Estimating Physical Properties of Oxidized Volatile Methylsiloxanes

Volatile methylsiloxanes (VMS) are well-documented contaminants on the International Space Station (ISS). Through gas-phase reactions initiated by hydroxyl radicals, VMS can form products with varying degrees of oxidation. Elemental analysis of SOA samples collected on board the ISS detected silicon, carbon, oxygen, and hydrogen, suggesting that oxidized VMS may be involved in aerosol formation and/or growth. This work explores the role of VMS in SOA formation on board the ISS by estimating physical properties of VMS as a function of degree of oxidation. Linear and cyclic VMS were chosen from previously published literature and NASA air quality data. The boiling point, vapor pressure, and saturation concentration for each species was estimated using group contribution methods. Trends in these physical properties suggest that as VMS are increasingly oxidized they become less volatile and more likely to exist in the particle phase. Future work will incorporate these estimated physical properties into a model of gas-particle partitioning of VMS oxidation products in the ISS atmosphere.