Gas-Particle Partitioning of Volatile Methyl Siloxane Oxidation Products in the New York City Airshed
JOSIE WELKER, Jeewani Meepage, Saeideh Mohammadi, Christopher Brunet, Hanalei Lewine, Rachel Marek, Keri Hornbuckle, Eleanor Browne, Charles Stanier, Elizabeth Stone, University of Iowa
Abstract Number: 645
Working Group: Aerosol Chemistry
Abstract
Decamethylcyclopentasiloxane (D5) is commonly used in personal care products and is highly volatile. In the atmosphere, D5 is oxidized primarily by OH radicals to form 1-hydroxynonamethylcyclopentasiloxane (D4TOH). D5 oxidation products can partition to the particle phase to form secondary organic aerosol (SOA). While the gas-particle partitioning of D4TOH has been studied in the laboratory with model aerosols, it has yet to be established in ambient air. To fill this gap, this study examines the gas and particle distribution of D4TOH in ambient air in Manhattan, New York City during the summer of 2022. Solvent extraction and gas chromatography mass spectrometry methods were used to analyze gas and particle samples for D4TOH and polycyclic aromatic hydrocarbons (PAHs). The fine particle fraction of PAHs increases as the molecular weight and number of carbons in the compound increase. The average fractions of PAH in the particle phase measured in New York City (n=13) were ~ 0% for 10-14 carbons (two-three rings: naphthalene, acenaphthene, and phenanthrene), 3% for 16 carbons (four rings: pyrene), 21% for 18 carbons (four rings: benzo(ghi)fluoranthene), 94% for 20 carbons (five rings: benzo(e)pyrene), and 100% for 22 carbons (five rings: picene). The fine particle fraction of D4TOH ranged from 23% to 69% (for n=13, x̅: 52%, σ: 0.13), with the remainder in the gas phase. Using the absorptive partitioning model, D4TOH is predicted to have an average fine particle fraction of < 1%. These results show that D4TOH partitions to the particle phase at a higher fraction than would be predicted by absorptive partitioning, suggesting that particle composition and/or adsorption substantially increase its partitioning to the particle phase.