Air quality simulations are performed to determine the impact of changes in future climate and emissions on regional air quality in the South Coast Air Basin (SoCAB) of California. The perturbation parameters considered in this study include (1) increased temperatures, (2) increased absolute humidity, (3) increased biogenic VOC emissions due to increased temperatures, and (4) increased pollutant concentrations at the western inflow boundary. All parameters are first perturbed individually. In addition, the impact of simultaneously perturbing more than one parameter is analyzed. Air quality is simulated over a three-day period with meteorology representative of a summertime ozone pollution episode using both a baseline 2005 emissions inventory and a future emissions projection for the year 2023. Different locations within the modeling domain exhibit varying degrees of sensitivity to the perturbations considered. Afternoon domain wide average ozone concentrations are projected to increase by 13-18% as a result of changes in future climate and emissions. Afternoon increases at individual locations range from 10-36%. The change in afternoon PM levels is a strong function of location in the basin, ranging from -7.1% to +4.7% when using 2005 emissions and -8.6% to +1.7% when using 2023 emissions. Afternoon SOA concentrations for the entire domain are projected to decrease by over 15%, and the change in SOA levels is not a strong function of the emissions inventory utilized. Temperature increases play the dominant role in determining the overall impact on ground-level ozone, PM, and SOA concentrations in both the individual and combined perturbation scenarios.

The studies presented in this dissertation represent cutting-edge research in both fundamental and applied air quality modeling. In the first three chapters, a multidisciplinary approach is used to understand and investigate the importance of new chemistry that is not included in current air quality models. The results of laboratory experiments and theoretical calculations are used to develop and implement new chemical mechanisms into the UCI-CIT airshed model. In addition to the modifications to the model's chemical mechanism, changes to the model's inputs were also necessary prior to running air quality simulations. New chemical species that are unaccounted for in existing emissions inventories needed to be added, based on a combination of published information, experimental results, and field measurements conducted specifically for these studies. Additionally, emissions rates for existing species needed to be adjusted to better reflect present day conditions, or in some cases, explore potential future emissions scenarios. Results from these studies highlight the importance of collaborations between multidisciplinary teams of chemists and engineers to continuously improve air quality modeling capabilities. The novel chemistry included in these state-of-the-science modeling studies allows improved capability to simulate gas-phase pollutants, new particle formation, and aerosol aging in ways unachievable with other air quality models currently available. For example, results from Chapter 1 show that as fossil fuel combustion is phased out in an urban coastal area, particle formation will decrease substantially but still continue at a reduced rate due to the contribution of organosulfur compound (OSC) oxidation products. Furthermore, methanesulfonic acid generated simultaneously in OSC oxidation will become a significant contributor to particle formation, which should be taken into account in air quality and climate models. Chapter 2 demonstrates that indole is an effective precursor to SOA, and the UCI-CIT model showed significant potential for indole SOA formation driven by the oxidation of indole by OH. Indole SOA represents a previously unconsidered source of SOA that should be included in regional and global models, which tend to underestimate SOA concentrations. The air quality simulations conducted in Chapter 3 indicate that the inclusion of a previously unaccounted for sink for gas-phase ammonia intro an airshed model can significantly affect modeled ammonia and PM25 concentrations. Because inorganic particles comprise a large fraction of total PM2.5 mass, accurate modeling of gas-phase ammonia concentrations is essential for predicting future air quality. Models used in the development of air quality management strategies should account for changes in the concentration of ammonia due to its uptake by SOA to ensure accurate prediction of PM2.5 concentrations.

In the final chapter, a diverse team of industry personnel and researchers is assembled to assess the potential implementation of natural gas-fired distributed generation of electricity (DG) in the contiguous United States, including displacement of power from central power generation, and determine the potential impacts on air quality. DG market penetration estimates are translated into spatially and temporally resolved emissions and combined with other emissions projected to the year 2030 to conduct the air quality modeling. The Comprehensive Air Quality Model with Extensions (CAMx) is used to conduct the air quality simulations, which span several weeks and cover both summer and winter periods. This study explores a range of plausible scenarios and provides a modeling framework and methodology that can be applied in future studies to assess the potential implementation and impacts of DG. Results indicate that the operation of central power generation units can be affected by increased DG penetration and that most DG units in the U.S. emit at relatively high levels and significant emission reductions can be achieve through the implementation of stricter emissions limits. Overall, air quality impacts from DG are found to vary greatly based on meteorological conditions, proximity to emissions sources, the number and type of DG installations, and the emissions factors used for DG units.

While there is increasing evidence for unique chemical reactions at interfaces, there are fewer data on photochemistry at liquid-vapor junctions. This paper reports a comparison of the photolysis of molybdenum hexacarbonyl, Mo(CO)(6), in 1-decene either as liquid droplets or in bulk-liquid solutions. Mo(CO)(6) photolysis is faster by at least three orders of magnitude in the aerosols than in bulk-liquids. Two possible sources of this enhancement are considered: (1) increased light intensity due to either Morphology-Dependent Resonances (MDRs) in the spherical aerosol particles and/or to increased pathlengths for light inside the droplet due to refraction, which are termed physical effects in this paper; and (2) interface effects such as an incomplete solvent-cage at the gas-liquid boundary and/or enhanced interfacial concentrations of Mo(CO)(6), which are termed chemical effects. Quantitative calculations of the first possibility were carried out in which the light intensity distribution in the droplets averaged over 215-360 nm was obtained for 1-decene droplets. Calculations show that the average increase in light intensity over the entire droplet is 106%, with an average increase of 51% at the interface. These increases are much smaller than the observed increase in the apparent photolysis rate of droplets compared to the bulk. Thus, chemical effects, i.e., a decreased solvent-cage effect at the interface and/or enhancement in the surface concentration of Mo(CO)(6), are most likely responsible for the dramatic increase in the photolysis rate. Similar calculations were also carried out for broadband (290-600 nm) solar irradiation of water droplets, relevant to atmospheric conditions. These calculations show that, in agreement with previous calculations by Mayer and Madronich [B. Mayer and S. Madronich, Atmos. Chem. Phys., 2004, 4, 2241] MDRs produce only a moderate average intensity enhancement relative to the corresponding bulk-liquid slabs when averaged over a range of wavelengths characteristic of solar radiation at the Earth's surface. However, as in the case of Mo(CO)(6) in 1-decene, chemical effects may play a role in enhanced photochemistry at the aerosol-air interface for airborne particles.