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https://doi.org/10.5194/egusphere-2023-1033
https://doi.org/10.5194/egusphere-2023-1033
12 Jun 2023
 | 12 Jun 2023

Volatile Oxidation Products and Secondary Organosiloxane Aerosol from D5 + OH at Varying OH Exposures

Hyun Gu Kang, Yanfang Chen, Jiwoo Jeong, Yoojin Park, Thomas Berkemeier, and Hwajin Kim

Abstract. Siloxanes are composed of silicon, oxygen, and alkyl groups and are emitted from consumer chemicals. Despite being entirely anthropogenic, siloxanes are being detected in remote regions and are ubiquitous in indoor and urban environments. Decamethylcyclopentasiloxane (D5) is one of the most common cyclic congeners, and smog chamber and oxidation flow reactor (OFR) experiments have found D5 + OH to form secondary organosiloxane aerosol (SOSiA). However, there is uncertainty about the reaction products, and the reported SOSiA mass yields (YSOSiA) appear inconsistent. To quantify small volatile oxidation products (VOP) and to consolidate the YSOSiA in the literature, we performed experiments using a Potential Aerosol Mass OFR while varying D5 concentration, humidity, and OH exposure (OHexp). We use a proton transfer reaction time-of-flight mass spectrometer to quantify D5, HCHO, and HCOOH, and detect other VOP, which we tentatively identify as siloxanols and siloxanyl formates. We determine molar yields of HCHO and HCOOH between 52 – 211 % and 45 – 127 %, respectively. With particle size distributions measured with a scanning mobility particle sizer, we find YSOSiA to be < 10 % at OHexp < 1.3 × 1011 s cm-3 and ~20 % at OHexp corresponding to that of the lifetime of D5 at atmospheric OH concentrations. We also find that YSOSiA is dependent on both organic aerosol mass loading and OHexp. We use a kinetic box model of SOSiA formation and aging (aging-VBS model) to reconcile the YSOSiA values found in this study and the literature. The model uses a volatility basis set (VBS) of the primary oxidation products as well as an aging rate coefficient in the gas phase, kage,gas, of 2.17 × 10-11 cm3 s-1, and an aging rate coefficient in the particle phase, kage,particle, which is ten times smaller. The combination of primary VBS and OH-dependent oxidative aging predicts SOSiA formation much better than a standard-VBS parameterization that does not consider aging (R2 = 0.970 vs. 0.847). The need for an ageing-dependent parameterization to accurately model SOSiA formation shows that concepts developed for secondary organic aerosol precursors, which are able to form low-volatile products at low OHexp, do not necessarily apply to D5 + OH. The resulting yields of HCHO and HCOOH and the parameterization of YSOSiA may be used in larger scale models to assess the implications of siloxanes on air quality.

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Journal article(s) based on this preprint

20 Nov 2023
Volatile oxidation products and secondary organosiloxane aerosol from D5 + OH at varying OH exposures
Hyun Gu Kang, Yanfang Chen, Yoojin Park, Thomas Berkemeier, and Hwajin Kim
Atmos. Chem. Phys., 23, 14307–14323, https://doi.org/10.5194/acp-23-14307-2023,https://doi.org/10.5194/acp-23-14307-2023, 2023
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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D5 is an emerging anthropogenic pollutant that is ubiquitous in indoor and urban environments...
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