The ultrafine aerosol condensation nucleus counter (UFACNC) developed by Stolzenburg and McMurry, 1990 has been improved. The improved instrument can detect particles as small as 30 Angstroms in nitrogen at a sample flow rate of 2.5 cm3.sec.-1. This flow rate is five times that of the earlier design. The increased sample flow rate provides an increase in the arrival rate of detected particles. The result is an improved accuracy in estimating the true particle concentration in ultraclean gas systems. The improved UFACNC was used to measure the concentration of ultrafine contaminant particles in two ultraclean nitrogen systems. The nitrogen was found to contain as few as 1.28 × 10-4 particles per cm3 (3.62 per cubic foot). The higher flow rate of the improved counter resulted in a relatively small standard deviation of the measured particle concentration. Therefore, the small difference between the process and instrument noise background count rates was accurately resolved. The improved UFACNC was also used to demonstrate a new technique for size classifying ultrafine particles using only a UFACNC, i.e., without a separate size classifier. The results demonstrate that particle size distributions can be obtained directly from a stand-alone UFACNC.