The research described in this paper extends the body of work contained in earlier publications in which aerosol sampling aspiration efficiency was studied in a small wind tunnel for simple, idealized sampler geometries. These studies are aimed at providing generic knowledge about the physical and functional behaviors of such systems that can then be generalized to sampling systems of more practical interest. In this paper, attention is focused on the little-researched case of a blunt sampler facing away from the prevailing wind velocity. The results show, that for angles greater than 90degrees with respect to the wind, aspiration efficiency and particles losses inside the sampler close to the entry are constant as a function of a. This in itself may have some practical usefulness since it enables the definition of a regime of aerosol performance that is relatively "well-behaved". These results are in marked contrast to what has been learned from the same body of work for sampler behavior when facing forward with respect to the wind. Here not only does aspiration efficiency vary sharply with orientation, but so too does the particle entry loss. From this it is concluded that no general model of aspiration efficiency can be available so long as experimental results embody unknown entry loss biases. With this in mind, caution is recommended in field sampling or in laboratory experiments where the measurement does not explicitly include such entry losses. (C) 2002 Elsevier Science Ltd. All rights reserved.