Intercomparison Between Commercial Condensation Nucleus Counters and an Alternating Temperature Gradient Cloud Chamber

Three commercial CNC counters nSI models 30]0, 3022A, and 3025A) are compared with an alternating temperature gradient cloud chamber (ALGR). Electrically size classified aerosols of sodium chloride and silver are used. Diffusional losses within the ALGR are much larger than for the TSI instruments: therefore the ALGR concentrations are corrected for internal diffusional losses, but the TSI instruments are not. The particle size range tested is 4-90 nm. For sodium chloride, the TSI concentrations agreed fairly well with the corrected ALGR at larger sizes, and were below the corrected ALGR at smaller sizes. The electrical aerosol classifier (EAC) setting at which a particular TSI instrument concentration is 500/0 of the corrected ALGR concentration is denoted D50% For the 3010 and 3022A, the values of D50% are ]4.5 and 7.9 nm, respectively. For the 3025A the extrapolated D50% is 3.5 nm. These D50% values are close to those reported for comparisons between concentrations measured by the TSI instruments and concentrations determined with an aerosol electrometer. However, the count ratio rises to unity less quickly as size increases for the present intercomparisons as compared with intercomparisons with aerosol electrometers. The ALGR can produce at most a 30% water supersaturation, corresponding to a Kelvin diameter of 8.4 nm. Thus, since silver particles are water insoluble, the ALG R detected small silver particles less efficiently than the TSI model 3025A. The data with silver aerosols allowed determination of the relation between size and critical supersaturation over the size range 16-30 nm diameter. The silver aerosol exhibited critical supersaturations higher, by a factor of 1.2 to 1.8, than those given by the Kelvin equation