10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Wide Size Range Number Concentration Calibration with Low Uncertainties
ANSSI JÄRVINEN, Jorma Keskinen, Jaakko Yli-Ojanperä, Tampere University of Technology
Abstract Number: 1264 Working Group: Instrumentation
Abstract Aerosol instruments operate in a wide size range, which is a challenge in the instrument calibration. Different particle generation methods are used, and these require different concentration references. Instrument number concentration calibration in the small particle sizes, below 30 nm, is rather straightforward with different generation methods, bipolar charging and electrical classification with a Differential mobility analyser (DMA). Because at these sizes, particles acquire practically only one elementary charge, the Faraday-cup aerosol electrometer (FCAE) may be used as a reliable number concentration reference. The calibration size range can be extended towards larger particle sizes if multiply charging is minimized, but µm range is not typically achieved. The µm sized calibration particles are typically generated with completely different means. These methods do not produce singly charged particles, and as a result, different concentration references are used, for instance optical particle counters.
In traceable high accuracy calibrations, the step from the electrical reference to the optical reference is somewhat troublesome, because multiply charging limits the accuracy of the electrical method and the particle sizes below approximately 1 µm are problematic for optical particle counters as the detection efficiency may decrease. In addition, particle counters suffer from coincidence at high particles concentration, which limits the calibration concentration range. Thus, there is a need for a single calibration setup, which covers the size range from the smallest nm particles up to µm sizes.
To improve calibration accuracy in this problematic size range and to enable high concentration calibrations in µm size range, we have developed a calibration system relying on singly charged particles (Järvinen et al. 2018). The size range spans from nanometers up to micrometers, while the reference concentration is acquired from a FCAE. The developed system is an evolution of the Singly charged aerosol reference (SCAR, Yli-Ojanperä et al. 2010). The main principle is that singly charged seed particles are grown into larger sizes by condensing diethylhexyl sebacate (DEHS) on them. At large particle sizes, the process may also generate neutral particles, which must be removed with a DMA. To generate and classify µm sizes particles, a large sized DMA was designed and constructed. In a calibration measurement, large particles are divided between the instrument and the reference, which is sometimes challenging due to inertial losses. In our case, the concentration is equalized using a static mixer, which is followed by a flow splitter.
The operation of the calibration setup components was studied carefully. The particle growth was studied and DMA transfer function was measured. The major factors affecting the number concentration uncertainty were minimized and confirmed. The flow splitting was studied thoroughly with two identical FCAEs, and the bias between the inlet ports was observed to be small, less than ±1% from 3.6 nm up to 5.3 µm with uncertainty less than ±1% (k=2) at most of the size range at 1.5 l/min flow rate.
We used the developed setup in a calibration of a Condensation particle counter (CPC) detection efficiency. The detection efficiency was measured in the same size range with the bias, between 3.6 nm and 5.3 µm. The calibration uncertainty (k=2) was less than ±5% over the entire size range and less than ±2% between 5 nm and 4.3 µm.
References [1] Järvinen, A., Keskinen, J., Yli-Ojanperä, J. (2018) submitted to Aerosol Sci. Technol. [2] Uin, J., Tamm, E., Mirme, A. (2009) Aerosol Sci. Technol. 43, 847-853. [3] Yli-Ojanperä, J., Mäkelä, J., Marjamäki, M., Rostedt, A., Keskinen, J. (2010) J. Aerosol Sci. 41, 719-728.