Field evaluation of a wind tunnel-impactor system for sampling ambient aerosols

The wind tunnel-impactor aerosol sampling system consists of a horizontal duct in which a cascade impactor is mounted. Inside this wind tunnel a constant air speed is maintained, equal to the air velocity in the impactor inlet, and the whole system can be aligned with the ambient wind direction. In this way, sampling biases due to anisokinetic and anisoaxial conditions can be largely reduced. The efficiency of this sampling system, in field conditions, was evaluated by operating a Battelle cascade impactor inside the wind tunnel, in parallel with an identical impactor sampling from the free wind. The elemental concentrations of the aerosol samples collected with these two sampling systems were measured by PIXE and compared by means of a paired t-test. It was shown that, when the weather conditions are not too extreme, the wind tunnel-impactor system is more efficient for the collection of particles > 4 mu m d(ae), than the impactor operating in the free wind. Differences of up to 150% were noted. (C) 1999 Elsevier Science Ltd. All rights reserved.