Mechanisms of atmospheric secondary aerosol formation in the gas and aqueous phase

Abstract

Investigations into the processes driving aerosol formation and evolution are critical because aerosols impact climate and human health. Specifically, uncertainty exists regarding aerosol formation from the oxidation of functionalized volatile organic compounds and the role aerosols and cloud droplets play in providing a substrate for reactions. This dissertation details experiments and analyses that examine aerosol formation via gas-phase oxidation and aqueous-phase kinetics studies relevant to aerosol evolution. Chapter 2 examines the impact of ether and hydroxyl functional groups on the oxidation and resulting aerosol production from the volatile glycol diether, 2-(2-ethoxyethoxy)ethanol (2-2-EEE;carbitol). We conducted chamber studies as part of the Secondary organic aerosol Chamber Experiments on Non-Traditional Species (SCENTS) project and monitored the chemical evolution with a suite of gas and particle-phase instruments. Our chamber observations indicate that highly functionalized hydroperoxy carbonyl products contribute to aerosol production. We use these observations in conjunction with existing structure-activity relationships to propose oxidation mechanisms and use box modeling to demonstrate how the fate of peroxy radicals (RO2) determines 2-2-EEE fate. We detail how the oxygen-containing functional groups promote RO2 H-shifts, making them competitive with bimolecular reactions and providing a pathway to low-volatility products and aerosol formation. Chapter 3 expands on the work of Chapter 2 to compare the oxidation and resulting aerosol production of branched glycol diethers to their linear counterparts. Our results show that branched glycol diethers have decreased yields of SOA mass and low-volatility products compared to linear species. We combine chamber observations with simple box models to propose oxidation mechanisms, highlighting ways branching alters oxidation. Specifically, we discuss how branching alters the fate of the first RO2 formed, the alkoxy radical fate, and the fate of RO2 formed after carbonyl addition to the parent glycol diether. We detail how the presence of branching decreases RO2 H-shift prevalence, how this decrease is exacerbated by the addition of a carbonyl, and how the specific location of the branch influences RO2 fate. Chapter 4 summarizes experiments quantifying the oxidation of methanesulfinic acid by ozone in the aqueous phase. We outline two approaches to study the kinetics of this reaction using relative rates: a multiphase system and a bulk aqueous phase system. We report a rate of 1.8(±2) × 107 M−1 s−1 using the bulk aqueous phase approach and detail how factors such as surface activity may have hindered rate determination with our multiphase approach.