Size resolution and capillary centering in sheathed condensation particle counters studied with a new variable saturation condensation particle sizer

The variable-saturation condensation particle sizer (VSCPS) of Gallar et al. (2006) determines aerosol condensation (Kelvin) size distributions by measuring optically the concentration of grown particles, while quickly scanning over the saturation ratio. A time-varying yet spatially uniform saturation is achieved by injecting the aerosol with a capillary placed at the center of a larger flow of sheath gas, created by mixing variable proportions of a saturated and a dry flow. Prior studies with other VS CPSs with centered capillary injectors had shown excellent sizing resolution when scanning over the saturator or the condenser temperature. However, some VSCPSs exhibited poorer resolution. Using the VSCPS of Gallar et al. (2006) we observe here that a chief reason for resolution variability is an imperfect capillary centering. A new VSCPS design preserving an excellent centering upon assembling and disassembling is then tested, systematically achieving excellent resolution. We also widen the cross sections of the aerosol and the sheath flow at their merging region to reduce the corresponding Reynolds numbers, and hopefully maintain laminar flow at larger aerosol and sheath gas flow rates. This goal is also largely achieved, except at small aerosol flows, though at the cost of an increased response time.