Wildfires have shown an increase in occurrence and severity in recent years. These biomass burning events release volatile organic compounds (VOCs) and particulate matter in the atmosphere which impact air quality, human health, and Earth’s radiative balance. The oxidation of these wildfire-emitted VOCs at nighttime by the nitrate radical (NO3) can lead to the formation of secondary organic aerosol (SOA) and light-absorbing brown carbon (BrC). Despite their significance, the understanding of NO3-initiated oxidation mechanisms of wildfire-emitted VOCs leading to SOA and BrC in previous studies have been very limited. The major SOA constituents from these reactions have remained a challenge to uncover largely due to limitations in analytical techniques used to uncover the gas- and particle-phase chemical composition. In the following projects, we use a suite of analytical methods to elucidate the major reactions leading to the formation of SOA and BrC from representative wildfire-emitted VOCs. In Chapter 2, we studied the NO3 oxidation of seven phenolic VOCs. From this study, we discovered many products that were previously undiscovered from this system and found nitrophenol products were very dominant adding to evidence that these are a major class of compounds responsible for significant light absorption in BrC. We also found evidence of diphenyl ether dimers forming from NO3 oxidation of each of the studied phenolic VOCs. In Chapter 3, we investigated the NO3 oxidation of limonene. From this project, we discovered that the primary nitrooxy peroxy radical formed from limonene can rapidly undergo autoxidation leading to highly oxidized organonitrates. We also identified the formation of several dinitrate compounds highlighting the importance of sequential oxidation for limonene. In Chapter 4, we investigated the NO3 oxidation of selected N-containing heterocyclic VOCs: pyrrole, 1-methylpyrrole (1-MP) and 2-methylpyrrole (2-MP). From the observed product distribution in the SOA from these systems, we concluded that the presence of an easily abstractable hydrogen in precursor structure regulates the mechanism of initial NO3 oxidation and has significant effect on light absorption of the SOA. Furthermore, we propose a novel gas-phase mechanism for the addition of three NO2 groups to the backbone of pyrrole.

Organic aerosols (OA) contribute to a substantial fraction of atmospheric fine particulate matter (PM2.5) that significantly impact the Earth’s radiative forcing, air quality, and human health. OA are formed from direct emissions or by secondary formation through oxidation of volatile organic compounds (VOC). Once formed in the atmosphere, OA can undergo various chemical and physical evolution throughout their lifetime on the order of ~ 10 days, constantly changing their compositions and properties. Thus, elucidating these changes is crucial for understanding their environmental impacts, and a comprehensive suite of state-of-art mass spectrometry techniques have been coupled to achieve an isomer-resolved picture of the OA compositions during the evolution. The dissertation focuses on examining (1) chemical aging at the gas-particle interface initiated by gas-phase oxidants (e.g., O3 and OH), and (2) evolution in the condensed-phase caused by ambient temperature and relative humidity (RH) change. The following projects have been studied to address underexplored scientific questions on OA chemical evolution and transformation, including: (I) Criegee intermediate dynamics during heterogeneous ozonolysis of endocyclic unsaturated OA proxies were examined to highlight the importance of particle-phase water and the rapid later-generation reactions that together govern the products. (II) Interfacial dimer formation during the heterogeneous OH-initiated oxidation of OA surrogates were demonstrated, elucidating new mechanisms of dimerization by organic radical (i.e., peroxy and alkoxy radicals) cross reactions at the gas-particle interface. (III) OA molecules with the same formula but different branching structures could affect the overall heterogeneous OH oxidation and control the oxidized OA composition via site-specific mechanisms, which results largely distinct functionalization, fragmentation, and oligomerization. (IV) Exploration of SOA from α-pinene ozonolysis in a smog chamber as the temperature and RH cycle determined that changing ambient conditions can dictate kinetics and the extent of gas-particle partitioning and impact diverse reversible and irreversible reactions between monomers and oligomers during the changes. (V) Influence of heating on the composition of OA was investigated because thermal desorption of OA is often applied to vaporize the particles for subsequent real-time mass spectrometry analysis. How thermal desorption leads to chemical decomposition of the OA constituents were studied.

The Earth’s atmosphere is composed of an enormous variety of chemical species associated with trace gases and aerosol particles whose composition and chemistry have critical impacts on the Earth’s climate, air quality, and human health. Reactive volatile organic compounds (VOCs) and organic aerosols (OAs) in the atmosphere can be oxidized by oxidants in the atmosphere through a number of pathways, generating critical intermediate products such as the peroxy radicals (RO2•), as well as the subsequent closed-shell oxygenated species which are a highly variable class of organic mixtures with diverse functional groups, such as ketone, alcohol, carboxylic acid, hydroperoxide, etc. Due to the significance to atmospheric environment, it is imperative to elucidate RO2•-centered chemistry and understand chemical compositions of the resultant OAs on the molecular and even isomeric level. Mass spectrometry analysis as a powerful and popular analytical technique has been widely developed and applied in atmospheric chemistry for decades. In combination with a comprehensive set of mass spectrometry instrumentation and kinetic simulations, this dissertation aims to help better understand OA formation and evolution and provide new insights in the realistic understanding of atmospheric components.Chapter 1 introduces and reviews recently developed mass spectrometry techniques that allow for effective detection, identification, and quantification of a broad range of organic and inorganic chemical species with high sensitivity and resolution. Chapter 2, 3, 4 focus on probing RO2•-centered bimolecular reactions in the gas phase and the particle phase that have been understudied in previous research. In Chapter 2, we demonstrate interferences caused by secondary ion chemistry in iodide-adduct chemical ionization mass spectrometry (I−-CIMS) which has been a popular analytical technique to measure a wide range of oxygenated VOC (OVOCs) due to its low selectivity. Moreover, we apply the secondary ion chemistry to inform OVOCs’ functionalities and hence formation mechanisms. Chapter 3 illustrates that condensed-phase bimolecular autoxidation largely accelerates OA aging under atmospheric oxidant concentrations and impacts aerosol compositions, reversing a conventional view that multiphase (i.e., heterogeneous) oxidative aging is a slow process. Chapter 4 expands the heterogeneous study to the organic hydroperoxide formation from RO2• + HO2• reactions at aerosol particle interface via hydrogen-deuterium exchange mass spectrometry. This dissertation provides novel advancements in mass spectrometry applications and a better understanding of the atmospheric chemical mechanisms. It may help interpret existing datasets, develop effective experimental approach to simulate atmospheric OA processes, and improve models to accurately predict OA transformation in the atmosphere.