Secondary organic aerosol formation from the oxidation of decamethylcyclopentasiloxane at atmospherically relevant OH concentrations

Abstract

Abstract. Decamethylcyclopentasiloxane (D5, C10H30O5Si5) is measured at parts per trillion (ppt) levels outdoors and parts per billion (ppb) levels indoors. Primarily used in personal care products, its outdoor concentration is correlated to population density. Since understanding the aerosol formation potential of volatile chemical products is critical to understanding particulate matter in urban areas, the secondary organic aerosol yield of D5 was studied under a wide range of OH concentrations and, correspondingly, OH exposures using both batch-mode chamber and continuously run flow tube experiments. These results were comprehensively analyzed and compared to two other secondary organic aerosol (SOA) yield datasets from literature. It was found that the SOA yield from the oxidation of D5 is extremely dependent on either the OH concentration or exposure. For OH concentrations of ≲ 107 molec.cm-3 or OH exposures of ≲ 2 × 1011 molec.scm-3, the SOA yield is largely < 5 % and usually ∼ 1 %. This is significantly lower than SOA yields previously reported. Using a two-product absorptive partitioning model for the upper bound SOA yields, the stoichiometric mass fraction and absorptive partitioning coefficients are, for the first product, α1 = 0.056 and KOM,1 = 0.022 m3 µg−1; for the second product, they are α2 = 7.7 and KOM,2 = 4.3 × 10−5 m3 µg−1. Generally, there are high SOA yields (> 90 %) at OH mixing ratios of 5 × 109 molec.cm-3 or OH exposures above 1012 molec.scm-3.

Comparison of the Yield and Chemical Composition of Secondary Organic Aerosol Generated from the OH and Cl Oxidation of Decamethylcyclopentasiloxane

“…Representative error bars indicate ±1σ uncertainty in binned SOA yield values, ±50% uncertainty in OH exp exposure values, and ±70% uncertainty in Cl exp exposure values. Additional figure notes: a Wu and Johnston, b Janechek et al, c Charan et al, d Han et al, 1 Cl generated using OFR369-iCl 2 under humid conditions, 2 Cl generated using OFR369-iCl 2 under dry conditions, and 3 Cl generated using OFR254/313-iC 2 Cl 2 O 2 under dry conditions.…”

“…First, we characterized the yield and chemical composition of SOA obtained from OH oxidation of D 5 and compared our results to previous studies. 14,16,22 OFRs use residence times that are on the order of minutes and oxidant concentrations that are typically 100−1000 times higher than ambient levels. Their ability to access photochemical aging time scales of up to several days was critical in previous studies examining D 5 + OH SOA formation as a result of the relatively long OH lifetime of D 5 .…”

“…Important early-generation gas-phase D 5 + OH oxidation products include siloxanol/silanol compounds . Compounds with similar functional groups have been detected in laboratory D 5 SOA particles. , Given the lower volatililty and higher emission factors of D 5 relative to other VMS, D 5 is a presumptive precursor to particulate Si in ambient aerosols. − Recent studies investigating the OH oxidation of D 5 and its early-generation oxidation products have shown that the yield of SOA increases significantly beyond ≈10 days of atmospheric OH exposure. − In addition to OH, chlorine atoms (Cl) may influence the atmospheric lifetime of D 5 in certain source regions, including the marine boundary layer, polluted coastal cities, and indoors following bleach washing. − D 5 reacts almost 2 orders of magnitude faster with Cl than with OH and generates similar early-generation gas-phase oxidation products . However, SOA generation following Cl oxidation of D 5 has not been studied.…”

Yields and molecular composition of gas phase and secondary organic aerosol from the photooxidation of the volatile consumer product benzyl alcohol: formation of highly oxygenated and hydroxy nitroaromatic compounds

“…The contribution of VCPs to ambient OA is not fully understood and only limited modeling studies have been reported (Mohr et al, 2015;Vlachou et al, 2018;Pennington et al, 2021;Qin et al, 2021;Seltzer et al, 2021). Additionally, few experimental and chamber studies of VCPs have been conducted with limited characterization of aerosol products Johnston, 2016, 2017;Harrison and Well, 2012;Charan et al, 2020Charan et al, , 2021Humes et al, 2022). For example, the analysis of SOA from the oxidation of cyclic methyl siloxanes Johnston, 2016, 2017;Fu et al 2020;Alton and Browne, 2020;Charan et al, 2021) and cyclic siloxanes (Janechek et al, 2019) has been conducted.…”

“…Additionally, few experimental and chamber studies of VCPs have been conducted with limited characterization of aerosol products Johnston, 2016, 2017;Harrison and Well, 2012;Charan et al, 2020Charan et al, , 2021Humes et al, 2022). For example, the analysis of SOA from the oxidation of cyclic methyl siloxanes Johnston, 2016, 2017;Fu et al 2020;Alton and Browne, 2020;Charan et al, 2021) and cyclic siloxanes (Janechek et al, 2019) has been conducted. Kinetic studies with limited products characterization have been reported for the oxidation of benzyl alcohol (BnOH) by hydroxyl radicals (Bernard et al, 2013;Wang, 2015;Well, 2009, 2012).…”