Photoenhanced Radical Formation from Secondary Organic Aerosols and Surrogate Mixtures Derived from Biomass Burning Organic Aerosol Components

LENA GERRITZ, Meredith Schervish, Pascale Lakey, Tim Oeij, Jinlai Wei, Sergey Nizkorodov, Manabu Shiraiwa, University of California, Irvine

     Abstract Number: 42
     Working Group: Aerosol Chemistry

Abstract
The photochemical aging of biomass burning organic aerosol (BBOA) by exposure to sunlight changes the chemical composition over its atmospheric lifetime, affecting toxicological and climate-relevant properties of BBOA particles. This study applies electron paramagnetic resonance (EPR) spectroscopy with a spin trapping agent BMPO, high resolution mass spectrometry (HRMS), and kinetic modeling to study photosensitized formation of reactive oxygen species (ROS) and free radicals in model BBOA. In mixtures of benzoquinone and levoglucosan, known BBOA tracer molecules, EPR analysis revealed substantial formation of hydroxyl radicals (•OH), hydrogen radicals (H•), and both carbon- and oxygen-centered organic radicals, which were generated by benzoquinone photochemistry, decomposition of semiquinone radicals, and •OH oxidation of levoglucosan, respectively. HRMS analysis confirmed the presence of free radicals derived from both benzoquinone and levoglucosan. Kinetic modeling of the processes in the irradiated mixtures successfully reproduced the time evolution of the observed formation of the BMPO adducts of •OH and H• observed with EPR. We used the same approach to investigate ROS formation from the irradiation of secondary organic aerosols from BBOA relevant molecules, phenol , α-pinene, α-terpineol, and toluene. We found substantial enhancements of radical formation most likely due to decomposition of labile organic hydroperoxides, as we observed their reduction of peroxide contents based on the iodometric−spectrophotometric method. These results imply that photoirradiation of aerosols containing photosensitizers induces ROS formation and secondary radical chemistry to drive photochemical aging of BBOA in the atmosphere.