Photochemical Production of Light-absorbing Syringol Secondary Organic Aerosol in Droplets using an Atmospheric Simulation Chamber

LELIA HAWKINS, Ellie Smith, Linden Conrad, Jacob Weber, David De Haan, Christian Carmona, Duncan Ugland, Matthew-Khoa Tran, Jean-François Doussin, Mathieu Cazaunau, Aline Gratien, Michael R. Giordano, Mikael Ehn, Frans Graeffe, Liine Heikkinen, Peter F. DeCarlo, Matthieu Riva, Mario Contin, Harvey Mudd College

     Abstract Number: 719
     Working Group: Aerosol Chemistry

Abstract
The photooxidation of syringol, a substituted phenol emitted primarily from lignin pyrolysis during wildfires, was used to explore the formation of secondary brown carbon (BrC) under dry, moist, and cloud-like conditions in aerosol and droplet-phase reactions using the CESAM multiphase atmospheric simulation chamber at the University of Paris-Est LISA in Créteil, France. Optical properties were monitored using a particle-into-liquid (PILS) waveguide with total organic carbon (TOC) analysis system while chemical properties were interrogated using two high-resolution aerosol time-of-flight mass spectrometers (HR-ToF-AMS, Aerodyne). Syringol oxidation produced brown, water-soluble products in deliquesced ammonium sulfate (AS) aerosol and in cloud droplets; the limited secondary organic aerosol (SOA) produced on dry AS seed aerosol did not absorb visible light. Browning occurred in simulated sunlight both with and without OH radicals generated by hydrogen peroxide photolysis, as well as in dark conditions with hydrogen peroxide. Brown products formed under dark conditions were different from those formed under light, while products formed in simulated sunlight were chemically and optically very similar whether or not HOOH was present. The aqueous BrC formed without light featured an absorbance peak at 470 nm consistent with a dimer observed previously in this chemical system and disappeared immediately upon illumination. HR-ESI-MS of chamber filter extracts indicates that most products detected by this technique contain N although HR-AMS spectra indicate only a very small contribution from N-containing fragments. UV/visible absorbance spectra and aerosol mass spectra suggest that the products formed in sunlit, OH-mediated reactions are highly similar to those formed under light without an OH radical source, suggesting that direct syringol photolysis may be capable of initiating similar radical-driven chemistry in the absence of additional oxidants like OH or 3C*. However, the actinic flux of the wavelengths of light necessary for this direct mechanism of syringol SOA formation (λ< 280 nm) is likely too small in the lower atmosphere to be relevant for most biomass burning plumes. Our findings from droplet phase reactions support earlier bulk phase studies suggesting that syringol is capable of forming light absorbing products rapidly in sunlit reactions, especially when liquid water is available.