New experimental studies to directly measure aspiration efficiencies of aerosol samplers in calm air

Typical air movements in many indoor living and working places are as low a s, or less than, 0.1 m/s, and so conditions might best be described in term s of calm, as opposed to moving air-or perhaps some combination of the two. However, the history of aerosol sampling science has been dominated by stu dies of the aspiration efficiencies of sampling devices in moving air, and there have been very few definitive studies of calm air sampling, even for very simple sampling systems. With this in mind, this paper sets out the ba sis for the development of a larger body of experimental work, and describe s an experimental method by which unambiguous measurements may be made for a range of simple aerosol sampling systems (i.e., shape and orientation). T he method, referred to as the "direct method"', owes much to the 'thread-of -droplets' approach proposed by Lipatov ct al. tJ. Aerosol Sci. 17 (1986) 7 63) for studies of aspiration efficiency in moving air. In the new method, monodisperse large droplets of olive oil with aerodynamic diameter in the r ange from 40 to 70 mum, were, generated using a vibrating orifice aerosol g enerator and the experimental conditions were controlled very carefully so that a well-defined, vertically-falling droplet stream was obtained. By the application of appropriate tracking apparatus, visual observation of this droplet stream was used to map the pattern of particle trajectories near a given sampling inlet and to identify limiting particle trajectories in orde r to calculate aspiration efficiency. In an initial application of the new method, measurements of aspiration efficiency were carried out for upwards- facing thin-walled sampling tubes, and the results were found to be in sati sfying agreement with the numerical results of Agarwal and Liu (1980, in th e Am. Ind. Hyg. Assoc. J. 41 (1980) 191) and the numerical and experimental results of Yoshida et al. (Kagaku Koqaku Ronbunshu 4 (1978) 123). The resu lts of this initial study provide confidence that the method described may now be applied to a wider range of sampling systems. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.