In this work the fluid flow and particle trajectories for a thin-walled sampling probe facing the wind are considered. The flow around the sampler and into the sampler has been determined numerically and the paths of the particles in the flow are then traced and the efficiency of the sampler investigated. A variety of operating conditions have been considered, in particular large values of the velocity ratio. R. which is equal to the ratio of the undisturbed free stream air velocity to the average sampling velocity. The situation of large R values is becoming increasingly important as samplers are developed with low flow rates. Previous experimental results have shown that the empirical model developed for sampling in moving air does not accurately predict the efficiency of samplers operating at these high values of R. The numerical results show that the aspiration efficiency for high R is not significantly affected by gravitational effects for the majority of cases of interest but it is dependent upon the magnitude of the reversal of the flow within the sampling tube. (C) 2004 Elsevier Ltd. All rights reserved.