UC Irvine

Despite a wealth of available techniques and preexisting data to measure and estimate atmospheric aerosol abundance, sources, and formation, many uncertainties persist. These uncertainties, in particular for aqueous aerosol and polar water-soluble organic gases (WSOCg), hinder accurate prediction of atmospheric fine particulate matter (PM2.5) and its subsequent health and radiative impacts. Emissions of ammonia (NH3), a key precursor to the formation of hygroscopic aerosol mass, are poorly constrained relative to other controllable sources of aerosol precursors such as sulfur dioxide (SO2) and nitrous oxides (NOx). Aerosol liquid water (ALW) is a ubiquitous component of aerosol mass that directly impacts light extinction and radiative forcing, and facilitates the formation of aqueous secondary organic aerosol (aqSOA). ALW is difficult to directly measure at ambient conditions, and observational estimates are subject to errors and biases in measurements of speciated aerosol mass and meteorological variables. Partitioning of WSOCg to the aqueous phase and aqSOA formation are modulated by ALW content. Ambient WSOCg is rarely measured, and changes in the solubility of gas-phase inventories as a function of changing chemical climatology has not been systematically evaluated. My dissertation aims to utilize holistic approaches, incorporating publicly accessible datasets, remote sensing, field measurements, and modeling to understand sources and spatiotemporal trends in aqueous aerosol and its physicochemical properties. Specifically, I (1) relate surface-level particulate air quality and animal inventories with satellite retrievals of NH3 to examine air pollution from concentrated animal feeding operations (CAFOs) in the contiguous U.S.; (2) evaluate the sensitivity of thermodynamically-derived ALW observational estimates to meteorological and chemical variables; and (3) investigate WSOCg ambient concentrations in NOx-limited and NOx-saturated chemical regimes. Remotely sensed ammonia is spatially and temporally linked to animal unit density at medium-to-large sized CAFOs, with co-located increasing trends noted from 2002 to 2017. It is difficult to quantify the impacts on PM2.5 mass in locations where animal density and ambient NH3 is highest due to a dearth of regulatory monitors. Yet, results suggest air quality improvements lag behind the national average in rural agricultural regions. ALW predictions are more sensitive to ammonium and dust ions than to tested meteorological variables (temperature, relative humidity). Despite uncertainties, long-term trends of ALW mass consistently exhibit decreasing trends across the contiguous U.S. due to decreasing mass concentrations of hygroscopic aerosol species. Application of ALW observational estimates helps explain: (1) aerosol optical depth and Ångström exponent-derived size during cloudy and clear sky conditions in Bondville, IL; and (2) changes in surface temperature modulation related to shortwave radiation in the Southeast U.S. warming hole region over two decades. WSOCg mass concentrations in Irvine 2024 (NOx-limited), are significantly less than those in Pasadena 2010 (NOx-saturated). Differences in VOC emissions and actinic flux contribute to differences in WSOCg abundance between locations. WSOCg concentrations are positively correlated with NOx and O3 in Pasadena. In sharp contrast, Irvine WSOCg is not correlated with NOx, and is negatively correlated to O3. EPA’s Community Multiscale Air Quality model (CMAQ) simulations of WSOCg, NOx, and O3 fail to accurately reproduce observed associations in both Pasadena and Irvine. These findings suggest WSOCg abundance is related to O3 production chemical regimes, and that the solubility of VOC oxidation products may change with reduced anthropogenic emissions. The results of this dissertation demonstrate the ability of multidisciplinary approaches to understand ambient aerosol water, and highlights the need to chemically characterize gas- and aerosol-phase atmospheric trace species.