Design and Calibration of the EPA PM_2.5Well Impactor Ninety-Six (WINS)

Abstract

The EPA well impactor ninety-six (WINS) was designed and calibrated to serve as a particle size separation device for the EPA reference method sampler for particulate matter under 2.5 m aerodynamic diameter.The WINS was designed to operate downstream of a PM 10 inlet at a volumetric ow rate of 16.7 Lpm.For design simplicity and ease of construction, fractionation of the aerosol in the WINS is provided by a single-stage, single-jet, round-hole impactor.Particles >2.5 m (aerodynamic diameter) have suf cient inertia to be impacted upon a circular 37 mm diameter glass ber lter immersed in 1 mL of a low volatility oil.The relatively large amount of oil is intended to minimize substrate overloading and subsequent particle bounce experienced by some conventional impactors and to represent an easily eld dispensable quantity of de ned tolerance.The novel geometry of the impaction reservoir (or well) is designed to capture any reentrained material from the impaction surface and to prevent loss of oil should the unit be inadvertently turned over or onto its side.The penetration curve of the nal WINS design has a 50% cutpoint diameter equal to 2.48 m and a geometric standard deviation of 1.18.During development, several nozzle designs and well geometries were evaluated to optimize the performance of the WINS.Additionally, two candidate oils (Neovac and Dow Corning 704 diffusion pump oils) and three types of lters (glass ber lters, drain discs, and polycarbonate membrane lters) were evaluated for use as impaction substrates in the WINS.The performance of the WINS was similar for the two oils in combination with a glass ber lter and a drain disc; however, a polycarbonate lter demonstrated elevated penetration values.Based on these tests, a Gelman Type A/E glass ber lter immersed in Dow Corning 704 diffusion pump oil was selected as the best impaction substrate.Further testing showed that the penetration curve was essentially the same when operated with quantities of oil ranging from 0.75 to 3 mL.