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.
Airborne particles damage human health, alter the Earth’s climate, and reduce visibility. Because of these effects, scientists and policymakers have worked for decades to understand and reduce the sources of particles to the atmosphere. In our research we examined a new mechanism for particle formation involving gaseous phenols emitted from wood burning. This process involves three steps: (1) phenols dissolve into a cloud or fog drop, (2) the dissolved phenols react with oxidants inside the drop to form large, low volatility products (also known as secondary organic aerosol or SOA), and (3) the SOA products remain as a particle once the water of the drop evaporates. To understand whether these types of reactions are a significant source of particles we studied the aqueous reactions of phenols with two oxidants - hydroxyl radical and triplet excited states of organic compounds - in the laboratory. We measured the rates of the phenol oxidation reactions for the five main types of phenols emitted from wood combustion, determined how efficiently these reactions form SOA, and investigated the chemical composition and evolution of the SOA products. We found that both of the oxidants, including the poorly studied triplet excited states, very rapidly oxidize aqueous phenols. Our measured SOA yields are typically near 100%, meaning that the mass of SOA formed was approximately equal to the mass of phenol reacted. We also examined the composition of the SOA products and their stability during continued atmospheric reactions. The initial products formed include dimers and higher oligomers, which are created by coupling two or more individual phenol molecules. These products are significant because they have extremely low vapor pressures and thus will exist nearly exclusively on particles rather than in the gas phase. Phenol oxidation reactions also form carbonyl species, which can absorb light and undergo secondary reactions. Subsequent aging reactions modify the initial SOA by first adding more oxygen atoms, and eventually by breaking up the phenol rings to form smaller, and more oxygenated products, some of which are volatile. As the final step of our project we put our laboratory data into a simple model to assess the potential significance of phenol oxidation as a source of SOA. The model simulated phenol chemistry under wintertime foggy conditions typical of California’s Central Valley during a wood burning stagnation event. Our preliminary results show that aqueous reactions of phenols are a rapid, significant source of SOA under foggy of cloudy conditions in areas with significant wood combustion. Excited triplet states are a major oxidant for the aqueous phenols, indicating that these little-studied oxidants can be a significant pathway for reactions in atmospheric drops and particles. Gas-phase reactions of phenols are also a source of SOA in our model, but these are much less important than the aqueous reactions. Overall, the amount of SOA formed is a significant percentage of the mass of particles directly emitted by wood combustion, thereby adding to the pollutant burden caused by burning wood and other biomass.
