Spatiotemporal Analysis of Particulate Matter Chemical Composition: Liquid Water Impacts over the United States

Abstract

Organic compounds and aerosol liquid water (ALW) are two poorly-characterized components of fine particulate matter (PM_2.5), a criteria pollutant that negatively impacts human health and affects Earth’s radiation budget. Organic compounds influence particle hygroscopicity, yet this is not well-constrained because of the difficulties inherent to characterizing the complex organic matrix, which is composed of hundreds of thousands of individual species. ALW impacts aerosol radiative properties, and aerosol growth due to water affects the formation of clouds, which are important drivers of weather and climate. Despite this, the influence of atmospheric water is minimized in field studies, laboratory experiments, and routine measurements. A quantitative understanding of organic speciation, ALW, and interactions between particle chemical components is necessary for accurate prediction of future air quality and climate. My dissertation aims to increase understanding of interactions between water and particle chemical components, especially organics. Specifically, I 1) investigate the potential of routinely-measured organic carbon (OC) and associated hygroscopicity to reconcile discrepancies between surface-measured PM_2.5 and remotely-sensed aerosol optical thickness (AOT), 2) investigate differences in chemical composition, particle hygroscopicity, and ALW between cloudy and clear sky days, and 3) analyze decadal trends in organic aerosol by combining measurements and model output to lend insights into changing organic speciation or formation pathways. I find that ALW qualitatively explains patterns in satellite-measured vertical extinction within the planetary boundary layer. Routinely-measured OC is unable to reconcile PM_2.5 and AOT measurements, largely due to organic hygroscopicity uncertainties and extinction aloft, which cannot be extrapolated from surface measurements. I also find that ALW mass concentrations are significantly altered between cloudy and clear sky days. In a case study of the Mid South region, aerosol growth due to water is highest during cloudy times, when current techniques are least able to characterize particle mass and impacts. Finally, I find that organic aerosol chemical composition in both humid and arid regions across the US has been changing over time in ways related to ALW chemistry. This work highlights the need for a quantitative understanding of water-mediated chemistry in order to accurately characterize atmospheric aerosol burden and impacts.