Molecular Composition, Volatility, and Formation Mechanisms of Biogenic Secondary Organic Aerosol

Abstract

The research herein seeks to answer the following question: what are the main processes governing the formation and properties of secondary organic aerosol (SOA) from oxidation of biogenic volatile organic compounds (BVOC) in both remote and anthropogenically influenced regions? To study this gas-to-particle conversion, a Time-of-Flight Iodide-adduct chemical ionization mass spectrometer has been coupled to a newly developed inlet manifold, the Filter Inlet for Gases and AEROsols (FIGAERO), which provides alternating, online, chemically speciated measurements of both the gas- and particle-phases. The FIGAERO also provides information on effective vapor pressures of each detected composition which can be used to test gas-particle partitioning theories or group contribution methods and thus gain insights into the chemical and physical states of the aerosol. This allows the chemical composition and volatility of bulk aerosol and individual compounds to be examined. By applying this field-deployable instrumentation to a variety of environments, in particular atmospheric simulation chamber experiments, where individual reactant and oxidant pairings can be targeted and examined, as well as the ambient atmosphere, a piece-by-piece molecular understanding of SOA formation from BVOC can be generated. This work is explicitly focused on the oxidation of isoprene and α-pinene, the two most abundantly emitted BVOC globally which have been shown to form SOA via distinctly different mechanisms, in atmospheric simulation chambers. In chapter 2, I present measurements of a low volatility compound, C5H12O6, that plays a significant role in SOA formation via gas-particle partitioning and that the presence of NOx, a common anthropogenic pollutant, will decrease the amount of SOA formed by this mechanism. In chapter 3, I apply a detailed mechanistic model to these measurements and find that the pathway to very low volatility material is minor under typical atmospheric conditions. Instead, autoxidation is a more significant pathway in the atmosphere, resulting in higher volatility material than bimolecular reactions. Chapter 4 focuses on the volatility and phase state of SOA from α-pinene ozonolysis. I found that the physical age of the aerosol, as opposed to the oxidative age, determines the volatility of the aerosol and that reversible oligomerization must be invoked to model our observations.