An ultrafine,water-based condensation particle counter(U-WCPC,TSI Model 3786)has been compared to a butanol-based ultrafine counter(U-BCPC,TSI Model 3025)for measurement of atmospheric and freeway-tunnel aerosols.The U-WCPC utilizes a warm,wet-walled growth tube to activate and grow particles through water condensation in a laminar-flow.It has an aerosol sampling rate of 0.3 L/min,and a nominal detection limit near 3 nm.Several field comparisons were made to the butanol-based instrument with the same nominal detection limit.For measurements of size-selected aerosols with diameters of 5 nm and larger the two instruments generally agreed,with a mean response within 5.At 3 nm particle size differences were observed,and these differences varied with the data set.Measurements of ambient aerosol in Boulder,Colorado showed higher counting efficiency at 3 nm with the U-BCPC,while in a California freeway tunnel the opposite trend was observed,with higher counting efficiencies at 3 nm observed by the U-WCPC.For direct measurement of atmospheric aerosols,the two types of instruments yielded equivalent concentrations,independent of particle number concentration.
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