Measuring snow scavenging of two airborne cyclic volatile methylsiloxanes under controlled conditions

The objective of this study was to determine snow scavenging of cVMS and its potential effect on the cVMS concentrations in snowmelt water and surrounding soil. Snow scavenging of two cVMS, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), was examined in two steps. First, sorption and desorption of D4 and D5 on snowflakes, including snow sorption coefficients (K_iA), were measured against a benchmark compound, cyclopentanone, at different temperatures from 0 to -20 °C. Measurements were made using a custom-made snow chamber and ¹⁴C-labeled D4 and D5. In addition, the transfer of snow-bound cVMS to snowmelt water and surrounding soil was studied with ¹⁴C-D4 and ¹⁴C-D5-spiked snowpack placed both in a closed snow chamber and on top of a layer of frozen soil in an open chemical hood. K_iA values measured in both sorption and desorption processes were very small (<10^-2 m). They increased with decreasing temperature and were higher for the D5 compared to D4. The calculated gas scavenging of D4 and D5 was small because of the small K_iA values, while particle scavenging of cVMS is predicted to be negligible due to their low octanol/air partition coefficients (K_OA). Most importantly, almost all ¹⁴C-D4 and ¹⁴C-D5 sorbed by a snowpack was lost during the snow melting process through re-volatilization and hydrolysis and became non-detectable in snowmelt water. In short, the experimental measurements demonstrated that snow scavenging could not be a valid deposition mechanism for these volatile hydrophobic compounds.