Sorption and desorption kinetics and isotherms of volatile methylsiloxanes with atmospheric aerosols.

Abstract

This study investigated sorption and desorption behaviors of airborne volatile methylsiloxanes (VMS) such as octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) on nine major primary and secondary atmospheric aerosols at a relative humidity (RH) of 30%. It was found that sorption and desorption of VMS took place via a two-phase process, which included an initial rapid step, followed by slower subsequent step. The initial rapid step was favored especially at low concentrations. Equilibrium sorption isotherms were slightly better fitted to Polanyi-Manes sorption model than Langmuir model except D4 on black carbon and D5 on sea salt. Values of apparent aerosol-air partition coefficients ranged 0.09-50.4 L/m(2) for D4 and 2.1-284 L/m(2) for D5 with carbon black having the largest values. Some of aerosols such as carbon black and sea salts reversibly interacted with D4 and D5 whereas other aerosols such as kaolinite and sulfates showed highly irreversible sorption for the VMS, especially at low concentrations. As sorption density of D4 and D5 on kaolinite was decreased from 1100 to 250 µg/m(2), the irreversible fraction was increased from 27% to 80%.The mechanism responsible for these differences is essential for a better understanding and prediction of atmospheric chemistry of VMS as affected by the presence of atmospheric aerosols.