APPLICATION OF A RADIAL DIFFUSION-MODEL TO DESCRIBE GAS PARTICLE SORPTION KINETICS

A radial diffusion model is adapted to describe the kinetics of the exchange of organic compounds between the gas and particulate phases in the atmosphere. A Crank–Nicolson finite difference scheme with variable time stepping is used in applying the model to polydisperse particle size distributions. Two sets of conditions are simulated: (1) airborne particles, as in the ambient atmosphere, and (2) filter-bound particles, such as those found in an air filtration apparatus. The time scale of the intraparticle diffusion reaction, TD, is greatly dependent upon the atmospheric partition coefficient of the compound, Kp (m3/μg), and the particle size distribution of the aerosol. For most compounds typically determined in atmospheric samples, sorption equilibrium with airborne particles is approached rapidly. Sorption reactions that take place on /in filters are controlled not only by TD but also by an equilibrium mass-transfer time scale, which is a function of Kp, the amount of particulate mass, Mp, and the volumetric flow rate, f. The kinetics of sorption reactions on/in filters are predicted to play an important role in the determination of both the probability and the magnitude of sorption-related artifacts within field-derived values of Kp. © 1990, American Chemical Society. All rights reserved.