| 25 Sep 2026
A metrological calibration facility for particle number counters used in emission measurements
Abstract. A metrological calibration facility for particle number counters (PNCs) has been developed at the Physikalisch-Technische Bundesanstalt (PTB) as a primary reference standard for this metric required in the quantification of solid soot emission as described in national and European regulatory frameworks. This study describes the calibration of a Condensation Particle Counter (CPC) using PTB’s primary aerosol number standard, a Faraday Cup Aerosol Electrometer (FCAE). The calibration facility enables high accuracy determination of CPC counting efficiency for particles with mobility diameters between 17 and 200 nm, as well as linearity measurements of particle number concentrations ranging from 1 cm⁻³ to 20 000 cm⁻³. Soot aerosol particles were generated using a miniCAST 5303 and subsequently treating them in a Catalytic Stripper to remove volatile components before homogenisation in an integrated mixing unit. The aerosol flow was distributed via a ten-port manifold, allowing simultaneous evaluation of up to eight CPCs and a FCAE connected to two different ports simultaneously. The results demonstrate that the facility provides state-of-the-art metrological capabilities for CPC calibration, supporting improved reliability and comparability of particle number measurements in emission control applications.
This manuscript describes PTB's calibration facility for particle counters used in emission control. Although I greatly appreciate the effort that has been put into setting up and validating the system, the manuscript merely describes a method that has been standardised within ISO 27891 for almost 15 years. As the authors acknowledge, several other national metrology institutes and calibration laboratories around the world have developed and validated similar setups and at least two international intercomparisons have taken place (Högström et al., 2014) (Brown et al., 2023).
After reading the reports of the intercomparisons (Högström et al., 2014; Brown et al., 2023), it became clear to me that several other metrology institutes in Europe and Asia have smaller measurement uncertainties and better agreement with the reference value than PTB. Surprisingly, this issue has not been discussed in the manuscript.
Moreover, a quick literature search revealed that there are at least two other papers describing methods to calibrate CPCs and other particle counters down to 1 cm-3 in a traceable manner (Sakurai et al., Metrologia 60(5), 2023, 10.1088/1681-7575/acf5f1; Owen et al. Aerosol Science & Technology 46, 2012, https://doi.org/10.1080/02786826.2011.637998) which the authors have not cited. Note that the paper by Owen et al. dates back to 2012, showing that NIST had developed a CPC calibration method based on FCAE and dilution units already 15 years ago.
The suitability of miniCAST generators as a source of combustion particles has been examined and confirmed in numerous studies. I cannot cite here all the papers, but please see, for instance, Ess et al. (Aerosol Science and Technology, 53(1), 2019, https://doi.org/10.1080/02786826.2018.1536818) and references therein. Therefore, there is nothing novel in this regard either.
So much work has been done in the last 15-20 years regarding aerosol/combustion generators, design and validation of multiple-flow splitters, calibration of particle counters (CPCs and diffusion chargers) and electrometers, yet the authors of this manuscript cite a very small number of papers, the majority of which has been co-authored by them. The most striking example is when the authors state that "The Faraday Cup Aerosol Electrometer (FCAE) has been established as the primary standard for PN determination (Molzahn et al., 2025)". It was not PTB (Molzahn et al.) who established FCAE as a primary standard in 2025. FCAEs have been used for more than 15 years as primary standards for number concentration all around the world!
To summarise, I respect the efforts invested in validating the PTB setup. However, this setup does not go beyond the-state-of-the-art in any way. The calibration method described in this manuscript was standardised within ISO 21789 more than 15 years ago and many other institutes have developed similar setups. I am astonished that the authors do not acknowledge previous work done in their field. I also have the impression that this manuscript serves more like a "marketing" strategy to advertise PTB calibration services rather than as a means to communicate new scientific knowledge that helps advance aerosol science. For these reasons, I do not recommend publication of this manuscript.