Calibration of a passive air sampler for volatile methyl siloxanes

The atmosphere is a key compartment for understanding the environmental fate and behavior of volatile methyl siloxanes (VMS). Due to their fairly high volatility, VMS are mainly emitted to, and largely remain in, the atmosphere where they are relatively persistent and subject to long-range transport. However, also because of their high volatility, the applicability of standard air sampling methods to VMS cannot be assumed and must be proven. The purpose of this study was to evaluate the suitability and to calibrate an existing, polystyrene-divinylbenzene co-polymeric resin based passive air sampler (XAD-PAS) for VMS. The XAD-PAS has been tested and applied for different compounds and environmental conditions, but has not yet been calibrated for more volatile compounds like VMS. To achieve this, a number of samplers were deployed at a suburban site in Toronto, Canada, and collected after different time points. Simultaneously the VMS concentration in air at the sampling site was measured using an active air sampling method based on solid phase extraction cartridges, previously validated and applied for VMS. The uptake in the XAD-PAS of octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and three linear VMS was linear throughout the whole deployment period of 98 days. The sampling rate was approximately 0.5 m3/day for all congeners. For hexamethylcyclotrisiloxane (D3) and dodecamethylcyclohexasiloxane (D6) the blank levels were too high relative to the concentrations of D3 and D6 in air at the site to assess the uptake in the sampler. Subsequent to the calibration study, XAD-PAS samplers were deployed at 26 sites in Toronto and the surrounding area to evaluate the influence of both population density and proximity to sewage treatment plants (STPs) on urban atmospheric levels of VMS. The sites could mainly be binned into three groups; two STP sites with strongly elevated levels of VMS in air, the urban sites with intermediate levels; and three rural background sites with very low levels of VMS.