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.
Highlights
Present in outdoor mixing ratios as high as ∼ 40 ppt, decamethylcyclopentasiloxane (D5, C10H30O5Si5) has been observed in cities, rural areas, and the Arctic (Buser et al, 2013, 2014; Ahrens et al, 2014; McLachlan et al, 2010; Xu et al, 2019)
The secondary organic aerosol (SOA) yields from the CPOT and chamber experiments vary from 0 % to 110 % (158 % at the upper bound of the wall-deposition-corrected value), an even wider range than that reported by the literature of 8 %–50 % (Table 1 shows all experimental conditions and SOA yields)
Since F9 and C1–8 have low SOA yields, there appears to be agreement between the SOA yields when viewed as a function of OH concentration
Summary
Present in outdoor mixing ratios as high as ∼ 40 ppt, decamethylcyclopentasiloxane (D5, C10H30O5Si5) has been observed in cities, rural areas, and the Arctic (Buser et al, 2013, 2014; Ahrens et al, 2014; McLachlan et al, 2010; Xu et al, 2019). Outdoor observations of D5 are population-dependent (Janechek et al, 2017; Gkatzelis et al, 2021), and this dependence is sufficiently reliable that it can be used to tease out personal care product emission patterns from other common urban emissions (Coggon et al, 2018). The impact of D5 does not stop at population centers; its long atmospheric lifetime means that it is even found in areas with low population densities. Even the international space station contains trace amounts of D5 in the air (Carter et al, 2015)
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