Concentration measurement and counting efficiency for the aerodynamic particle sizer 3320

The aerodynamic particle sizer (APS, Model 3320, TSI Inc., St. Paul, MN) is an instrument that counts and sizes particles by time-of-flight, an aerodynamic property, and/or by light-scattering intensity, an optical property. If the counting efficiency of the APS 3320, defined as the number of particles counted divided by the number sampled is not 1.0 for particles of all sizes, then the reported size distributions and particle concentrations will be biased. A laboratory aerosol was sampled with two APS 3320s alternating between collecting only time-of-flight data in summing mode and collecting simultaneous time-of-flight and light-scattering intensity data in correlated mode. Collecting data in correlated mode resulted in errors in the reported aerodynamic size distributions and concentrations. The magnitude of the concentration error was an inverse function of concentration, ranging from approximately 10% at 500 particles/ cm 3 to 45% at 50 particles/ cm 3 . Experiments were also conducted to determine the counting efficiency of the APS 3320 in summing mode by comparing size distributions obtained with the analyzer to those obtained with a cascade impactor. Counting efficiency increased from 30% for 0.5 μm particles to 100% for 0.9 μm particles, then decreased to 60% for 5 μm particles. For particles larger than about 5 μm , the size distribution reported by the APS 3320 was distorted by artificial particle counts. To determine accurate particle size distributions and concentrations, the values reported by this instrument must be adjusted for counting efficiency.