Intercomparison of 15 aerodynamic particle size spectrometers (APS 3321): uncertainties in particle sizing and number size distribution
- 1Leibniz Institute for Tropospheric Research, Leipzig, Germany
- 2Institute of Chemical Process Fundamentals, Academy of Science, Prague, Czech Republic
- 3Joint Research Center, Ispra, Italy
- 4Institute for Atmospheric Sciences and Climate, Bologna, Italy
- 5TSI GmbH, Particle Instrument, Aachen, Germany
- 6Federal Environment Agency, GAW Station Zugspitze/Hohenpeissenberg, Platform Zugspitze, Germany
- 7Federal Environment Agency, Platform Schauinsland, Germany
- 8Department of Physics, University of Helsinki, Helsinki, Finland
- 9National Observatory of Athens, Athens, Greece
- 10Department of Chemistry, University of Crete, Heraklion, Greece
Abstract. Aerodynamic particle size spectrometers are a well-established method to measure number size distributions of coarse mode particles in the atmosphere. Quality assurance is essential for atmospheric observational aerosol networks to obtain comparable results with known uncertainties. In a laboratory study within the framework of ACTRIS (Aerosols, Clouds, and Trace gases Research Infrastructure Network), 15 aerodynamic particle size spectrometers (APS model 3321, TSI Inc., St. Paul, MN, USA) were compared with a focus on flow rates, particle sizing, and the unit-to-unit variability of the particle number size distribution.
Flow rate deviations were relatively small (within a few percent), while the sizing accuracy was found to be within 10 % compared to polystyrene latex (PSL) reference particles. The unit-to-unit variability in terms of the particle number size distribution during this study was within 10 % to 20 % for particles in the range of 0.9 up to 3 µm, which is acceptable for atmospheric measurements. For particles smaller than that, the variability increased up to 60 %, probably caused by differences in the counting efficiencies of individual units. Number size distribution data for particles smaller than 0.9 µm in aerodynamic diameter should only be used with caution. For particles larger than 3 µm, the unit-to-unit variability increased as well. A possible reason is an insufficient sizing accuracy in combination with a steeply sloping particle number size distribution and the increasing uncertainty due to decreasing counting. Particularly this uncertainty of the particle number size distribution must be considered if higher moments of the size distribution such as the particle volume or mass are calculated, which require the conversion of the aerodynamic diameter measured to a volume equivalent diameter.
In order to perform a quantitative quality assurance, a traceable reference method for the particle number concentration in the size range 0.5–3 µm is needed.