Counting efficiency of condensation particle counters at low-pressures with illustrative data from the upper troposphere

The low-pressure counting efficiency of three modified condensation particle counters (CPCs, TSI Model 7610) was investigated. In this study, we describe the modifications of the counters, present a "new" low-pressure calibration set-up, and show the resulting CPC counting efficiency curves as function of operating pressure (160-1000hPa) and temperature difference between the saturator and condenser block (10.5-26.0°C). Furthermore, particle coincidence in the CPC optics was studied. The presented CPCs are able to measure particle number concentrations of up to 40,000 particles/cm3 at pressures down to 160hPa. The lower threshold diameter of the CPCs can be varied between 4 and 23nm.