Atmospheric Degradation of Cyclic Volatile Methyl Siloxanes: Radical Chemistry and Oxidation Products

Abstract

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 HO₂) 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.

Figure 1

Example time series of D3 oxidation with H₂O₂ as oxidant precursors. The shading signifies when the lights are on, and the dashed vertical line signifies when 10% of D3 had been oxidized. Signals averaged to 1 min time resolution are shown.

Figure 2

Example cVMS oxidation product structures.

Figure 3

Signal yield of different cVMS oxidation products when RO₂ was most likely to react with (a, d, g) NO, (b, e, h) HO₂, or (c, f, i) when unimolecular reactions are favored, divided by the signal of the siloxane lost at that point. Panels (a–c) are for D3, (d–f) for D4, and (g–i) for D5 oxidation. The signals for siloxanediol, ether hydroperoxide, and ether nitrate are multiplied by 10 and striped for visual clarity.

Scheme 1. Potential Reactions of cVMS in High NO_x/HO₂ Conditions

Scheme 2. Potential Reactions of cVMS in Low NO_x/HO₂ Condition

Figure 4

Signal yields of formate ester (a–c) and siloxanol (d–f) for all three cVMS from the experimental results and kinetic modeling using the mechanism presented, normalized to the results from the RO₂ + NO conditions. Panels (a) and (d) are for D3 oxidation, (b) and (e) are for D4 oxidation, and (c) and (f) for D5. Note the log axis on siloxanol results. Only the rate constants for reactions with OH/Cl and wall loss constants were changed between cVMS. In the model results that had a source of siloxanol from the RO₂ + NO reaction, 5% of the reaction product made siloxanol directly, and the other 95% made RO.

Figure 5

Estimated Henry’s solubility, estimated with HenryWIN v3.21, for D3 and the siloxanol oxidation products (dashed lines). The mass concentration labels on the solid lines correspond to the amount of liquid water in clouds or ambient aerosol. Only siloxanol products are shown as the hydroxyl groups change the Henry’s solubility most significantly. Larger cVMS oxidation products are estimated to have lower Henry’s solubility than D3. All values are listed in Table S6 of the Supporting Information. Adapted with permission from Daumit, K. E.; Carrasquillo, A. J.; Hunter, J. F.; Kroll, J. H. Laboratory Studies of the Aqueous-Phase Oxidation of Polyols: Submicron Particles vs Bulk Aqueous Solution. Atmos. Chem. Phys. 2014, 14 (19), 10773–10784. https://doi.org/10.5194/acp-14-10773-2014.