Design, calibration and testing of a parallel stage impactor

The fractionation of airborne particles according to aerodynamic size by a number of impactor stages operated in parallel offers significant advantages over the commonly employed cascade impaction technique when determining mass or chemical size distribution of an aerosol. A seven stage, round jet impactor based on the parallel stage concept was designed, manufactured, calibrated and tested. The stages operate at 25 liters per minute collecting fractions of zero to 0.35, 0.70, 2.0, 4.0 and 7.0 micrometers aerodynamic diameter. The last two stages have no impaction surface and collect aerosol over the entire size range. All stages are mounted on standard 47 mm filter holders. For each stage, particles larger than the cut-off size impact on a thick adhesive surface and are discarded while those smaller are collected on a filter. The ratio of the particle mass penetrating to the filter of a given stage to the mass of the entire aerosol represents the cumulative mass fraction associated with the cut-off diameter for that stage. Any losses associated with non-ideal behavior in the impactor become an inherent part of the calibration. Calibration of the impactor showed that stage cut-off sizes are close to the values predicted by impactor theory. A novel technique employing polydisperse aerosol was used in the calibration. Challenge of the impactor calibration with solid aerosols of known size distribution gave excellent results. Tests for particle bounce after four hours of operation in urban aerosol showed that accumulation of particles at the impaction surface does not increase bounce or blow-off. Microscopic examination of impaction surfaces immediately after sampling showed most of the particles to be embedded within the adhesive rather than on the surface.