Uptake on fractal particles 1. Theoretical framework

We develop a detailed framework for calculating uptake rates of gases on fr actal particles. Aggregates with a fractal dimension between 1.6 and 2.6 ar e generated numerically. Three-dimensional simulations of uptake on these a ggregates are performed for a large range of conditions. The numerical resu lts can be expressed by a single physically based formula as a function of the usual parameters (gas phase diffusion coefficient, uptake coefficient) and of morphological parameters (aggregate gyration radius, size of the mon omers composing the aggregate, fractal dimension). The expression for the u ptake flux on an aggregate is generalized to a lognormal distribution of ag gregates. The results for an isolated aggregate are compared to calculation s used for spherical particles. The importance of accounting for the fracta l character of aggregates in the calculation of the uptake flux is highligh ted. The magnitude of the errors also depends on which aggregate equivalent size is used as the characteristic radius for mass transfer.