Evaluation of PM2.5 size selectors used in speciation samplers

The separation characteristics of the PM2,5 aerosol size selectors used in speciation samplers developed for the U.S. EPA National PM2.5 Chemical Spec iation Trends Network were evaluated under clean renditions. Measurement of particle penetration versus aerodynamic diameter was conducted using an AP S 3320 in conjunction with a polydisperse test dust, The resulting penetrat ion curves were integrated with assumed ambient particle size distributions (40 CFR Part 53, Subpart F) to obtain an estimate of measured mass concent ration and to predict bias relative to the PM2.5 reference separator. The c utpoint of two sharp cut cyclones, from the family of cyclones developed by Kenny and Gussman (1997), compares favorably with the WINS, although posse ssing a slight tail that extends into the coarse particle mode. A second cy clone used by the Andersen Corp., AN 3,68 demonstrated the sharpest cut cha racteristics of the devices tested; however, it possesses a D-50 cut size o f 2.7 mum at its design flow rate. The separation characteristics of the Sp iral separator were observed to be the shallowest and were >2.5 mum when gr eased or ungreased. Estimated mass concentration bias relative to the FRM w as within +/-5% for the idealized fine and typical assumed ambient distribu tions 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 demons trates that a laboratory evaluation under clean conditions, while useful fo r design purposes, is not sufficient to predict separator behavior in the " real world".