Uptake on fractal particles 2. Applications

This first part of the paper is devoted to the validation of the theoretica l framework developed in the companion paper [Coelho et al,, this issue] ag ainst laboratory data. Although there are limited data suitable for a full- scale validation of the model, model-calculated surface to mass ratios of s oot aggregates are found to be consistent with laboratory measurements. The n we use the framework to estimate errors which can be generated in the der ivation of fractal aggregate properties from measurements of equivalent siz es. It is shown that the derivations of the aggregate mass and the surface area enhancement factor can be in error by at least an order of magnitude. The calculations are substantially improved when the fractal character of s oot is accounted for. Since the gyration radius is a key parameter of the m odel, useful relationships are provided for converting aggregate equivalent sizes (mass transfer equivalent radius, hydrodynamic radius) into gyration radius. Finally, uptake of chemical species on atmospheric soot is investi gated for the lower stratosphere and the boundary layer. The uptake is foun d to be mostly reaction limited, justifying the assumption of proportionali ty between the soot surface area and the uptake rate. However, the uptake o ccurs in the transition regime for the relatively compact aggregates found in urban areas.