Heterogeneous uptake of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) onto mineral dust aerosol under variable RH conditions

We have carried out kinetic studies to characterize the heterogeneous decay of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in the presence of representative mineral dust aerosol in order to obtain a better understanding of the atmospheric fate of these siloxanes. The heterogeneous chemistry of D4 and D5 with various mineral dusts was studied in an environmental aerosol reaction chamber using FTIR absorption spectroscopy to monitor the reaction. The apparent heterogeneous uptake coefficient, γapp, for D4 and D5 with various mineral dusts was measured under dry conditions and as a function of relative humidity (RH). In addition, the effect of initial D4 and D5 concentration on the rate and yield of the reaction was examined. The uptake coefficient, γapp, for D4 and D5 was similar for the most reactive aerosols tested, with kaolinite ≈hematite > silica. Limited uptake onto carbon black and calcite surfaces was observed for either siloxane. Reaction with hematite and kaolinite resulted in multilayer coverages, suggesting extensive polymerization of D4 and D5 on the aerosol surface.