Development of Methods to Study Secondary Organic Aerosol

Citation

Huang, Yuanlong (2019) Development of Methods to Study Secondary Organic Aerosol. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/783S-K686. https://resolver.caltech.edu/CaltechTHESIS:05232019-231457899

Abstract

Secondary organic aerosol (SOA) in the atmosphere contributes significantly to air pollution and has profound impacts on regional and global climate change, as well as human health. SOA, as opposed to directly emitted particles, refers to those particles formed from oxidation of gas-phase compounds followed by nucleation and/or gas-particle partitioning, as well as those modified by gas-phase oxidants (e.g., O₃, OH radical, and NO₃ radical) through heterogeneous reactions within their lifetime in the atmosphere. Investigations of SOA formation in the laboratory have been carried out in batch reactors (e.g., environmental smog chambers) and continuous flow reactors (e.g., oxidation flow reactors). Compared with the real atmosphere, the reactors in the laboratory have boundaries and defined residence times under different operation conditions. To better constrain the experimental results and derive reliable parameters for aerosol models (e.g., yields of volatile organic compounds), a full understanding of the role of the reactors on the gas-phase components and suspended particles is needed.

In this thesis research, a number of studies were carried out to understand the role of the reactor itself on the behavior of SOA-forming systems. This includes the effect of the Teflon-walled Caltech Environmental Chamber on vapor molecules and characterization of the newly-built Caltech PhotoOxidation Flow Tube reactor (CPOT) for atmospheric chemistry studies.

Vapor-wall interactions in Teflon-walled environmental chambers have been studied; however, conflicting results existed in the literature concerning the basic timescales of vapor-wall loss in environmental chambers. The competition between vapor-particle and vapor-wall interactions determines the fate of vapor molecules in the reactor. A unified theory and empirical equations have been developed in this thesis to explain the observed vapor-wall interaction timescales. About 100 compounds have been studied to verify this theory. In characterizing the flow reactor performance, computational fluid dynamics (CFD) simulations have been combined with residence time distribution (RTD) experiments, revealing, among others, the importance of the inlet design of the reactor and the effect of temperature gradients on radial mixing in the reactor. An axial-dispersed plug flow reactor (AD-PFR) model framework was developed as a basis on which to simulate photochemistry occurring in the CPOT. An analytical solution for the cumulative RTD, which uses data during the transition period to a steady state, can be applied to diagnose the dispersion condition inside the flow rector.

Since SOA formation involves interactions among gas-phase molecules, particle surfaces, and particle bulk phases, a state-of-the-art experimental technique (field-induced droplet ionization mass spectrometry, FIDI-MS) and a comprehensive model coupling gas-surface-aqueous multiphase transport and chemical reactions have been applied to investigate the gas-phase OH-initiated oxidation of pinonic acid (PA) at the air-water interface. The interfacial oxidation mechanism has been found to differ from that of homogeneous reactions, and the kinetics depend on both OH diffusion from gas-phase to the interface and aqueous-phase reaction of pinonic acid + OH. The model calculation shows that, under typical ambient OH levels, PA is oxidized exclusively at the air-water interface of droplets with a diameter of 5 µm, demonstrating the critical importance of air-water interfacial chemistry in determining the fate of surface-active species.

Thesis Files