Modeled Global Impacts of Chlorine Oxidation and Temperature Dependence on the Atmospheric Lifetime and Concentrations of Volatile Methyl Siloxanes

Volatile methyl siloxanes (VMS) are high production volume chemicals found in a wide range of consumer items such as personal care products. VMS have attracted scrutiny due to long-range environmental transport (LRET) concerns. However, their emissions, lifetimes, and concentrations remain uncertain, in part because of limitations in previous atmospheric modeling. Herein, we describe the global modeling of siloxanes: D4 (octamethylcyclotetrasiloxane), D5 (decamethylcyclopentasiloxane), and D6 (dodecamethylcyclohexasiloxane) in the Community Earth System Model (CESM2-SLH) using an updated chemical mechanism that includes chlorine radical oxidation and temperature-dependent reaction rates. With these previously unconsidered factors, we predicted VMS lifetimes ranging between 2.7 and 6.7 days and annual average D5 near-surface concentrations as high as 4.5 ng m^-3 in the remote Arctic and 160 ng m^-3 in urban areas. These lifetimes and the degree of LRET were significantly lower than previously reported. OH remained the largest loss pathway, although it decreased at low temperatures relative to previous modeling. The temperature-induced lifetime increase was outweighed by the previously unconsidered chlorine oxidation channel. Model results and previous measurements agreed relatively well but exhibited negative normalized biases (-0.55 to -0.88), particularly in urban areas not well-resolved at global model resolution, and for passive sampler measurements.