A static particle size selective bioaerosol sampler for the ambient atmosphere

A static size-selective bioaerosol sampler for the ambient atmosphere was developed for studying the allergen carried by atmospheric aerosols smaller than intact pollen grains. The sampler consists of the University of Minnesota-type aerosol sampling inlet, a pre-impactor for water droplet and insect removal from the sample, a two-stage virtual impactor for the separation of intact pollen grains and selective sampling of fine and coarse ambient particles smaller than intact pollen grains, a condenser for vapour collection, and a flow control unit. The total flow rate of the sampler is 18.5 l min−1. Minor flow rate of both virtual impactor stages is 10% of the total flow through the corresponding stage. The virtual impactor was calibrated with solid ammonium fluorescein particles in the aerodynamic size range 1.7–14 μm. Aerodynamic cut diameters of the stages 1 and 2 of the virtual impactor were measured to be 7.2 and 2.4 μm, respectively. Maximum wall losses of stages 1 and 2 were 12 and 5%, respectively, occurring at the ‘cut-point’ particle sizes. When plotted as the function of the square root of the Stokes number, measured separation efficiencies collapsed on a single curve having √Stk50 = 0.60.