New Measurement Approaches to the Study of Secondary Organic Aerosol

Abstract

Aerosol particles constitute a class of atmospheric pollutants that are detrimental to human health and influence the Earth’s climate. A significant fraction of aerosol mass is composed of organic material, produced by photochemical reactions of organic trace gases which form secondary organic aerosol (SOA). However, the large diversity of volatile organic compounds (VOCs) makes it challenging to identify all of the chemical reactions contributing to SOA formation. In addition to this chemical complexity, our ability to identify and measure all of the relevant organic compounds, especially species present in aerosol particles, is limited by challenges in efficiently sampling and detecting the various classes of molecules formed. Improving knowledge of the chemical behavior of aerosol will improve our ability to predict how changing emissions and chemical conditions will impact the formation and properties of particulate matter in the future. This thesis will explore recent improvements in instrumentation and measurement techniques and apply them to laboratory studies of organic carbon and SOA. First, we adapt a technique for measuring total suspended carbon to laboratory chamber experiments, converting organic compounds with high temperature catalysis to carbon dioxide, which is then monitored in real time. This allows for a “top-down” constraint on the overall concentration of all organic species (including SOA) as experiments proceed, as some lower volatility products are lost to the surfaces of the laboratory chamber. Second, we compare the measurements of SOA from three chemical ionization mass spectrometers using different ionization and desorption methods to detect particle-phase species. Clear differences emerge in the detected formulas across instruments, highlighting variations in chemical sensitivities to different classes of compounds and the influence of fragmentation on the detected products. Finally, we explore how changing peroxy radical fate influences SOA formation by monitoring SOA composition with extractive electrospray ionization (EESI) mass spectrometry. Differences in particle-phase products, particularly nitrates, hydroperoxides, and dimers, make the dependence of initial SOA composition on peroxy radical pathways clear. Over time, we observe a convergence of SOA spectra formed under different peroxy radical regimes, suggesting the influence of secondary products and particle-phase chemistry, though some differences persist from the initial gas-phase peroxy radical fate. Overall, this thesis demonstrates improved tools for constraining and investigating VOC oxidation pathways leading to particle-phase organic species.