The separation characteristics of the PM25 aerosol size selectors used in speciation samplers developed for the U.S. EPA National PM2.5 Chemical Speciation Trends Network were evaluated under clean conditions. Measurement of particle penetration versus aero- dynamic diameter was conducted using an APS 3320 in conjunc- tion with a polydisperse test dust. The resulting penetration curves were integrated with assumed ambient particle size distributions (40 CFR Part 53, Subpart F) to obtain an estimate of measured mass concentration and to predict bias relative to the PM2.5 refer- ence separator. The cutpoint of two sharp cut cyclones, from the family of cyclones developed by Kenny and Gussman (1997), com- pares favorably with the WINS, although possessing a slight tail that extends into the coarse particle mode. A second cyclone used by the Andersen Corp., AN 3.68, demonstrated the sharpest cut characteristics of the devices tested; however, it possesses a D5o cut size of 2.7 JLm at its design flow rate. The separation characteristics of the Spiral separator were observed to be the shallowest and were > 2.5 JLm when greased or ungreased. Estimated mass concentra- tion bias relative to the FRM was within :i:5 for the idealized fine and typical assumed ambient distributions for all separators. For the idealized coarse distribution, estimated bias ranged between +4 and +8. A comparison of these results with actual field measurements made by the samplers that employ these separators demonstrates that a laboratory evaluation under clean conditions, while useful for design purposes, is not sufficient to predict separa- tor behavior in the "real world".
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