The Role of Acidity, Viscosity, and Morphology on Atmospheric Aerosol Physicochemical Properties and Impacts

Abstract

Atmospheric aerosol plays a critical role in Earth’s climate by scattering or absorbing solar radiation, acting as cloud condensation and ice nuclei, and impacting air quality and public health. The physicochemical properties of aerosols dictate their climate and health impacts yet are challenging to measure accurately and quantitatively due to the complex nature of atmospheric aerosol. Specifically, the chemical composition, size, morphology, acidity, and viscosity have great interparticle variation. Methods enabling detailed quantitative investigation of individual aerosol properties are needed to understand the chemical transformation and climate effects of atmospheric aerosol. In this dissertation, atmospherically-relevant aerosol particles were examined using various state-of-the-art microspectroscopic techniques to measure the acidity, morphology, and viscosity of individual submicron particles, allowing better prediction of the climate and health impacts. The acidity of aerosol is a critical property that affects the chemistry and composition of the atmosphere. However, there are challenges with quantifying aerosol acidity in individual particles due to the extremely small volumes of fine aerosol particles that have limited pH measurements. A novel single-particle acidity measurement was explored using the degradation of a pH-sensitive polymer. Submicron particles of known pH values (0 or 6) were deposited on a polymer thin film to erode the film. Particles were then rinsed off and the degradation of the polymer was characterized using atomic force microscopy (AFM) and Raman microspectroscopy. Acidic particles (pH=0) caused the polymer to degrade while near neutral particles (pH =6) did not. As particle size decreased, polymer degradation increased, indicating an increase in aerosol acidity at smaller particle diameters. To further understand the impacts of aerosol acidity on the formation and evolution of secondary organic aerosol (SOA), inorganic sulfate particles with varying acidities (pH 1, 2, 3, and 5) reacted with gaseous isoprene-derived epoxydiols (IEPOX) for a range of times (30, 60, and 120 minutes). The morphology and chemical composition were systematically characterized at a single-particle level using AFM with photothermal infrared spectroscopy (AFM-PTIR) and Raman microspectroscopy. Core-shell morphology of SOA particles was observed under acidic conditions after the IEPOX uptake and increasing aerosol acidity led to an increase in SOA viscosity and higher yield of organosulfates. These physicochemical properties have the potential to significantly alter the climate properties of the SOA particles. To examine the effects of physicochemical properties in more complex atmospheric particles, the morphology and viscosity of submicron SOA from four different volatile organic compounds precursors (α-pinene, β-caryophyllene, isoprene, and toluene) were characterized before and after exposure to IEPOX. Dramatic morphological modifications were observed after the reactive uptake of IEPOX. SOA derived from α-pinene and β-caryophyllene were less viscous after IEPOX reactive uptake, while the viscosities did not change for isoprene and toluene-derived SOA. Additionally, a new glass transition temperature measurement was developed to reveal the viscosity of individual particles. The glass transition temperatures of atmospheric particles were measured for the first time under ambient atmospheric conditions using AFM-PTIR with thermal analysis. The methods developed in this dissertation and their application to the study of atmospheric aerosol yield a wide range of possibilities to connect the physicochemical properties of aerosol particles with their chemical transformation processes and climate effects. Such characterization of individual submicron SOA particles provides new insights into the multiphase atmospheric processes and the ice nucleation/cloud formation of complex particles in the atmosphere.