Atmospheric Chemistry of Volatile Methyl Siloxanes: Kinetics and Products of Oxidation by OH Radicals and Cl Atoms

Abstract

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.