Volatile methyl siloxanes (VMS) are ubiquitous anthropogenic pollutants that have recently come under scrutiny for their potential toxicity and environmental persistence. In this work, we determined the rate constants for oxidation by OH radicals and Cl atoms at 297 ± 3 K and atmospheric pressure in Boulder, CO (∼860 mbar) of hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), decamethyltetrasiloxane (L4), dodecamethylpentasiloxane (L5), hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5). Measured rate constants with OH radicals were (1.20 ± 0.09) × 10–12, (1.7 ± 0.1) × 10–12, (2.5 ± 0.2) × 10–12, (3.4 ± 0.5) × 10–12, (0.86 ± 0.09) × 10–12, (1.3 ± 0.1) × 10–12, and (2.1 ± 0.1) × 10–12 cm3 molec–1 s–1, for L2, L3, L4, L5, D3, D4, and D5, respectively. The rate constants for reactions with Cl atoms with the same compounds were (1.44 ± 0.05) × 10–10, (1.85 ± 0.05) × 10–10, (2.2 ± 0.1) × 10–10, (2.9 ± 0.1) × 10–10, (0.56 ± 0.05) × 10–10, (1.16 ± 0.08) × 10–10, and (1.8 ± 0.1) × 10–10 cm3 molec–1 s–1, respectively. Substituent factors of F(−Si­(CH3)2OR) and F(−SiCH3(OR)2) are proposed for use in AOPWIN, a common model for OH radical rate constant estimations. Cl atoms can remove percentage levels of VMS globally with potentially increased importance in urban areas.

Cyclic volatile methyl siloxanes (cVMS) are anthropogenic chemicals that have come under scrutiny due to their widespread use and environmental persistence. Significant data on environmental concentrations and persistence of these chemicals exists, but their oxidation mechanism is poorly understood, preventing a comprehensive understanding of the environmental fate and impact of cVMS. We performed experiments in an environmental chamber to characterize the first-generation oxidation products of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) under different peroxy radical fates (unimolecular reaction or bimolecular reaction with either NO or HO 2 ) that approximate a range of atmospheric compositions. While the identity of the oxidation products from D3 changed as a function of the peroxy radical fate, the identity and yield of D4 and D5 oxidation products remained largely constant. We compare our results against the output from a kinetic model of cVMS oxidation chemistry. The reaction mechanism used in the model is developed using a combination of previously proposed cVMS oxidation reactions and standard atmospheric oxidation radical chemistry. We find that the model is unable to reproduce our measurements, particularly in the case of D4 and D5. The products that are poorly represented in the model help to identify possible branching points in the mechanism, which require further investigation. Additionally, we estimated the physical properties of the cVMS oxidation products using structure−activity relationships and found that they should not be significantly partitioned to organic or aqueous aerosol. The results suggest that cVMS first-generation oxidation products are also long-lived in the atmosphere and that environmental monitoring of these compounds is necessary to understand the environmental chemistry and loading of cVMS.

Cyclic volatile methyl siloxanes (cVMS) that are emitted from industrial processes and consumer products often dominate the burden of volatile organic compounds (VOCs) in occupied spaces. cVMS may contribute to oxygenated VOC and secondary organic aerosol (SOA) formation following oxidation by gas-phase radicals in both indoor and outdoor source regions. Several recent studies examined the SOA formation potential of decamethylcyclopentasiloxane (D 5 ) following exposure to hydroxyl radicals (OH) and found that this reaction generates SOA in high yield following multiple days of oxidative aging. Chlorine atoms (Cl) may compete with OH for the oxidative loss of D 5 in indoor and outdoor source regions with active chlorine chemistry, but the SOA formation potential of D 5 + Cl reactions has not been studied. Here, we characterized the yield and chemical composition of SOA generated from Cl oxidation of D 5 in an oxidation flow reactor (OFR) under dry [relative humidity (RH) < 5%] and humid (RH = 40%) conditions and compared results to the yield and composition of SOA generated from OH oxidation of D 5 . D 5 was oxidized using integrated OH and Cl exposures (OH exp and Cl exp ) ranging from 1.1 × 10 12 to 8.2 × 10 12 cm −3 s and from 1.6 × 10 10 to 1.6 × 10 12 cm −3 s, respectively. Like OH, Cl facilitated multistep SOA oxidative aging over the range of OFR conditions that were studied, with maximum SOA mass yields of 1.5 and 1.3 obtained following OH and Cl oxidation of D 5 under humid conditions. These results suggest that indoor and outdoor source regions that are significantly influenced by chlorine chemistry may enhance the atmospheric SOA formation potential of D 5 .