Experimental and Theoretical investigations of the Effect of the Calibration Aerosol Material on the Counting Efficiencies of TSI 3790 Condensation Particle Counters

摘要

The counting efficiencies of two TSI 3790 Condensation Particle Counters (CPCs) were investigated experimentally for graphite, poly-(alpha)-olefin (PAO), tetradecane (C14), and hexadecane (C16) particles at saturator-to-condenser temperature differences spanning from 5.6° C to 11.3°C. The efficiencies determined with PAO, C14, and C16 particles were broadly similar, while tests with graphite particles resulted in systematically lower counting efficiencies. The differences between PAO and graphite particles were reduced at elevated temperature differences, i.e., as the saturation ratios inside the condenser increased. The possibility to predict measured counting efficiencies by heterogeneous nucleation theory was also assessed. The results for PAO, C14, and C16 were representative of perfectly wettable particles, while graphite data could only be reproduced with a contact angle of 6-12°, for all temperatures examined. Line tension fits revealed a linear correlation with the graphite mobility diameter for all operating temperatures (5 x 10~(-11) N at 15 nm to 4 x 10~(-10) N at 70 nm). This could actually indicate that the mobility diameter underestimates the contact line for these complex geometry graphite aggregates. An examination of the calculated activation regions inside the condenser indicated that the upper part of the counting efficiency curve (> 50 %) is very sensitive to flow and temperature nonidealities. This observation is in quantitative agreement with systematic deviations observed between theoretical predictions and experimental data. Numerical calculations for a range of working fluids suggested that for a given affinity of the calibration particle to the examined vapors (i.e., for a finite contact angle), the benefit from shifting to a fluid alternative to butanol is limited. Further investigations on the reduction of the material dependence should focus on the identification of working fluids exhibiting greater affinity for different particle materials (e.g., lower contact angle).