UC Irvine

Atmospheric aerosols, including secondary organic aerosols (SOA), are ubiquitous constituents in the atmosphere. During their transport over hundreds of kilometers from their sources, aerosols can undergo a variety of chemical and physical changes depending on the atmospheric conditions and interactions with other compounds. The SOA formation and aging processes–including the influence of acidity on these processes–are the leading sources of uncertainty in aerosol radiative forcing in global climate models. The main objective of this thesis is to explore the influence of sulfuric acid on the chemical composition and optical properties of organic aerosols. This is achieved through laboratory studies that offer a detailed molecular-level understanding of the interactions between monoterpene oxidation products and inorganic compounds like sulfuric acid (H2SO4). Exploring the chemical reactions between organic aerosols and acids is crucial for understanding the evolution of SOA in the atmosphere.

The effect of water vapor and aerosol liquid water on the molecular composition of SOA derived from biogenic volatile organic compounds was studied to understand the chemical changes that can occur in SOA particles in the absence of sulfuric acid. SOA was generated and then aged by dissolving it in liquid water and keeping it in solution for 1-2 days or exposing it to water vapor for 1-2 weeks. The composition was monitored with mass spectrometry, and patterns of peak intensities and observed molecular formula were examined for evidence of water-mediated chemistry in the dark. The presence of water led to changes in the chemical composition of these particles, but the extent of these changes was small, suggesting that hydrolysis and hydration is not a major aging mechanism for atmospheric aerosols.

The second project discussed in this thesis tested the effect of sulfuric acid on the chemical composition and optical properties of α-pinene SOA. α-Pinene SOA was formed in an oxidative flow reactor and then aged in aqueous solutions containing various concentrations of H2SO4. Composition changes were analyzed using mass spectrometry, while optical properties were studied using UV-Vis and fluorescence spectroscopy. This study found that SOA aged in moderately (pH 0 to 4) acidic conditions experienced relatively small changes in composition, while SOA aged in a highly (pH -1 to 0) acidic environment experienced more dramatic changes in composition. The aged SOA had compounds containing sulfur (accounting for 30% of the relative sample), as well as light-absorbing and fluorescent compounds. The findings in this study demonstrated that concentrated sulfuric acid plays a crucial role in the chemical composition and optical properties of aerosols in regions where high concentrations of H2SO4 persist, such as the upper troposphere and lower stratosphere.

To gain improved mechanistic understanding of the SOA acid-aging processes, two common monoterpene oxidation products from α-pinene, cis-pinonic acid and cis-pinoaldehyde, were then aged in highly concentrated sulfuric acid. All resulting reactions were analyzed using UV-Vis spectroscopy, high resolution mass spectrometry, and nuclear magnetic resonance spectroscopy to identify the products formed. Under highly concentrated sulfuric acid, it was found that cis-pinonic acid forms homoterpenyl methyl ketone, while cis-pinonaldehyde forms two regioisomers of 1-(4-isopropylcyclopenta-1,3-dien-1-yl)ethan-1-one.

The effect of highly concentrated sulfuric acid on the fluorescent properties of SOA from volatile organic compounds of anthropogenic and biogenic origin was investigated in the last study of this thesis. SOA was created with different combinations of oxidants and VOC precursors, followed by the analysis of absorption and fluorescence. The appearance of strongly light-absorbing and fluorescent compounds at pH = ∼ -1 confirms that sulfuric acid is a major driver of SOA aging. The aged SOA from biogenic precursors (d-limonene and α-pinene) resulted in stronger fluorescence than aged SOA from anthropogenic toluene and xylene. The absorption spectra of aged SOA from biogenic precursors changed drastically in shape upon dilution, whereas the shapes of the fluorescence spectra remained the same, suggesting that fluorophores and chromophores in SOA are separate sets of species. Additionally, the fluorescence spectra of aged SOA and primary biological aerosol particles exhibited significant overlap, indicating that when subjected to highly concentrated sulfuric acid environments, aged SOA may be mistakenly identified as primary biological aerosol particles using fluorescence-based techniques.

SOA can exist under a range of acidities, providing a range of acid-catalyzed and acid-driven reactions. This thesis demonstrates that sulfuric acid is a strong driver in aging mechanisms for biogenic and anthropogenic SOA. By integrating these laboratory studies along with future field measurements and model predictions, informed decisions can be made to tackle the environmental issues associated with air quality and climate change.