Counting Efficiency of the TSI Model 3020 Condensation Nucleus Counter

The counting efficiency of the TSI model 3020 condensation nucleus counter (CNC) was determined as a function of aerosol flow rate and trigger level using aerosols of known size and an aerosol electrometer. When the aerosol flow rate dropped from 300 to 200 mL/min, counting efficiencies increased significantly in the single-particle counting mode for particles with diameter < 20 nm while those for larger particles remained constant. However, the photometric mode counting efficiency for particles with diameter > 20 nm increased and exceeded unity. When the aerosol flow rate was reduced to 100 mL/min, the counting efficiencies for both counting modes decreased regardless of particle size. Varying the trigger level of the CNC did not influence the photometric mode counting efficiency. However, the counting efficiency of the single-particle counting mode increased with decreasing trigger level, especially for particles < 20 nm in diameter. Characteristics for individual instruments need to be measured because counting efficiencies of two CNCs with the same trigger level and flow rate were not identical.