UC Davis

Ground-level ozone (O3) is widely recognized as a significant air pollutant that affects public health across the globe. O3 is formed from precursor emissions of oxides of nitrogen (NOx) and volatile organic compounds (VOCs) that react in the atmosphere in the presence of sunlight. Understanding the source contributions to O3 formation is a critical step in the design of efficient O3 control strategies. However, due to the nonlinear nature of O3 chemistry, it is challenging to identify the major source contributions to O3 concentrations. The research in this thesis addresses such issue by using O3 source apportionment calculations within a chemical transport model (CTM), which is a specialized approach to resolve the source contributions to O3.In Chapter 2, a novel O3 source apportionment technique is developed to explicitly resolve the contributions from both NOx and VOC sources to O3 formation. The new technique is flexible and can be configured to identify the original source of precursors that contribute to O3 formation or the most recent source depending on the choice of the tagging method. The detailed features of the new technique are demonstrated during a peak O3 event in September 2010 in Los Angeles, California, while trends in O3 source contributions over time are evaluated during two additional simulations in July 2005 and August 2015. Quality control checks show that the new source apportionment methodology does not alter predicted total O3 concentrations, and the detailed source apportionment information can be averaged to yield results that are consistent with traditional O3 source apportionment calculations. The detailed O3 source apportionment results during Sept 2010 show that the major NOx sources that contribute to O3 are gasoline and diesel engines from on-road vehicles, diesel combustion in off-road engines, and soil-derived NOx, while the majority of VOCs are VOCs emitted from upwind boundary conditions that are outside the domain and biogenic VOCs within the domain. The formaldehyde to NO2 ratio (FNR) suggest that the chemical regime was VOC-limited, but given the uncontrollable nature of the identified major VOC emissions, the source apportionment results suggest that NOx emission controls would have been the preferred emission control strategy to reduce O3 concentration in Los Angeles at that time, with the understanding that some period of O3 disbenefits would need to be tolerated until the emissions control program shifts the atmospheric chemistry back into the NOx-limited regime. In Chapter 3, the newly developed O3 source apportionment is applied to resolve a puzzle found in Zapata et al. (2018a) that O3 concentrations within major California cities in the year 2050 are predicted to increase and continue to violate the 8-h O3 standard despite the adoption of low-carbon energy. This O3 penalty is largely due to the persistent NOx-rich chemistry within urban cores that causes O3 concentrations to increase when traditional combustion sources are replaced by renewable energy sources and reductions in NOx emissions are not enough. By simulating Zapata’s case again but employing a CTM equipped with the novel O3 source apportionment technique, the major sources that contribute to the O3 violations are identified and three cumulative Supplemental Control Steps are developed based on Zapata’s low-carbon energy scenario. All three Control Steps successfully lower O3 concentrations significantly in the South Coast Air Basin (SoCAB), with residual O3 violations mainly predicted in non-populated wilderness areas. The significant reductions in O3 produced by the Supplemental Control Steps decrease the predicted incidence of hay fever/ rhinitis, asthma, and all-cause mortality by an order of magnitude compared to the previous scenario where low-carbon energy is adopted. The predicted public health savings associated with the Supplemental Control Steps are valued at $19.70-30.36B/yr. Chapter 4 extends the scope of Chapter 3 by analyzing the environmental justice aspect within the Supplemental Control Steps designed in Chapter 3. The exposure disparities, defined as departures from the population average exposure, are calculated for O3 and NO2 in Southern California. Future air quality fields are simulated using a CTM under five emission scenarios including the business-as-usual (BAU) scenario, Greenhouse Gasses reduction (GHGAi) scenario, and the three Control Steps. We find that traditional O3 control strategies reduce O3 exposure disparities by <1.6 % and reduce NO2 exposure disparities by <9 % in Southern California. For the Black and African residents living in the urban core of Los Angeles, the relative NO2 exposure disparities increase from +23.1 % to +66.2 % and O3 exposure disparities increase from -3.3 % to +0.1 % due to NOx emissions reductions mainly in outlying regions and the NOx-rich environment in the urban core. To reduce the exposure disparities, additional analysis is performed with complete elimination of NOx emissions from Los Angeles International Airport (LAX), which is not practical but able to reduce the NO2 exposure disparities by up to 50 %.