Aqueous reaction kinetics and secondary organic aerosol formation from atmospheric phenol oxidation.

摘要

Organic aerosols (OA) are a dominant fraction of particulate mass in the atmosphere, and much is secondary in nature. Secondary organic aerosol (SOA) is formed in the atmosphere from volatile organic compound precursors. Traditional SOA formation pathways involve primarily gas-phase processes: Oxidation reactions of organic gases result in low-volatility products that condense to the particulate phase, increasing aerosol mass. However, in recent years heterogeneous processes, including aqueous reactions, have gained more attention as gas-phase processes often fail to accurately predict observed mass loadings of aerosol in the atmosphere. Aqueous SOA formation is the result of a volatile organic species partitioning to the aqueous phase (clouds, fogs, aqueous aerosols), where they are chemically converted into a non-volatile species that remains in the particulate phase upon water evaporation. In this work we explore the aqueous chemical reaction kinetics and the SOA formation potential of phenols, which are released in large quantities from biomass combustion.;Phenols are a broad class of organic compounds with intermediate volatilities (102 -- 106 microg m-3 at 20°C) and moderate to high Henry's Law Constants (103 -- 10 9M atm-1), indicating significant partitioning to atmospheric aqueous phases. We begin in chapters 2 and 3 by investigating the aqueous oxidation of the compounds phenol (compound with formula C6H 5OH), guaiacol (2-methoxyphenol), syringol (2,6-dimethoxyphenol), and three dihydroxybenzenes (catechol, resorcinol, hydroquinone). For each phenol we examined reactions with two oxidants: hydroxyl radical (*OH) and the triplet excited state of 3,4-dimethoxybenzaldehyde, which is also emitted from biomass combustion. Triplet excited states (3C*) have been widely studied in surface waters (oceans and lakes) but are a novel oxidation pathway in atmospheric aqueous phases. The precursors for triplet excited states are essentially brown carbon: organic molecules high amoutns of conjugation (or nitrogen hetero atoms) that can absorb solar radiation, resulting in an excited molecule with a high oxidative potential. We find that the 3C*-mediated aqueous oxidations of phenols are rapid and can dominate over *OH at low pH (< 5), with *OH becoming dominant as pH increases. Oxidation of phenols by •OH and 3C* both efficiently produce low-volatility products, with mass yields of aqueous SOA that approach unity.;In chapter 4 we expand our investigation to the aqueous chemistry of phenolic carbonyls, another major class of phenols. These compounds possess a carbonyl functional group in conjugation with the aromatic ring of a phenol. This conjugation allows these compounds to absorb large amounts solar radiation, and as a result their dominant aqueous sink is rapid direct photodegradation. We found that these compounds also form a triplet state that can oxidize non-carbonyl phenols. As in chapters 2 and 3, the direct photodegradation of phenolic carbonyls efficiently produces low-volatility compounds, with SOA mass yields ranging from 60%--120%.;Chapter 5 takes the reaction kinetic and SOA mass yield results from chapters 2-4 and combines them into a simple model comparing aqueous- and gas-phase SOA formation during a heavy biomass burning event in Bakersfield, CA. This model uses gas- and aqueous-phase concentrations of typical atmospheric oxidants (*OH, 3C*, ozone) and measured phenol concentrations for foggy or deliquesced aqueous aerosol water conditions. We find that under heavy biomass burning and foggy conditions, aqueous reactions are the dominant source of phenol-derived aerosol. Under aqueous aerosol conditions, the amounts of gas-phase and aqueous-phase SOA formed are comparable. Overall, this work demonstrates that aqueous-phase oxidation of phenols can be a significant source of aerosol mass during fog and cloud events, and suggests the same is true (though to a lower extent) within aqueous aerosols.