INTERCOMPARISON BETWEEN COMMERCIAL CONDENSATION NUCLEUS COUNTERS AND AN ALTERNATING TEMPERATURE-GRADIENT CLOUD CHAMBER

Three commercial CNC counters (TSI models 3010, 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 f or internal diffusional losses, but the TSI instruments are not. The p article size range tested is 4-90 nm. For sodium chloride, the TSI con centrations agreed fairly well with the corrected ALGR at larger sizes , and were below the corrected ALGR at smaller sizes. The electrical a erosol classifier (EAC) setting at which a particular TSI instrument c oncentration is 50% of the corrected ALGR concentration is denoted D-5 0%. For the 3010 and 3022A, the values of D-50% are 14.5 and 7.9 nm, r espectively. For the 3025A the extrapolated D-50% is 3.5 nm. These D-5 0% values are close to those reported for comparisons between concentr ations measured by the TSI instruments and concentrations determined w ith an aerosol electrometer. However, the count ratio rises to unity l ess quickly as size increases for the present intercomparisons as comp ared with intercomparisons with aerosol electrometers. The ALGR can pr oduce at most a 30% water supersaturation, corresponding to a Kelvin d iameter of 8.4 nm. Thus, since silver particles are water insoluble, t he ALGR detected small silver particles less efficiently than the TSI model 30256 The data with silver aerosols. allowed determination of th e relation between size and critical supersaturation over the size ran ge 16-30 nm diameter. The silver aerosol exhibited critical supersat u rations higher, by a factor of 1.2 to 1.8, than those given by the Kel vin equation.