Formation of Secondary Organic Aerosol by Aqueous-phase Reactions of Phenols

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Atmospheric particles have a wide range of impacts, from damaging human health to altering the earth's climate. And yet a great deal is still not known about the sources of these particles, especially for carbon-containing particles formed in the atmosphere, i.e., secondary organic aerosols (SOA). Until recently it was thought that SOA was only formed from reactions of organic gases in the atmosphere, but more recent work has shown that SOA can also be formed from reactions in cloud and fog drops and aerosol particles containing small amounts of water. One group of compounds that is likely to make a significant contribution to SOA via aqueous phase reactions are the phenols. Large amounts of phenols are emitted from the combustion of wood and other biomass through sources such as residential wood combustion and wild fires. These compounds are quite water soluble and react rapidly in solution to efficiently form large, low volatility products. Given these properties, it is likely that phenols are a significant source of aqueous-phase SOA. The few available studies of atmospheric phenols are consistent with this idea, but there has been no thorough study of the aqueous-phase reactions of phenols as a source of secondary organic aerosol. The research will thoroughly characterize aqueous phenol reactions as a source of SOA and will estimate the contribution of this pathway to the atmospheric burden of particles. This will be accomplished through four objectives: 1) Measure the rates of SOA formation from aqueous phenols under a variety of conditions to mimic both cloud/fog drops and aqueous aerosol particles; 2) Quantify the yields of SOA from aqueous-phase reactions of phenols; 3) Chemically characterize the SOA using three complementary techniques; and 4) Use the rate, yield, and characterization data to estimate the significance of phenol oxidation in aqueous phases as a source of SOA. Most of the research will be performed using well-defined laboratory solutions of individual and mixed phenols studied in the laboratory using specific reaction pathways that are important in atmospheric water drops and water-containing particles. The formation of SOA from aqueous reactions of the complex mixture of gases emitted from the combustion of wood will also be studied. SOA yields will be determined using gravimetric mass, high-resolution Aerosol Mass Spectrometry (HR-AMS), and Fourier Transform Infrared spectrometry (FTIR). These latter two techniques, in conjunction with liquid chromatography coupled to high-resolution mass spectrometry, will also be used to characterize the elemental, functional group, and molecular characteristics of the SOA.Three graduate students will be mentored through the research. The principal investigators will also integrate some of the approaches and results from this research project into their undergraduate and graduate teaching, including a new graduate course on advanced atmospheric instrumentation. The project will also include an outreach component in conjunction with the University of California - Davis Air Quality Research Center: students (primarily in middle school) will come to campus for a one- or two-day program to receive hands-on lessons about atmospheric particles and air pollution.