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

The wind tunnel-impactor aerosol sampling system consists of a horizontal d uct in which a cascade impactor is mounted. Inside this wind tunnel a const ant air speed is maintained, equal to the air velocity in the impactor inle t, 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 condi tions, was evaluated by operating a Battelle cascade impactor inside the wi nd tunnel, in parallel with an identical impactor sampling from the free wi nd. The elemental concentrations of the aerosol samples collected with thes e two sampling systems were measured by PIXE and compared by means of a pai red t-test. It was shown that, when the weather conditions are not too extr eme, the wind tunnel-impactor system is more efficient for the collection o f particles > 4 mu m d(ae), than the impactor operating in the free wind. D ifferences of up to 150% were noted. (C) 1999 Elsevier Science Ltd. All rig hts reserved.