Effect of ozone and relative humidity on the heterogeneous uptake of octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane on model mineral dust aerosol components.

We have carried out kinetic and reaction yield studies to determine the effect of O(3) on the heterogeneous reaction of two cyclic volatile methylsiloxanes (cVMS), octamethylcyclotetrasiloxane (D(4)) and decamethylcyclopentasiloxane (D(5)), with model mineral dust aerosol in order to obtain a better understanding of the atmospheric fate of cVMS. The heterogeneous chemistry was studied in an environmental reaction chamber using FT-IR spectroscopy to monitor the reaction progress. The uptake kinetics and the reaction extent for D(4) and D(5) in the presence of O(3) were quantified for two components of mineral dust aerosol, hematite and kaolinite. Some experiments with a carbonaceous particulate, carbon black, were also performed for D(5). The relative humidity (RH) inside the chamber was varied to investigate the influence of surface adsorbed water on the heterogeneous chemistry of the dust samples. With the dust samples, but not carbon black, the coadsorption of O(3) introduced a new reaction pathway, characterized by a linear, zero-order, decay of both gas phase cVMS and ozone. The new pathway does not saturate on the time scale of our experiments. Elevated RH was observed to decrease the total uptake of cVMS and ozone by the end of the experiment, but the characteristic linear decay was still present. The atmospheric loss of cVMS due to heterogeneous uptake is enhanced due to O(3), even at higher RH values, but the overall loss rate is reduced at RH values typical of the troposphere.