Development of a sharp-cut cyclone for ambient aerosol monitoring applications

Aerosol samplers for ambient PM2.5 are required to possess a steep aerodyna mic particle size selection curve, i.e., a 'sharp cut' at 2.5 mu m aerodyna mic diameter. For long-term and continuous PM monitors the selector system also requires low maintenance and the ability to operate at high loadings. While a sharp cut is easier to achieve with an impactor-based selector, the other requirements are more easily met with a cyclone. Four alternative PM 2.5 selectors were tested against these criteria: two were pre-existing com mercial designs and two were novel cyclone prototypes. The main aim of the work was to assess the characteristics of the selectors when clean and unde r various loading conditions. The aerodynamic size-selection characteristics of the PM2.5 selectors were tested before and after loading,vith dust, under both laboratory and field conditions. Aerosol penetration measurements were made using an Aerodynamic Particle Sizer. Many repeat tests were performed on two specimens of the W ell Impactor Ninety-Six (WINS), two specimens of a novel Sharp-Cut Cyclone (SCC), one member of the GK cyclone family (GK4.39), and one University Res earch Glassware (URG) cyclone. Four loadings of the WINS and SCC were made in the laboratory using a narrow-fraction alumina dust. The penetration cur ves were measured after each loading. Five cumulative outdoor loadings were made by setting up four PM2.5 samplers, two with the WINS and two with the SCC, in a suburban garden during the summer months. The penetration curves were measured at weekly intervals after sampling times ranging from 96 to 132 h. Three further cumulative loadings were tested in a similar experimen t in a city-center underground car park When clean, all three PM2.5 size se lectors were shown to have 50% penetration (D-50) values close to 2.5 mu m, although the penetration curve shape differs for the three selector design s. Under loading the D-50 value for both the WINS and SCC fell, with the de crease being largest for the WINS. With high loadings the SCC D-50 fell to 2.35 mu m and the WINS D-50 fell to 2.15 mu m. The WINS deviation was large enough to potentially lead to ender sampling of PM2.5. The SCC cyclone was seen to provide a sharp cut for ambient air sampling ap plications that is less affected than the WINS by loading. Additionally, th e SCC is a dry system whereas the WINS uses an oiled substrate. While the W INS cut point is unlikely to shift to an unacceptable degree during 24 or 9 6 h sampling periods, it would perform less well than the SCC over extended sampling periods.