Development and application of a dual-impedance radial diffusion model to simulate the partitioning of semivolatile organic compounds in combustion aerosols

The fate of semivolatile organic compounds in the atmosphere is largely dependent on their partitioning between the gas phase and sorption to particulate matter. Since real atmospheres have been shown to deviate significantly from gas-particle equilibrium under certain conditions, dynamic mass transfer models are needed to accurately predict partitioning. In this work, a dual-impedance radial diffusion model is presented that is able to simulate the partitioning of deuterated fluoranthene in wood and diesel soot atmospheres generated in a large outdoor Teflon film chamber. It is shown that the dual-impedance model produces significantly better fits to experimental results than a one-layer model and that surface mass transfer is not rate limiting in these systems. The sensitivity of optimized apparent diffusion coefficients to key input parameters is also explored. This work lays the foundation for incorporating dynamic gas-particle partitioning models into larger atmospheric models, such as urban airshed models. By conducting experiments under various conditions (e.g., temperature and humidity), values for apparent diffusivities as a function of compound, particle source, and atmospheric conditions may be developed. After including photochemical reactions, the model may be used to predict the fate of semivolatile organic compounds in real atmospheres.