Emissions of Volatile Methyl Siloxanes (VMSs) from Vehicles

Volatile Methyl Siloxanes (VMSs) are anthropogenic molecules emitted from personal care products and industrial chemical products. Recent studies have found that concentrations of VMSs tend to be higher in densely populated urban areas, although research in large urban centers remains limited. Toxicity of such molecules remain uncertain with often conflicting reports. Previous literature has also reported the presence of long chain siloxanes in the particle phase from traffic emissions, and represented the first identification and quantification of siloxanes mostly likely emitted from vehicles.

In this study, the chassis dynamometer measurements were conducted to characterize the gaseous components from a variety of vehicles, including gasoline passenger cars, diesel passenger cars, and scooters. Time-resolved volatile organic compound (VOC) emissions during the cold start through a full driving cycle from different vehicles were chemically characterized by the Vocus proton-transfer-reaction time-of-flight mass spectrometer (VOCUS PTR-TOF-MS). The emission factors (EFs) of Hexamethylcyclotrisiloxane (D₃), Octamethylcyclotrisiloxane (D₄) and Decamethylcyclotrisiloxane (D₅) were determined, and will be discussed with only specific types of vehicles emitting significant quantities of VMSs. Our results will also be compared to recent tunnel measurements, which demonstrated elevated concentrations of VMSs. Additionally, our results offer new insights into characterization and source appointment siloxanes in the atmosphere.

Figure 1. Real-time concentrations of D₃, D₄, D₅, acetone, benzene, toluene, and driving cycle for a) a gasoline engine and b) a diesel engine.