Sensitivity analysis of the USEPA WINS PM2.5 Separator

Abstract

Factors affecting the performance of the USEPA WINS PM2.5 separator have been systematically evaluated. In conjunction with the separator's laboratory calibrated penetration curve, analysis of the governing equation that describes conventional impactor performance was used to predict changes in cutpoint as a function of impactor dimensions, flow rate, uncertainties in ambient temperature and pressure measurement, and the temperature and pressure of the sampled air volume. By integrating the resulting performance curves with three idealized ambient aerosol size distributions, the effect of these parameters on measured PM2.5 concentration was predicted. Results showed that allowable variations in impactor jet width, flow rate, diffusion oil volumes, and ambient temperature and pressure measurement result in relatively minimal PM2.5mass concentration measurement biases, Loading of the WINS well with previously collected particles slightly reduces the separator's cutpoint and thus slightly reduces expected PM2.5 mass concentrations. Variations in ambient pressure produce negligible changes in the performance of the WINS. While not causing a true measurement bias as defined by the regulations, low ambient temperatures naturally affect the airstream's properties and inherently shifts the WINS' cutpoint to slightly lower values. Laboratory-induced crystallization of the DOW 704 diffusion oil produced no appreciable changes in either the position or shape of the WINS separation curve.