Toward Community-Scale Modeling of Air Pollution: Transportation Equity Applications and Updated Ship Emission Factors
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Toward Community-Scale Modeling of Air Pollution: Transportation Equity Applications and Updated Ship Emission Factors


Air pollution emitted by transportation sources such as cars, trucks, buses, trains, and ships negatively impacts human health, and the exposure burden is not shared evenly across populations. Therefore, working towards environmental justice, or the equitable distribution of environmental risks and benefits, requires accurate modeling of air pollution at the community scale. High-resolution air pollution models can help identify local pollution hotspots within cities, assess environmental health risks of residential developments near polluting transportation corridors, and predict or evaluate the impact of regulatory interventions or other pollution mitigation measures. The studies presented in this dissertation broadly relate to how air pollution emissions from transportation sources can contribute to local-scale heterogeneities in pollutant concentrations and resulting inequities in environmental health impacts, with a focus on how improved community-scale modeling can help assess these inequities. Chapter 1 describes how near-source air pollution impacts have historically been approximated by the distance from the source and argues that this approach should be replaced with a more nuanced modeling approach to better capture local-scale concentration gradients, particularly when evaluating the impact of transportation emissions on new residential developments. Some of the challenges and gaps in current modeling approaches are elaborated, including the need for accurate and spatiotemporally specific emission factors. To mitigate inequities in transportation planning and funding allocations, some metropolitan planning organizations (MPOs) have adopted equity analysis frameworks for their regional transportation plans (RTPs). Air pollution exposure related to transportation can exacerbate other transportation-related inequities, but in most equity analyses it is only addressed superficially, if at all. Chapter 2 assesses how three Californian MPOs consider traffic-related air pollution (TRAP) burden in equity analyses of RTPs. Patterns of exurbanization and gentrification have, in some regions, displaced communities outside of urban core public transit networks, and therefore excluded them from the benefits of reduced local vehicle miles traveled (VMT). In these exurban communities, investments tend to be skewed towards freeway expansion and improvement rather than public transportation. An analysis of the RTPs of three Californian MPOs shows that current equity metrics do not adequately capture the historical and current burdens on people in outer suburban and exurban communities. One notable way that MPOs miss the inequities experienced by communities outside the urban core is by assessing compliance with air pollution standards and VMT reduction targets solely at the regional level, despite significant local variation in dimensions of transportation equity such as TRAP exposure and long car commutes. The MPOs that did consider more local variations approximated TRAP exposure solely by distance from a roadway. However, it is argued in this chapter that this is not sufficient for comparing urban core communities living near transit hubs to exurban communities living near commuter freeways. This study suggests that equity analyses of transportation projects would benefit from improved methods for modeling TRAP exposure at the community scale. Ships contribute substantially to air pollution in port communities, but most modeling studies in these areas use emission inventories developed from decades-old measurements of ship engines. Chapter 3 updates brake-specific particulate matter (PM) mass emission factor (EFPM) estimates from ships via a meta-analysis of 55 lab and field studies. This meta-analysis incorporates newer studies that capture changing fuel standards and incorporates more measurements of operating modes and ship types associated with near-shore emissions that contribute to urban air pollution in port communities. The primary influence examined is the dependence of EFPM on fuel sulfur content (FSC), which has been targeted by the International Maritime Organization as well as local jurisdictions in efforts to reduce PM emissions from shipping. The results support the assumption that limiting FSC has the potential to significantly reduce EFPM and demonstrate a statistically significant linear dependence of EFPM on FSC, although the incorporation of new studies weakens the relationship relative to an earlier meta-analysis. Across the included studies, switching from a low-grade, high sulfur fuel to a high-grade, low sulfur fuel reduces EFPM by 50-80%. The updated emission factors for high sulfur fuels and low sulfur fuels are not significantly different than those derived from earlier studies and corroborate the literature. Some studies of EFPM and engine load show increased EFPM at very low engine loads relative to medium engine loads, likely due to reduced combustion efficiency. The greater heterogeneity of EFPM as compared to emission factors for gaseous pollutants suggests the need for additional studies to investigate the influence of individual factors. Additionally, more studies of ship types (such as tugs and pilots) and operating conditions (such as low speeds and idling) typical of near-shore ship operation would better support high-resolution modeling of ship contributions to PM pollution in port communities. The results of this chapter provide evidence to validate policy interventions that lower the maximum allowable FSC to mitigate PM emissions from shipping, corroborate EFPM estimates used in studies based on previous measurements, and identify areas where further measurements are warranted.

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