Taekyu Joo Ph.D. Defense

Title: SECONDARY ORGANIC AEROSOL FORMATION FROM THE OXIDATION OF FURANS EMITTED FROM BIOMASS BURNING AND THEIR POSSIBLE IMPACTS ON RADIATIVE FORCING

Committee members:

Dr. Nga Lee (Sally) Ng, CHBE & EAS & CEE, Advisor

Dr. Rodney J. Weber, EAS

Dr. Greg Huey, EAS

Dr. Pengfei Liu, EAS

Dr. Matthew J. Alvarado, Atmospheric and Environmental Research

Abstract: Biomass burning is a significant source of gas- and particle-phase carbon in the atmosphere. It emits thousands of volatile organic compounds (VOCs) that can contribute to the formation of secondary organic aerosol (SOA). Recent studies have shown that furans are an important class of VOCs and constitute a significant portion of biomass burning plumes. Furans are also proposed to potentially form a considerable amount of SOA due to their high reactivity towards atmospheric oxidants, and thus, understanding the oxidation chemistry of furans and their possible impact on the atmosphere is highly important. This dissertation investigates the oxidation of major furans from biomass burning by reaction with hydroxyl (OH) and nitrate (NO3) radicals, focusing on the characterization of SOA chemical composition and gas-phase oxidation mechanisms that can lead to the SOA formation via gas-to-particle partitioning of low-volatility compounds. First, the 3-methylfuran reaction with NO3 radicals, which is the major nighttime atmospheric oxidant, is investigated and more than half of the SOA mass is generated after the depletion of 3-methylfuran, highlighting the importance of higher-generation and multiphase reactions to aerosol formation. Particle-phase organic nitrates are observed to contribute 39.4% of organics and their volatility is found to be higher than that of the non-nitrate organics. Second, SOA formation by photooxidation of furfural, 2-methylfuran, and 3-methylfuran under different humidity conditions is investigated, and it is found that 2-furaldehyde (furfural) have higher SOA forming potential than 2- and 3-methylfuran from the reaction with OH radicals. Under humid conditions (50% RH), SOA mass yield and mass concentration are enhanced, and identified oxidation products are found to be less volatile and to exist more as in particle phase compared to the SOA formed under dry conditions (<5% RH). The degree of such enhancement is intensified from furfural photooxidation among others, where the SOA formation rate also increases drastically under humid conditions. Furans generate high yields of carbonyl compounds during the oxidation and humid conditions enhance the uptake of first-generation products onto pre-existing aqueous particles followed by oligomerization. It is further noted that continuous formation of maleic anhydride (C2H4O3) until the end of the experiment is observed during the photooxidation of all type of furans tested, suggesting this compound can be a marker of aged biomass burning plume as reported from previous studies. Finally, light-absorbing properties of SOA from furans is examined, and only furfural generates light-absorbing SOA that is comparable to the light absorption of atmospheric brown carbon. Stronger light absorption and higher nitrogen-containing organic fraction in total organics are observed from the SOA formed under dry conditions compared to humid conditions. Imine and imine intermediate can be formed via the interaction of multi-carbonyl compounds with ammonium from ammonium sulfate seed aerosol, and this process would have been stimulated under dry conditions. Results from this dissertation provide detailed oxidation chemistry of major biomass burning-emitted furans and can improve our understanding of their impact on SOA and ozone formation and climate change.