10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Secondary Organic Aerosol Formation and Aging in a Flow Reactor in the Forested Southeast US during SOAS
WEIWEI HU, Brett Palm, Pedro Campuzano-Jost, Douglas Day, Suzane de Sá, Benjamin Ayres, Danielle C. Draper, Juliane L. Fry, Amber Ortega, Lina Hacker, Astrid Kiendler-Scharr, Aki Pajunoja, Annele Virtanen, Jordan Krechmer, Manjula Canagaratna, Samantha Thompson, Laxmi Narasimha Yatavelli, Harald Stark, Douglas Worsnop, Scot T. Martin, William Brune, Steven S. Brown, Jose-Luis Jimenez, CIRES, University of Colorado, Boulder
Abstract Number: 440 Working Group: Oxidation Flow Reactor: Development, Characterization, and Application to Aerosols
Abstract A major field campaign (Southern Oxidant and Aerosol Study, SOAS) was conducted in summer 2013 in a forested area in Centreville Supersite, AL (SEARCH network) in the southeast U.S. To investigate secondary organic aerosol (SOA) formation from biogenic volatile organic compounds (BVOCs), 3 oxidation flow reactors (OFR) were used to expose ambient air to oxidants and their output was analyzed by state-of-the-art gas and aerosol instruments including a High-Resolution Aerosol Mass Spectrometer (HR-AMS), a HR Proton-Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-TOFMS), and two HR-TOF Chemical Ionization Mass Spectrometers (HRToF-CIMS). Ambient air was exposed 24/7 to variable concentrations of each of the 3 main atmospheric oxidants (OH, NO3 radicals and O3) to investigate the oxidation of BVOCs (including ambient isoprene-derived epoxydiols, IEPOX) and SOA formation and aging. Effective OH exposures up to 1×1013 molec cm-3 s were achieved, equivalent to over a month of aging in the atmosphere, although the peak SOA formation was observed for equiv. exposures of about 1 day. Multiple oxidation products from isoprene and monoterpenes including small gas-phase acids were observed in OH OFR. High SOA formation of up to 12 µg m-3 above ambient concentrations of 5 µg m-3 was observed under lower OH exposures, while very high OH exposures led to destruction of ~30% of ambient OA, indicating shifting contributions of functionalization vs. fragmentation, consistent with results from urban and terpene-dominated environments. The highest SOA enhancements were 3-4 times higher than ambient OA. More SOA is typically formed during nighttime when terpenes are higher and photochemistry is absent, and less during daytime when isoprene is higher, although the IEPOX pathway is suppressed in the OFR. SOA is also observed after exposure of ambient air to O3 or NO3, although the amounts and oxidation levels were lower than for OH. Formation of organic nitrates in the NO3 oxidation will also be discussed.