Single Particle Microscopic and Spectroscopic Chemical Analysis of Primary and Secondary Aerosols

Abstract

Atmospheric aerosols have significant impacts on climate and human health. Particle physicochemical properties including chemical composition, distribution of chemical species (i.e. mixing state), morphology, and phase have been tied to their climate and health-related effects, yet our understanding of these properties is still limited. In this dissertation, aerosol particles from the southeastern U.S. were collected and studied using single particle microscopy and spectroscopy methods. In addition, new methods were developed to improve our understanding of their chemical composition and to better predict climate-relevant properties. The Southern Oxidant and Aerosol Study (SOAS), completed in the summer of 2013, was held at a rural, forested location in Alabama impacted by regional pollution. Particles collected during SOAS were analyzed using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), Raman microspectroscopy, and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure (STXMNEXAFS) to identify particle sources and degree of chemical aging during transport. Although mixed organic carbon with ammonium sulfate particles dominated number concentrations during SOAS, time periods with high concentrations of mineral dust and sea spray aerosol (SSA) were identified. Chemical mixing state calculations for submicron and micron-sized particles quantified the degree of internal mixing during three distinct time periods, showing that the degree of aerosol aging varied throughout SOAS. Additionally, during two SSA-rich events, SSA was frequently aged by reactions with nitric acid and sulfuric acid leading to chloride depletion within particles, though 24 % of SSA still contained chloride. The frequent observation of SSA at this inland site and the range of chloride depletion observed suggest that SSA may represent an underappreciated inland sink for NOx/SO2 oxidation products and source of halogen gases that can act as oxidants. To more thoroughly characterize organic aerosol from SOAS, a Raman fingerprint was identified for organosulfates derived from isoprene oxidation products, a significant component of secondary organic aerosol (SOA) in the southeastern U.S. In this analysis, the vibrational modes of key organosulfates were identified using Raman microspectroscopy and density functional theory (DFT), allowing organosulfates to be distinguished from inorganic sulfate within complex ambient SOA. Complementary to Raman microspectroscopy which provides molecular information for particles > 1 µm, atomic force microscopy-infrared spectroscopy (AFM-IR) was applied to aerosol particles for the first time to detect trace organic and inorganic species and probe chemical variation within individual particles down to 150 nm in diameter. With its high spatial resolution, AFM-IR has the potential to advance our understanding of aerosol impacts on climate and health for particles < 500 nm in diameter by improving analytical capabilities to study water uptake, heterogeneous reactivity, and viscosity. Finally, organicsulfate SOA particles collected during SOAS were observed to have liquid-liquid phase separation with a range of internal morphologies, including core-shell and more complex internal morphologies. Backward air mass trajectory modelling indicated that SOA morphology was dependent on aerosol lifetime, as well as temperature and relative humidity history. Taken together, these analyses of aerosol particle sources, atmospheric aging, internal morphologies, and chemical compositions provide an increased understanding of particle chemistry, mixing state, and internal structure in the southeastern United States.