Diffusion Dependence on Aerosol Composition by Analytical Solution: Derivation, Implementation and Limitation Inside a Multi-process Model

The evidence for diffusion limitation inside atmospheric particulates indicates a potential kinetic limitation to equilibrium between the condensed and gas phase: a 10 nm particle with glassy phase state is expected to take approximately 1 month to equilibrate (Koop et al. 2011). However, to fully explore the potential effect of condensed phase diffusion limitation, and to simulate all real scenarios, a solution to diffusion with a diffusion coefficient dependent on particle composition is required. This poster presents a method for the analytical solution to diffusion when diffusivity depends on composition and the concentration of the diffusing component at the particle surface is constant. A partial differential equation (pde) solution is used as the benchmark for accuracy. The analytical solution was implemented in the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC), which is widely used for aerosol process simulation in regional coupled models. Unlike the benchmark pde solution, which is too computationally expensive to be of use in a regional coupled model, the analytical solution may be readily utilized as demonstrated by a cost analysis. Finally, the limitations of making the analytical solution applicable to a variable concentration of the diffusing component at the surface are discussed, and alternative methods are proposed. Koop, T., Bookhold, J. Shiraiwa, M., Pöschl, U.: Glass transition and phase state of organic compounds: dependency on molecular properties and implications for secondary organic aerosols in the atmosphere, Phys. Chem. Chem. Phys., 13, 19238-19255, 2011.