A Variable Supersaturation Condensation Particle Sizer

Abstract

A prototype variable supersaturation condensation particle sizer (VSCoPS) capable of measuring particle size distributions from 5 to 30 nm has been developed. This system design is adapted from existing condensation particle counter (CPC) technology with three significant differences: (1) the working fluid is a perfluorinated organic compound that is nonreactive toward, and not an effective solvent for, most laboratory or ambient particle compositions; (2) the vapor pressure of the working fluid is controlled by dilution of saturated air with vapor-free air at the same temperature; and (3) the optical block and condenser are located below the saturator, so that fluid condensed on the condenser walls does not flow back toward the saturator. By using fast-response flow controllers to vary the ratio of saturator and dilution air while keeping total flows and temperatures constant, the vapor saturation ratio in the condenser can be controlled with time constants of ∼ 1 s. The nucleation threshold diameter is changed by stepping through small increments in saturation ratio. The particle size distribution can be recovered by inverting the measured concentration using the known instrument response for each saturation ratio. Further development of the system may allow size distribution measurements to smaller particle diameters and scan times of < 30 s at total particle concentrations as low as ∼ 100 cm^{− 3} .

This research was supported through the Innovative Research Program of the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado. We thank Darin Toohey, Shelly Miller, and David Henderson for their contributions to an earlier prototype of this instrument.

Notes

¹Taper 25^ˆ from 1.9 cm to 0.64 cm.

¹At 1013 hPa and 0°C.

²Sum of saturator and dilution flows.