Investigating SOA Formation from Volatile Methyl Siloxanes

HANALEI LEWINE, Jeewani Meepage, Saeideh Mohammadi, Carlos Gutierrez, Charles Stanier, Elizabeth Stone, Eleanor Browne, University of Colorado Boulder & CIRES

     Abstract Number: 419
     Working Group: Chemicals of Emerging Concern in Aerosol: Sources, Transformations, and Impacts

Abstract
Volatile methyl siloxanes (VMS) are anthropogenic organosilicon molecules that are used in numerous applications including in personal care products. Approximately 90% of the decamethylcyclopentasiloxane (D5) in cosmetics is released to the atmosphere. This source is believed to be one of the most important sources for D5 in both indoor and outdoor environments. VMS have recently been identified as precursors to aerosol in urban areas. The dominant D5 oxidation product, 1-hydroxynonamethylcyclopentasiloxane (siloxanol), in which one methyl group is replaced by a hydroxyl group, has been observed in lab and ambient aerosol, however absorptive partitioning alone cannot explain the amounts measured. Previous lab studies of D5 oxidation and SOA formation found a wide range of SOA yields as a function of OH exposure. Improving our understanding of SOA formation from D5 requires further investigation of the mechanism, influence of seed aerosol, and consideration of wall effects. Here, we performed chamber experiments under carefully controlled conditions such that RO2 primarily reacted with HO2, conditions which are likely to be important in the ambient atmosphere given the several-day lifetime of D5. In these experiments we investigated how the presence of seed aerosols impacts D5 SOA formation and if multigenerational chemistry is required for D5 SOA. We measured the gas phase products of D5 oxidation using chemical ionization mass spectrometry, measured particle size distribution, and characterized particulate and semi-volatile products by liquid chromatography mass spectrometry. We find that at lower OH exposures, with only one generation of oxidation chemistry, we still make SOA in the presence of ammonium sulfate seed aerosol . The major product observed in both the gas phase and the aerosol is the siloxanol. We use a kinetic box model of D5 oxidation and SOA formation to investigate the mechanism and predict SOA yields.