM2.5 in this region have not improved substantially for the past decade, despite a reduction in vehicular emissions of their precursors, NOx and volatile organic compounds (VOCs). This reduction in traditional sources has made the current emission mixture of air pollutant precursors more uncertain. To map and quantify emissions of a wide range of VOCs in this urban area, we performed airborne eddy covariance measurements with wavelet analysis. VOC fluxes measured include tracers for source categories, such as traffic, vegetation, and volatile chemical products (VCPs). Mass fluxes were dominated by oxygenated VOCs, with ethanol contributing ∼29% of the total. In terms of OH reactivity and aerosol formation potential, terpenoids contributed more than half. Observed fluxes were compared with two commonly used emission inventories: the California Air Resources Board inventory and the combination of the Biogenic Emission Inventory System with the Fuel-based Inventory of Vehicle Emissions combined with Volatile Chemical Products (FIVE-VCP). The comparison shows mismatches regarding the amount, spatial distribution, and weekend effects of observed VOC emissions with the inventories. The agreement was best for typical transportation related VOCs, while discrepancies were larger for biogenic and VCP-related VOCs.
          Cover page: Comparison between Spatially Resolved Airborne Flux Measurements and Emission Inventories of Volatile Organic Compounds in Los Angeles.
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2018)
          Concentrations of atmospheric trace species in the United States have changed dramatically over the past several decades in response to pollution control strategies, shifts in domestic energy policy and economics, and economic development (and resulting emission changes) elsewhere in the world. Reliable projections of the future atmosphere require models to not only accurately describe current atmospheric concentrations, but to do so by representing chemical, physical and biological processes with conceptual and quantitative fidelity. Only through incorporation of the processes controlling emissions and chemical mechanisms that represent the key transformations among reactive molecules can models reliably project the impacts of future policy, energy and climate scenarios. Efforts to properly identify and implement the fundamental and controlling mechanisms in atmospheric models benefit from intensive observation periods, during which collocated measurements of diverse, speciated chemicals in both the gas and condensed phases are obtained. The Southeast Atmosphere Studies (SAS, including SENEX, SOAS, NOMADSS and SEAC4RS) conducted during the summer of 2013 provided an unprecedented opportunity for the atmospheric modeling community to come together to evaluate, diagnose and improve the representation of fundamental climate and air quality processes in models of varying temporal and spatial scales. This paper is aimed at discussing progress in evaluating, diagnosing and improving air quality and climate modeling using comparisons to SAS observations as a guide to thinking about improvements to mechanisms and parameterizations in models. The effort focused primarily on model representation of fundamental atmospheric processes that are essential to the formation of ozone, secondary organic aerosol (SOA) and other trace species in the troposphere, with the ultimate goal of understanding the radiative impacts of these species in the southeast and elsewhere. Here we address questions surrounding four key themes: gas-phase chemistry, aerosol chemistry, regional climate and chemistry interactions, and natural and anthropogenic emissions. We expect this review to serve as a guidance for future modeling efforts.
            Cover page: Southeast Atmosphere Studies: learning from model-observation syntheses.Creative Commons 'BY' version 4.0 license
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