Design and calibration of the EPA PM2.5 well impactor ninety-six (WINS)

The EPA well impactor ninety-six (WINS) was designed and calibrated to serv e as a particle size separation device for the EPA reference method sampler for particulate matter under 2.5 mum aerodynamic diameter. The WINS was de signed to operate downstream of a PM10 inlet at a volumetric flow rate of 1 6.7 Lpm. For design simplicity and ease of construction, fractionation of t he aerosol in the WINS is provided by a single-stage, single-jet, round-hol e impactor, Particles >2.5 mum (aerodynamic diameter) have sufficient inert ia to be impacted upon a circular 37 mm diameter glass fiber filter immerse d in 1 mL of a low volatility oil. The relatively large amount of oil is in tended to minimize substrate overloading and subsequent particle bounce exp erienced by some conventional impactors and to represent an easily field di spensable quantity of defined tolerance. The novel geometry of the impactio n reservoir (or well) is designed to capture any reentrained material from the impaction surface and to prevent loss of oil should the unit be inadver tently turned over or onto its side. The penetration curve of the final WIN S design has a 50% cutpoint diameter equal to 2.48 mum and a geometric stan dard deviation of 1.18., During development, several nozzle designs and wel l geometries were evaluated to optimize the performance of the WINS. Additi onally, two candidate oils (Neovac and Dow Coming 704 diffusion pump oils) and three types of tilters (glass fiber filters, drain discs, and polycarbo nate membrane filters) were evaluated for use as impaction substrates in th e WINS. The performance of the WINS was similar for the two oils in combina tion with a glass fiber filter and a drain disc; however, a polycarbonate f ilter demonstrated elevated penetration values, Based on these tests, a Gel man Type A/E glass fiber filter immersed in Dow Corning 704 diffusion pump oil was selected as the best impaction substrate. Further testing showed th at the penetration curve was essentially the same when operated with quanti ties of oil ranging from 0.75 to 3 mL.