UC Irvine

The net effect of aerosols on the Earth’s climate is highly uncertain. Secondary organic aerosols (SOA) contribute heavily to this uncertainty due to their complex chemical composition. This uncertainty is enhanced by the fact that SOA can be physically and chemically transformed as it slowly disperses through the atmosphere, a process known as aging. These aging processes further complicate the interpretation of the climate effects of aerosols. The condensed-phase aging occurring inside aerosol particles is an active area of research, with condensed-phase photochemistry driven by ultraviolet (UV) solar radiation being an area of particular interest. Recently, a number of studies have found that condensed-phase photochemistry of SOA can have a profound effect on its chemical composition and subsequent effects on climate. However, the role of these aging processes in controlling the climate and air pollution is still highly uncertain.

In this dissertation, model SOA was generated in either an aerosol flow tube reactor or smog chamber from a variety of aerosol precursors and oxidants. Chapter 2 discusses the triplet reactivity of select carbonyl compounds that are known photosensitizers that can be found in SOA using nanosecond transient absorption spectroscopy. The photosensitized reactivity of SOA arising from the photooxidation of naphthalene was also investigated. Combining the study of these individual and known photosensitizers with those formed in the atmosphere demonstrates that tropospheric photosensitization may involve a large variety of compounds and will introduce previously unconsidered chemical pathways that impact atmospheric multiphase chemistry. Chapter 3 extends these ideas and discusses the photosensitized uptake of SO2 into naphthalene photooxidation SOA. The aerosol particles were observed to grow in size and sustain photosensitized processes resulting in the production of particle phase sulfate (0.2 – 0.3 μg m−3 h−1). As naphthalene and other polycyclic aromatics are important SOA precursors in the urban and suburban areas, these photosensitized reactions are likely to play an important role in sulfate and SOA formation.

Chapter 4 discusses the long-term photodegradation of SOA produced from biogenic and anthropogenic precursors. The experiments relied on a quartz crystal microbalance (QCM) to quantify the mass loss rate from SOA materials while being irradiated by a 305 nm UV LED. Long-term changes in the chemical composition of SOA were examined using high-resolution electrospray ionization mass spectrometry. These experiments confirm that condensed-phase photochemistry is an important aging mechanism for SOA during long-range transport. Chapter 5 builds on the result of Chapter 4, and investigates the viscosity and phase state of UV-irradiated SOA. The viscosity of UV-irradiated SOA was measured after an equivalent UV exposure of 6–14 days at midlatitudes in summer. Results indicate that the viscosity and characteristic mixing times of organic molecules within an SOA particle can be as much as five orders of magnitudes larger for aged SOA. The increase in viscosity likely leads to an increased abundance of glassy SOA particles that can act as ice nuclei in the atmosphere. Furthermore, the increase in viscosity and mixing times can impede gas-particle partitioning and heterogeneous chemistry. Overall, results clearly demonstrate that aging driven by condensed-phase photochemistry needs to be considered when predicting the environmental impacts of SOA.

Chapter 6 discusses preliminary work on quantifying the mass loss and volatile organic compound (VOC) production from ambient biomass burning organic aerosol (BBOA). BBOA was collected using an impactor onto QCM crystals during the December 2020 Santiago Canyon Bond Fire. Experiments involved irradiating the BBOA substrates using a 305 nm LED while simultaneously measuring the mass loss rates using a QCM and photoproduced VOCs using a proton transfer reaction mass spectrometer. These preliminary experiments indicate that photodegradation of ambient BBOA can be a large source of VOCs, particularly formic acid, in the atmosphere.

On the whole, this dissertation makes a strong case for the critical importance of condensed-phase photochemistry in determining climate-relevant properties of organic aerosols.