10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Calibration of Condensation Particle Counters Against an Aerosol Electrometer Over a Wide Range of Sizes with Minimal Charge State Uncertainty
JONATHAN SYMONDS, Cambustion
Abstract Number: 119 Working Group: Instrumentation
Abstract The use of a Faraday Cup Aerosol Electrometer (FCAE) as a reference standard for Condensation Particle Counter (CPC) calibration is a long-established technique which has been recently codified as part of ISO27891:2015. Using a FCAE is attractive as it exhibits high detection efficiency, independence from particle material and morphology, and is clearly traceable to primary standards. However, its accuracy relies on the assumption that each particle entering the FCAE possesses just one elementary charge. Given a Differential Mobility Analyzer (DMA, referred to in the standard as a Differential Electrical Mobility Classifier, DEMC) is used to size select particles prior to the FCAE and CPC under test, it is possible for particles of the intended electrical mobility, but of larger size and with more than one elementary charge to pass through the DEMC. These will not only be counted by the FCAE as more than one particle, but by being larger, will bias the size dependent counting efficiency curve. The standard describes mitigating this by taking great care as to the size distribution of source aerosol used, or to measure or calculate the fractions of multiply-charged particles. The technique below follows the mitigation route.
The primary source is a nebulizer, here filled with dioctyl sebacate giving a peak size ~320 nm and σg=1.9. Between the aerosol source and the “charge conditioner” (neutralizer) is placed an Aerodynamic Aerosol Classifier (AAC, Tavakoli and Olfert, 2013). This selects monodisperse aerosol at high resolution by aerodynamic diameter rather than electrical mobility, and is thus not affected by particle charge. The AAC is set to the same equivalent aerodynamic diameter as the DEMC’s mobility diameter (converting using the known aerosol density). The DEMC is hence only fed with monodisperse aerosol at the desired mobility size, and populations of particles at sizes greater than desired corresponding to higher charge states are effectively eliminated. Although it is possible to use a second DEMC and neutralizer in this role, this would only attenuate the higher charge populations in proportion to the charging efficiency of the neutralizer, still leaving uncertainty.
Data is presented showing a TSI 3775 CPC compared with a TSI 3068B FCAE over a size range of 50 nm–1.8 μm, showing a mean detection efficiency of 98%. This upper size, beyond the normal range of the TSI 3801 DMA used, is achieved by dropping its sheath flow. This resulting reduced resolution of the DEMC does not matter as the size accuracy and resolution desired to generate an efficiency curve is achieved by the AAC: the DEMC is required only to remove the uncharged and multiply-charged particles.
The technique is automatable given a single aerosol source can be used over a wide range, as there is no requirement for a close relationship between raw aerosol size and that selected in the DEMC. A continuously variable rotating disc diluter was used to keep the raw aerosol concentration within the optimal band for both the CPC and the FCAE. Although the lower limit of the current AAC instrument is 25 nm, below this multiple charging is not significant.
ISO27891:2015 also provides for calibration of a CPC against a reference CPC. In this case, there is no requirement for the aerosol to be charged, and an AAC can replace the DEMC. This is potentially useful in field calibrations where regulation prevents the use of radioactive or X-ray sources, and is possible up to the AAC size limit of ~6 μm. The technique is used to show that for particles >3 μm, single particle counting in the 3775 CPC breaks down.