Insights into the Sources of Atmospheric Aerosols and Greenhouse Gases in California

Citation

Schulze, Benjamin Christopher (2023) Insights into the Sources of Atmospheric Aerosols and Greenhouse Gases in California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zay2-1v59. https://resolver.caltech.edu/CaltechTHESIS:06012023-202236185

Abstract

A substantial fraction of atmospheric science research is motivated by uncertainties in the sources of urban particulate matter and greenhouse gases. Such a focus is justified, as particulate matter exposure is responsible for up to nine million annual premature deaths globally, while climate change is rapidly altering ecosystems across the world. Recognizing the urgency of these interrelated problems, regulatory agencies in the U.S. and elsewhere have sought to limit emissions contributing to air quality degradation and global warming.

In this dissertation, we use a combination of ambient measurements, statistical models, and computational models to identify the sources of urban particulate matter and methane in multiple locations in California. In Los Angeles, we investigated the effects of reductions in mobile source pollutant emissions (i.e., on-road and off-road vehicles) on ambient aerosol concentrations. Mobile sources have historically accounted for the dominant fraction of urban particulate matter in Los Angeles, but despite notable reductions in their emissions over the last decade, ambient aerosol concentrations have not declined appreciably. Measurements using an Aerosol Mass Spectrometer demonstrate the complex interplay of direct (i.e., intended) and indirect effects of simultaneous reductions in organic aerosol (OA) precursor and nitrogen oxide emissions from these sources. Mobile sources are found to account for a modest and declining fraction of the total aerosol burden, while the contributions of non-traditional sources such as volatile chemical products (e.g., paints and coatings, cleaning products, adhesives and sealants) have increased. Simulations of organic and inorganic aerosol formation informed by in-situ measurements are developed to identify possible targets of future regulatory efforts.

In the San Joaquin Valley, we used airborne measurements of methane fluxes to evaluate dairy emissions inventories used by state regulatory agencies for policy development. Dairy operations currently account for nearly half of the state’s methane emissions, and recent legislation has mandated a 40% reduction in emissions by 2030. Observed methane fluxes align well with emission inventory predictions and demonstrate the utility of airborne flux measurements to track emission reduction progress in the future. Factor analysis of a combined dataset of greenhouse gas and volatile organic compound concentrations indicates dairy operations account for ~65% of total methane emissions in the southern San Joaquin Valley, with the remainder attributed to fugitive oil and gas emissions.

Thesis Files