Atmospheric chemistry of volatile methyl siloxanes.

Volatile methyl siloxanes (VMSs) are high-production-volume, synthetic organosilicon compounds widely detected in the environment. Because they are hydrophobic and volatile, they readily partition to the atmosphere, where they can undergo long-range transport owing to their low gas-phase reactivity. As such, VMSs and their oxidation products are potential tracers of anthropogenic influence, which has caused concerns about their bioaccumulation in remote ecosystems. Multigenerational atmospheric oxidation, primarily by hydroxyl and chlorine radicals to form siloxanol, formate ester and hydroperoxide compounds, has a central role in determining the environmental fate of VMSs. Thermodynamic partitioning of VMS and its oxidation products to aerosol particles (via adsorption and absorption) and environmental media further influences their atmospheric fate. Elucidating oxidation mechanisms and product distributions remains challenging owing to slow reaction kinetics, analytical challenges, absence of authentic standards and the complexity of multiphase chemistry. Advances in analytical methods, chamber studies, flow tube experiments, field campaigns and modelling efforts have underscored the need for an updated assessment of VMS atmospheric chemistry and environmental behaviour. In this Review, we synthesize current knowledge focusing on developments in gas-phase chemistry, multiphase partitioning behaviour, atmospheric measurements and modelling approaches for prominent VMS species and their oxidation products.