Counting Efficiency of an Improved 30-Å Condensation Nucleus Counter

Abstract

The ultrafine aerosol condensation nucleus counter (UFACNC) developed by Stolzenburg and McMurry (1990) has been improved. This new instrument can detect particles as small as 30 Å in nitrogen at a sample flow rate of 2.5 cm3·s−1. This flow rate is five times that of the earlier design with the same particle counting efficiency. An increased sample flow rate causes an increase in the arrival rate of detected particles. This improves accuracy in estimating the true particle concentration in ultraclean gas systems. In order to demonstrate its high accuracy the new UFACNC was used to measure the concentration of contaminant particles in ultrafiltered nitrogen. This represents the first accurate determination of ultrafine contamination levels in large ultraclean gas systems. The nitrogen was found to contain as few as 1.28 × 10−4 particles/cm3 (3.62/ft3). The higher volumetric flow rate of the new counter reduced the sample standard deviation of the measured count rate. Therefore, in the case of a low particulate count rate, the difference between the measured and instrument noise background count rates could be more accurately resolved. In addition, the new UFACNC was used to demonstrate a new technique for size classifying ultrafine contaminant particles using only a UFACNC, i.e., without a separate size classifier. The results demonstrate that ultrafine particle size distributions can be obtained directly from a stand-alone UFACNC