the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Nov 2024
Interlaboratory comparison exercise for micro-aerosol size measurement by cascade impactor
Abstract. This study presents an interlaboratory comparison (ILC) exercise focused on measuring micro-aerosol size distributions using cascade impactors. The aerodynamic particle size distribution (APSD) is a critical parameter for understanding aerosol behaviour, particularly for health-related applications. The ILC conducted at the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) aims to assess the performances of participating instruments measuring aerodynamic diameter, cascade impactors and an Aerodynamic Particle Sizer (APS) for real time monitoring. The experiments were performed in a custom test bench able to generate aerosols in a size range from 0.2 to 4 µm within a controlled environment. Performance evaluations of the participating instruments considering five distinct aerosol size distributions were assessed, and two methods – Henry's method and lognormal adjustment – were used to calculate the mass median aerodynamic diameter (MMAD) and the geometric standard deviation (σg). Statistical analysis using ζ-score and Z'-score ensured the reliability of the results across participating instruments.
The findings demonstrates that most instruments performed within acceptable limits, though variations observed in some cases, particularly for smaller particle sizes. This work highlights the feasibility of standardized ILCs for APSD measurement and offers a framework for improving accuracy and consistency in aerosol size distribution assessments.
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Status: open (until 05 Jan 2025)
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RC1: 'Comment on ar-2024-30', Anonymous Referee #1, 21 Nov 2024
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The purpose of this article is to present the results obtained during an exercise, which the authors describe as an interlaboratory comparison, aimed at comparing the results produced by cascade impactors used to determine the mass median aerodynamic diameter (MMAD) of aerosols denoting different size.
While the article's style is pleasant, it's very difficult to identify its real purpose.
Thus, it would appear that the aim is not to compare the analytical capabilities of a number of different laboratories (in this case, only two laboratories from the same entity), but rather to evaluate the performance of 3 instruments using the same technology (atmospheric or low-pressure impactors) operated by a quite limited number of participants. While the real ambition of this article is to demonstrate that cascaded impactors (in this case from two IRSN laboratories) are capable of determining the mass median aerodynamic diameter of an aerosol, this does not represent a major scientific contribution.
Beyond this lack of clarity in the presentation of the article's objective, the analytical methodology presented raises a large number of questions, which you can find listed below, leading me not to recommend such a study for publication in the journal “Aerosol Research”.
Specific comments
- C1. Lines 13, 59 and 139: the authors mention a range between 0.2 and 4 µm, but Table 1 only covers a range between 0.3 and 4 µm
- Introduction
- C2. Line 33: the authors state that the cascade impactor is considered a “gold standard”. On what factual basis (ASTM/ISO standards, OECD guidelines or at least scientific articles) is this statement backed up? What's more, if the cascade impactor is considered a “gold standard”, what's the purpose of this article, which aims to demonstrate its performance?
- C3. Lines 43 to 47: the objective of the article is not clearly explained, and the rest of the article adds to the confusion by mentioning participants instead of instruments.
- Experimental setup
- The experimental configuration presented raises several questions:
- C4. Why was quiet air sampling chosen?
- C5. The authors mention Grishpun's work on sampling probes, but this work proposes an approach for assessing the aspiration efficiency of probes. What types of probes were considered in this study? Have the results been corrected for aspiration efficiency, which could lead to a change in the expected particle size distribution?
- C6. Please specify whether each instrument was attached to a different probe or to a single probe:
- If several probes were used, how were they chosen according to the sampling rates of the different instruments?
- If a single probe was used, how were differences in sampling rates taken into account?
- C7. Has homogeneity, in terms of particle size distribution, been assessed within the test chamber? If so, how, and were any disparities noted (which may justify the use of one or more sampling probes, see my last question)?
- C8. Was the relative humidity in the measuring chamber measured during the tests?
- C9. Did the authors make sure that the particles produced by nebulization were perfectly solid particles and not residual droplets?
- C10. KCl has been used as a source of particles by adding sodium fluorescein: what is the nature of the particles generated in the test chamber? Is it a heterogeneous mixture of KCl and sodium fluorescein particles, or KCl particles “tagged” by sodium fluorescein?
- C11. Have electron microscope images been taken, together with elemental analyses, to determine the shape and composition of the particles produced (see my last question)?
- C12. Table 2 - ILC participants: the title and content of this table are confusing. Indeed, we might have expected to see different laboratories (and therefore different participants) operating similar instruments. Here, the participants are actually instruments operated by two laboratories belonging to the same entity.
- The experimental configuration presented raises several questions:
- Analysis method
- C13. Lines 82 et 85: please explain how the uncertainties associated with Henry's methods and lognormal smoothing are obtained.
- C14. Please provide details of the methodology and software used for Henry's method and lognormal adjustment.
- C15. Beyond the methods for exploiting the data produced using cascade impactors (or the APS), the uncertainties inherent in the instruments themselves are not discussed or presented. How are the uncertainties inherent in the cut-off diameters of the impactor stages, and in quantifying the mass deposited on each stage, taken into account in the uncertainty budget?
- C16. Lines 88-89: the link between fluorescein concentrations and KCl masses on each stage is not clearly explained.
- C17. Has weighing the mass of each stage, which is the most conventional approach for cascade impactors, been considered (if possible)?
- C18. If the masses available on each stage could not be quantified by weighing, why not consider an ICP-MS analysis to directly obtain the mass of K constituent element of the particles generated?
- C19. The term “reference value” suggests an additional objective of the article, namely to propose a consensus MMAD value for the experimental conditions produced on this test bench. Is it really necessary to use this term for an average value derived from measurements performed by only 3 different instruments?
- C20. Doesn't the choice of two indicators seem useful? Isn't just one enough (e.g. the Z' score or even the Z score)? If not, please explain why both indicators are needed and useful?
- C21. The article by Amarouche (2015), cited by the authors, specifies a different range for the normalized deviation E with satisfactory suitability for a value less than or equal to 1, in which case why are the intervals identical for the two indicators (lines 103-105)
- C22. Table 3: Are 3 or 4 significant digits after the decimal point really relevant? This point ties in with my questions about the assessment of measurement uncertainties (inherent in the instruments) and MMAD determination uncertainties (according to Henry's method or lognormal adjustment). The standard uncertainties presented in this table appear very low (5 nm for case E)
- C23. Discussion of the results is almost non-existent, and is limited to lines 130 to 135. As it stands, this does not justify a scientific publication.
- C24. Lines 134-135: “to underestimate the experimental uncertainties” is not clear to me. What do the authors mean?
- Conclusion: C25. as the article is not sufficiently clear on its objectives, it is difficult to identify the real contribution of this article through the conclusion.
Technical corrections
- C26. Lines 32 and 33: I don't understand the use of “...”.
Citation: https://doi.org/10.5194/ar-2024-30-RC1
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