the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Nov 2025
An intercomparison study of good laboratory practices for aerosol number size distribution measurements using optical spectrometers
Abstract. An inter-laboratory comparison (ILC) involving optical particle counters (OPCs) was organized at the French national level. The aim of this study was to make an inventory of the metrological capabilities of particle number size distribution (PNSD) measurements using OPCs. This laboratory study took place over a period of 18 months and involved 16 partners and 35 OPCs. Rather than focusing on the actual capability of the tested OPCs, this paper aims to reveal good laboratory practices when using standard OPCs. For that, each partner applied the same pre-defined experimental protocol on the OPC(s) to be tested, operated together with a common control OPC. Three different powder-borne test aerosols were involved, and their PNSDs were measured: (1) – a monodisperse amorphous silica sample, (2) – glass beads and (3) – a green cornstarch powder. This article presents the measured PNSD using the 35 OPCs associated with the description of the experimental set-up, sample preparation protocol and comparison with Scanning Electron Microscopy measurements.
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RC1: 'Comment on ar-2025-39', Anonymous Referee #1, 08 Dec 2025
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AC1: 'Reply on RC1', Sebastien Bau, 11 Dec 2025
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We thank the reviewer for the time and attention devoted to evaluating our manuscript. We are fully aware of the points raised, including the fact that several observations made in our study are already known within the scientific community, particularly the dependence of optical diameter measurements on aerosol properties and the constraints associated with calibration procedures. On a first reading, we naturally agree with many of the reviewer’s statements, which essentially recall well-established principles in aerosol metrology.
However, the primary objective of our work was not to re-establish this existing knowledge. Instead, the study provided an opportunity to bring together a large number of partners at the national level on a topic that still requires further scientific investigation, especially regarding the access to an affordable and widely deployable calibration bench for multiple laboratories. Such approach has already been adopted in a previous work on Mobility Spectrometers (J Nanopart Res, 22, 2020). We acknowledge that coordinating the contributions of fifteen partners inevitably created challenges, including highlight more clearly the originality of our collective approach.
This originality lies in the fact that, despite using the same aerosol, the same generator, the same reference measurement device, the same experimental protocol and the OPS instruments from each participating laboratory, the reported results still differed in several cases. This outcome is scientifically meaningful: it shows that robust aerosol characterization is not as straightforward as might be assumed, and certainly not as easily guaranteed as some might suggest. It underlines the need for strengthened robustness in aerosol measurements and for a clearer understanding of the sources of variability when instruments are deployed under routine monitoring conditions.
From our perspective, this collaborative effort is still lacking in the current scientific literature: to our knowledge, no inter-laboratory comparison of such scale, involving the instruments commonly used by national monitoring networks, has been documented (not in France, but not elsewhere either). In this respect, the work carried out within ASFERA and its partners represents a unique and valuable contribution.
Although our database does not yet allow good laboratory practices to be proposed, it does provide the first systematic documentation on how these instruments perform under shared conditions, using resources that are realistically available for routine monitoring laboratories. In particular, elements to further interpret deviations related to particle optical properties are still required when OPS performance characterization is sought. The latter properties are particularly difficult to determine, especially when complex particles are involved (composition, shape, structure). Nonetheless, this study offers an essential starting point for future work that will need to address the more advanced calibration approaches mentioned by the reviewer and to further investigate how such methods could be implemented in practice by monitoring networks.
In this sense, we believe the study represents a necessary preliminary step, filling a current gap in the literature and supporting future methodological improvements.
Citation: https://doi.org/10.5194/ar-2025-39-AC1
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AC1: 'Reply on RC1', Sebastien Bau, 11 Dec 2025
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RC2: 'Comment on ar-2025-39', Anonymous Referee #2, 12 Dec 2025
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Review for Aerosol Research
https://doi.org/10.5194/ar-2025-39
General Comments
The manuscript written by Bau et al. deals with comparative measurements of a series of OPCs. The sheer number of measuring devices to be compared and the number of participating institutes are noteworthy. A considerable amount of time, 18 months, was spent on this project so that all partners could carry out the same experiment protocol using the same measuring instruments.
The motivation given was that good laboratory practices would be established and that no inter-laboratory measurements of OPC were done before. Unfortunately, the manuscript stopped being convincing at this point. Images from a scanning electron microscope were presented as the sole reference measurement method. An OPC was specified as the reference instrument which was referenced against all other OPCs, but its individual calibration points were not examined in detail.
Three different aerosols were used as samples, which are only suitable to a limited extent and whose repeatability has not been investigated. The evaluations of the measurements and instruments were not considered individually but rather as a block. Information on the last service date or calibration was generalised and dismissed in the running text as inconclusive. Unfortunately, there was a lack of in-depth analysis such as of the optics, or pulse heights. In summary, the manuscript consists of a preliminary experiment, which many measurement partners have repeated, and the results are roughly the same.
The importance of inter-laboratory comparison is emphasised in the manuscript, but in-depth analysis considering differences in measuring instruments (light intensity, angles, calibrations, monodisperse aerosols, multimodal…..) is not taken into account. In its current form, I do not recommend publication of this manuscript in Aerosol Research.
In addition, I recommend that authors read the following articles and familiarise themselves with the approaches described therein.:
An intercomparison study of optical particle counters by van der Meulen et al. [1980] for the lower particle sizes, since the manuscript did not analysis lower detection size limits. As well as other Intercomparison studies such as: Rosenberg et al. 2012, which focused on pulse high analysis.
Rosenberg et al. and Sang-Nourpour et al. 2019 (mentioned in this manuscript) have also examined measurement artefacts that arise with large particles. Namely, that with large particles, small particles are also reported even though they are not actually present. This can distort measurements, such as those made in this manuscript, but this was not examined.
A review study by Vasilatou et al. 2025 or by McMurry 2000 on aerosol calibrations, since no SMPS/ CPC number concentration validations and no PSL standards were used in this manuscript by Bau et al.
Dependencies on optics, such as the viewing/integration angles of the measuring instruments as in Szymanski and Liu 1986 (mentioned in manuscript) or Liu 1985.
Comparison with Mie theory regarding multiply OPC such as in Heim et al 2008 or Whitby et al 1967.
It would also have been interesting to investigate other, more atmospheric aerosols like in Crilley et al 2020, or using Nigrosin as an light absorbing aerosol with known refractive index as in Foster et al 2019.
References
van der Meulen, A. Plomp, F. Oeseburg, E. Buringh, R.M. van Aalst, W. Hoevers, Intercomparison of optical particle counters under conditions of normal operation, Atmospheric Environment (1967),Volume 14, Issue 4,1980,Pages 495-499,ISSN 0004-6981,https://doi.org/10.1016/0004-6981(80)90215-2.
Rosenberg, P. D., Dean, A. R., Williams, P. I., Dorsey, J. R., Minikin, A., Pickering, M. A., and Petzold, A.: Particle sizing calibration with refractive index correction for light scattering optical particle counters and impacts upon PCASP and CDP data collected during the Fennec campaign, Atmos. Meas. Tech., 5, 1147–1163, https://doi.org/10.5194/amt-5-1147-2012, 2012.
Sang-Nourpour, N. and Olfert, J. S.: Calibration of optical particle counters with an aerodynamic aerosol classifier, J Aerosol Sci, 138,https://doi.org/10.1016/j.jaerosci.2019.105452, 2019.
Konstantina Vasilatou, Kenjiro Iida, Mohsen Kazemimanesh, Jason Olfert, Hiromu Sakurai, Timothy A. Sipkens, Gregory J. Smallwood,Aerosol physical characterization: A review on the current state of aerosol documentary standards and calibration strategies,Journal of Aerosol Science,Volume 183,2025,106483,ISSN 0021-8502, https://doi.org/10.1016/j.jaerosci.2024.106483.
Peter H McMurry,A review of atmospheric aerosol measurements,Atmospheric environment,Volume 34, Issues 12–14,2000,Pages 1959-1999,ISSN 1352-2310, https://doi.org/10.1016/S1352-2310(99)00455-0.
Szymanski, W.W. and Liu, B.Y.H. (1986), On the Sizing Accuracy of Laser Optical Particle Counters. Part. Part. Syst. Charact., 3: 1-7. https://doi.org/10.1002/ppsc.19860030102
Liu B, Szymanski W, Ahn K. On Aerosol Size Distribution Measurement by Laser and White Light Optical Particle Counters. The Journal of Environmental Sciences. 1985;28(3):19–24. doi: 10.17764/jiet.1.28.3.k873425806586048
Michael Heim, Benjamin J. Mullins, Heinz Umhauer, Gerhard Kasper,Performance evaluation of three optical particle counters with an efficient “multimodal” calibration method,Journal of Aerosol Science,Volume 39, Issue 12,2008,Pages 1019-1031,ISSN 0021-8502, https://doi.org/10.1016/j.jaerosci.2008.07.006.
Kenneth T. Whitby and Richard A. Vomela Environmental Science & Technology 1967 1 (10), 801-814 DOI: 10.1021/es60010a002.
Crilley, L. R., Singh, A., Kramer, L. J., Shaw, M. D., Alam, M. S., Apte, J. S., Bloss, W. J., Hildebrandt Ruiz, L., Fu, P., Fu, W., Gani, S., Gatari, M., Ilyinskaya, E., Lewis, A. C., Ng'ang'a, D., Sun, Y., Whitty, R. C. W., Yue, S., Young, S., and Pope, F. D.: Effect of aerosol composition on the performance of low-cost optical particle counter correction factors, Atmos. Meas. Tech., 13, 1181–1193, https://doi.org/10.5194/amt-13-1181-2020, 2020.
Foster, K., Pokhrel, R., Burkhart, M., and Murphy, S.: A novel approach to calibrating a photoacoustic absorption spectrometer using polydisperse absorbing aerosol, Atmos. Meas. Tech., 12, 3351–3363, https://doi.org/10.5194/amt-12-3351-2019, 2019.
Citation: https://doi.org/10.5194/ar-2025-39-RC2 -
RC3: 'Reply on RC2', Anonymous Referee #2, 12 Dec 2025
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Dear authors, I apologise for my harsh words; I should have reconsidered my choice of words. Nevertheless, I would urge the authors to possibly change the focus of these study and, with regard to the literature references presented, to supplement the manuscript with further methods and theoretical approaches.
With a stronger focus on the generation of the measured aerosol and calibration data, the data obtained could still be relevant. As a great number of OPC instrument did not differ significantly from each other regardless the service status.Citation: https://doi.org/10.5194/ar-2025-39-RC3
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RC3: 'Reply on RC2', Anonymous Referee #2, 12 Dec 2025
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This manuscript describes an inter-laboratory comparison of optical particle size spectrometers organized at the French national level. The focus was on the measurement of particle size. No reference method for number concentration measurements was available. According to the authors, this study aimed to reveal good laboratory practices when using commercially available optical particle counters. The authors, however, conclude that "Our database does not allow good laboratory practices to be proposed yet".
In my opinion, this study simply confirms what is already known, namely that the measurement of optical diameter with optical particle counters depends on the properties of the aerosols (especially the refractive index). Since little to no information is provided on the calibration of the particle counters under test by the manufacturers or end users, the study is simply descriptive and the main conclusion is that the particle counters do not report the same particle size. That was expected!
The calibration of optical particle counters (OPCs) designed for clean room monitoring has already been standardised within the ISO standard 21501-4. Likewise, the calibration of optical particle size spectrometers (OPSS) for ambient measurements is described in ISO 21501-1 (a revised version is soon to be published).
Size-certified spheres (e.g. silica or polystyrene spheres) are typically used for size calibration. Since ambient aerosols contain particles of different shapes and complex refractive index, the measured particle size distributions must be post-corrected by assuming a complex refractive index for the local ambient aerosol which was monitored. Do the monitoring stations in France apply such a correction? The authors seem to imply that all monitoring stations should calibrate their OPSS with the same size-certified particles, so that the measurements are "harmonised". This is not necessarily true. Monitoring stations should choose a "realistic" calibration aerosol, i.e. an aerosol with a refractive index as close as possible to that of the local ambient aerosols and then apply, if necessary, a correction. Alternatively, the method based on an aerodynamic aerosol classifier (Sang-Nourpour & Olfert, 2019; https://doi.org/10.1016/j.jaerosci.2019.105452) and atmospheric particles as calibration particles can be used.
In my opinion, this manuscript does not build on existing knowledge nor does it propose any measures for improving existing calibration procedures (as the authors themselves conclude). I therefore don't recommend publication.