A compact multistage (cascade) impactor for the characterization of atmospheric aerosols

Abstract

This paper presents the development, laboratory characterization, and field evaluation of a compact multistage cascade impactor (CCI). The CCI operates at a flow rate of 30 LPM and consists of eight impaction stages equipped with rectangular slit-shaped acceleration nozzles. A 47 mm backup Teflon membrane filter is used downstream of the eighth stage to collect particles smaller than 0.16 μm . In each stage, particles are retained by impaction onto the inert polyurethane foam (PUF) substrate. The major feature of this novel sampler is its ability to both fractionate by size and collect relatively large amounts of particles (mg quantities) onto an inert polyurethane foam impaction substrates. Even though the impaction substrates are not coated with adhesives such as grease or mineral oil, particle bounce and re-entrainment losses were found to be insignificant. Impaction characteristics (cutpoint and sharpness of collection efficiency curve) of the PUF substrates are maintained for mass loadings of at least 25 mg , which is much higher than for other commonly used rigid, flat impaction substrates. The system was calibrated in laboratory experiments using polydisperse aerosols. The 50% cutpoints of the eight stages were 9.9, 5.3, 3.3, 2.5, 1.7, 1.0, 0.47 and 0.16 μm (aerodynamic diameter), with pressure drops of 0.02, 0.02, 0.04, 0.06, 0.08, 0.27, 1.57 and 5.73 kPa . These pressure drops are considerably lower than those obtained using flat rigid impaction substrates with comparable cutpoints. Particle losses for each stage were less than 10% for particles smaller than 7 μm and less than 20% for particles larger than 7 μm . The CCI was also compared with the collocated micro-orifice impactor (MOI) in laboratory-controlled experiments using artificially generated polydisperse aerosol. These laboratory tests showed that the mass concentrations measured by the MOI are considerably lower than those measured by the CCI (the average ratio of total mass concentration of MOI to CCI was 0.86), with the size distribution measured by the CCI closer to that measured using the real time particle sizing instruments (SMPS, APS). A field comparison of CCI, the Harvard impactor (HI) and the federal reference method (FRM) for fine particles showed a good agreement between the CCI and the reference samplers for particles smaller than 2.5 μm .