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

Date

2018-07-08

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Publisher

48th International Conference on Environmental Systems

Abstract

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 generation, reaction, and removal rates representing the ISS atmospheric condition report-ed in the 2017 work. The model incorporates reaction stoichiometry and the kinetic frame-work of second order rate constants reported in the literature to yield three product com-pounds from ·OH reaction with cyclic and linear VMS compounds: DMSD, formaldehyde, and methylsilanetriol (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 radi-cal 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 × 105 to 1.0 ± 0.3 × 106 mole-cules/cm3, consistent with estimated [·OH] in terrestrial indoor air environments (104–106 molecules/cm3). The role of the ·OH sink on DMSD production rates is explored based on the estimated values of [·OH] and the concentrations of volatile organic compound com-pounds (VOC) also present in the ISS cabin atmosphere that are known to react with ·OH.

Description

Dean Muirhead, Barrios Technology
Denyce K. Wicht, Suffolk University
Kelsey M. Stocker, Suffolk University
Jay L. Perry, NASA
Matthew J. Kayatin, NASA
ICES300: ECLSS Modeling and Test Correlations
The 48th International Conference on Environmental Systems was held in Albuquerque, New Mexico, USA on 08 July 2018 through 12 July 2018.

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Keywords

hydroxyl radical, volatile methyl siloxanes, dimethylsilanediol, atmospheric chemistry, ionizing radiation

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