Simulation of semivolatile organic compound microtransport at different time scales in airborne diesel soot particles

The microtransport of polycyclic aromatic hydrocarbons I (PAHs) in airborne diesel soot particles over time scales ranging from 0.18s to 11.9 h was Studied via a smog chamber experiment and simulated using radial diffusion models. The sorption of the PAHs by particles overlong time periods (i.e;, minutes to hours)was studied by monitoring the gas and particle-phase concentrations in the chamber over the course of the experiment (11.9 h). Additionally, the desorption of PAHs from particles over short time scales (i.e., tenths of seconds to several seconds) was observed by passing the aerosol through a: large gas-phase stripping device and a-sampling denuder to remove the aerosol's gas phase, thus causing the PAHs to desorb from the particles. The results of each experimental test. were compared to simulation results using a one-layer model and four dual-impedance radial:diffusion models. The dual-impedance models were able to closely reproduce the experimental results of each test, while the one-layer model produced poor fits to the experimental data, especially for the rapid desorption tests. The low values of the resulting optimized apparent diffusion coefficients (similar to 10(-19)-10(-11) cm(2)/s) indicate significant impedance to microtransport beyond simple free;liquid diffusion. The mechanisms responsible for these impedances are explored. The results of this study indicate that a dual-impedance radial diffusion model is a useful tool for predicting the microtransport of PAHs in :diesel soot particles land probably other types of carbonaceous particles containing a significant organic fraction) over a wide range of time scales.