The photochemical formation and gas-particle partitioning of oxidation products of decamethyl cyclopentasiloxane and decamethyl tetrasiloxane in the atmosphere

Decamethyl cyclopentasiloxane (D-5) and decamethyl tetrasiloxane (MD2M) wer e injected into a smog chamber containing fine Arizona road dust particles (95% surface area < 2.6 <mu>M) and an urban smog atmosphere in the daytime. A photochemical reaction - gas-particle partitioning reaction scheme, was implemented to simulate the formation and gas-particle partitioning of hydr oxyl oxidation products of D-5 and MD2M. This scheme incorporated the react ions of D-5 and MD2M into an existing urban smog chemical mechanism carbon bond IV and partitioned the products between gas and particle phase by trea ting gas-particle partitioning as a kinetic process and specifying an uptak e and off-gassing rate. A photochemical model PKSS was used to simulate thi s set of reactions. A Langmuirian partitioning model was used to convert th e measured and estimated mass-based partitioning coefficients (K-p) to a mo lar or volume-based form. The model simulations indicated that > 99% of all product silanol formed in the gas-phase partition immediately to particle phase and the experimental data agreed with model predictions. One product, D4TOH was observed and confirmed for the D-5 reaction and this system was modeled successfully. Experimental data was inadequate for MD2M reaction pr oducts and it is likely that more than one product formed. The model set up a framework into which more reaction and partitioning steps can be easily added. (C) 2000 Published by Elsevier Science Ltd.