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

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. s cm⁻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 K_(OM,1) = 0.022 m³ µg⁻¹; for the second product, they are α₂ = 7.7 and K_(OM,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. s cm⁻³.

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