A Simple Model to Estimate the Hydroxyl Radical Concentration and Associated DMSD Production Rates from Volatile Methyl Siloxanes in the ISS Atmosphere

摘要

Volatile methyl siloxane (VMS) compound reaction with hydroxyl radicals (·OH) in the International Space Station (ISS) cabin atmosphere to form dimethylsilanediol (DMSD) has been proposed as a dominant reaction pathway by Perry and Kayatin (2017). A simple model is derived in this paper based on the material balance accounting for VMS compound gener- ation, reaction, and removal rates representing the ISS atmospheric condition reported in the 2017 work. The model incorporates reaction stoichiometry and the kinetic framework of sec- ond order rate constants reported in the literature to yield three product compounds from ·OH reaction with cyclic and linear VMS compounds: DMSD, formaldehyde, and methylsilan- etriol (MST). The model is intended to provide a quantitative framework for understanding the potential benefits of VMS compound source reduction on DMSD production rates. The model is applied to VMS compound concentration data reported by the Air Quality Monitor (AQM) and whole air grab samples to estimate the hydroxyl radical concentration, [·OH], in the ISS cabin atmosphere. The estimated value for [·OH] aboard ISS was determined to be in the range of 3.5 ± 1.0 × 10~5 to 1.0 ± 0.3 × 10~6 molecules/cm~3 , consistent with estimated [·OH] in terrestrial indoor air environments (10~4 -10~6 molecules/cm~3 ). The role of the ·OH sink on DMSD production rates is explored based on the estimated values of [·OH] and the concen- trations of volatile organic compound compounds (VOC) also present in the ISS cabin atmos- phere that are known to react with ·OH.