Molecular Characterization and Quantification of Oxidized Volatile Methyl Siloxanes in Laboratory Generated and Ambient Aerosols
JEEWANI MEEPAGE, Josie Welker, Saeideh Mohammadi, Hanalei Lewine, Charles Stanier, Eleanor Browne, Elizabeth Stone, University of Iowa
Abstract Number: 254
Working Group: Chemicals of Emerging Concern in Indoor and Outdoor Aerosol: Sources, Vectors, Reactivity, and Impacts
Abstract
Decamethylcyclopentasiloxane (D5), a common ingredient in personal care products, is emitted to the atmosphere where it undergoes oxidation to form 1-hydroxynonamethylcyclopentasiloxane (monosiloxanol, D4TOH) and other products. To date, the abundance of D5 oxidation products and their contribution to ambient aerosol formation remains poorly understood. To address this, a novel liquid chromatography–tandem mass spectrometry method was developed and validated for the sensitive detection and quantification of oxidized D5 in gas and particle phases. Laboratory oxidation experiments in a smog chamber confirmed that D4TOH is the dominant oxidation product, primarily residing in the gas phase under urban plume conditions (~1 day of atmospheric aging). Oxidation flow reactor experiments were conducted extending OH exposures beyond four days, allowing investigation of more oxidized products formed from D5. Field measurements conducted in New York City during the summer of 2022 identified D4TOH and the disiloxanol as the most abundant forms of oxidized D5 in atmospheric aerosols. Moreover, the lower positive artifacts and larger particle phase fractions of disiloxanol compared to D4TOH indicates that it is a more suitable molecular marker to track D5-derived secondary organic aerosols (SOA) in ambient aerosols. By comparing the fraction of D4TOH to total organic carbon in field and laboratory studies, the contribution of oxidized D5 to organic carbon was estimated to be <0.05%, indicating that D5 is a very minor contributor to SOA in NYC during summertime. This work presents a new analytical method for quantifying oxidized molecular tracers in the atmosphere, enabling improved source attribution of SOA and advancing understanding of the atmospheric fate of D5. Further, the applied methodology enables bottom-up estimates of D5-derived SOA, providing new insights into the atmospheric fate of this compound.