Much of what we think we know about aqueous aerosol chemistry - reaction rates, products, mechanisms, and photolytic pathways - comes from extrapolating bulk aqueous-phase lab simulations to atmospheric conditions. Based on this approach, it is now commonly assumed that small, water-soluble aldehydes can react at night with ammonium salts to slowly form light-absorbing brown carbon (BrC). These BrC products are thought to be quickly destroyed by sunlight. When aqueous aerosol processes are studied in aqueous aerosol particles, however, this common narrative turns out to be only partially true. In this talk, results will be presented from recent chamber studies on ammonium and amine-containing aerosol particles as they interact with aldehyde species, solar simulator lamps, and clouds. In some cases, sunlight actually accelerates BrC formation during cloud processing. Chemical analysis of the aerosol produced in these experiments suggests that mechanisms initiated by photolytically-produced radical species are the dominant source of oligomers, and by extension, of BrC.
David DeHaan is a Professor of Chemistry in the College of Arts and Sciences at the University of San Diego. He is currently on sabbatical at CU Boulder.
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