This first part of the paper is devoted to the validation of the theoretical framework developed in the companion paper <Coelho et al., this issue> against laboratory data. Although there are limited data suitable for a full-scale validation of the model, model-calculated surface to mass ratios of soot aggregates are found to be consistent with laboratory measurements. Then we use the framework to estimate errors which can be generated in the derivation of fractal aggregate properties from measurements of equivalent sizes. 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 soot is accounted for. Since the gyration radius is a key parameter of the model, 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 investigated for the lower stratosphere and the boundary layer. The uptake is found to be mostly reaction limited, justifying the assumption of proportionality between the soot surface area and the uptake rate. However, the uptake occurs in the transition regime for the relatively compact aggregates found in urban areas. ¿ 2000 American Geophysical Union