The aspiration efficiency of a thin-walled, cylindrical aerosol sampler facing vertically upwards in a slow moving vertical air stream is numerically investigated using both a potential flow model and a viscous flow model. In order to predict the air flow around the sampler, for the potential flow model we use a boundary element method whereas for the viscous flow model we use a control volume method. The motion of the particles is then predicted by considering both the drag and gravitational forces. We have found that both numerical models produce similar predictions for the aspiration efficiency and the predictions reveal a more complex sampling behaviour when the sampler is operated in a slow moving air environment, where the air velocity is comparable with the magnitude of the particle settling velocity, than for faster moving air flows. The comparison of the numerical predictions with the only available experimental data indicates that the aspiration efficiency is in qualitative agreement but further investigations are required in order to fully reveal all the sampling characteristics in slow air flows. (C) 2003 Elsevier Ltd. All rights reserved.