the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Development of a cascade impactor optimised for size-fractionated analysis of aerosol metal content by total reflection X-ray fluorescence spectroscopy (TXRF)
Claudio Crazzolara
Andreas Balthasar Held
Abstract. A new cascade impactor has been developed with the arrangement of the classifying nozzles optimised for analysis of the collected particles by total reflection X ray fluorescence (TXRF). TXRF offers detection limits in the range of a few pg of absolute mass and therefore poses great potential for the elemental analysis of heavy metals in aerosol particles. To fully exploit this high sensitivity, particles have to be collected in the effective analysis area of the TXRF instrument, which is often smaller than typical deposition patterns of commercial impactors or filter samplers. This is achieved by a novel, compact arrangement of the classifying nozzles within a circular area of a diameter of less than 5 mm. A decreasing density of the nozzle spacing from the inside to the outside of the nozzle cluster allows for constant cross flow conditions, minimising the mutual influence of the individual nozzles. The design of a multi-stage cascade impactor is presented, to individually sample PM10, PM2.5 and PM1 size fractions. Considering the high sensitivity of TXRF analysis, constructive measures have been taken to prevent attrition of impactor material which might lead to methodical blank values. Experimental validation confirms that neither attrition nor cross-contamination can be observed. Furthermore, a new spin-coating method has been developed which makes it possible to apply a thin and defined adhesive layer of grease to the sample carrier with good repeatability. Application of the impactor in a case study at an urban site at Potsdamer Platz, Berlin, Germany shows that at a moderate sampling volume flow rate of 5 litres per minute, the particle mass collected in 30 minutes or less is sufficient for reliable TXRF analysis of heavy metal concentrations (Fe, Zn, Cu, Mn, Pb, Ni) in ambient aerosol. This high time resolution enables snapshot sampling, e.g. to quantify variations in particle source strengths. Overall, the new impactor optimised for TXRF analysis bears great potential to improve the quantification of particulate trace metals and other elements in PM10, PM2.5 and PM1 with high time resolution.
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Claudio Crazzolara and Andreas Balthasar Held
Status: open (until 05 Oct 2023)
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RC1: 'Comment on amt-2023-182', Anonymous Referee #1, 11 Sep 2023
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- Does the paper address relevant scientific questions within the scope of AMT? Yes
- Does the paper present novel concepts, ideas, tools, or data? Yes
- Are substantial conclusions reached? Partly, see major points of criticism.
- Are the scientific methods and assumptions valid and clearly outlined? Partly, see major points of criticism.
- Are the results sufficient to support the interpretations and conclusions? Partly, see major points of criticism.
- Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? Partly, see major points of criticism.
- Do the authors give proper credit to related work and clearly indicate their own new/original contribution? Yes
- Does the title clearly reflect the contents of the paper? Yes
- Does the abstract provide a concise and complete summary? Yes
- Is the overall presentation well-structured and clear? Yes
- Is the language fluent and precise? Yes
- Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? Overall, yes
- Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? Some parts need clarification and additional detail information. See below.
- Are the number and quality of references appropriate? Yes
- Is the amount and quality of supplementary material appropriate? Yes
Major points of criticism:
- Line 55 to 59 describes the technical challenge to which the paper is dedicated: Increasing the detection efficiency of TXRF by complete excitation of the deposit and complete detection of the fluorescence response. The authors present a solution by shrinking the lateral diameters of centric deposit patterns on cascade substrates (carriers) to less than 5 mm and describe an appropriate cascade impactor design. To the reader the 5 mm criterion seems to be an arbitrary choice and a justification is lacking. A justification could e. g. be derived from an excitation beam profile analysis along with a description of the detector’s aperture and lateral efficiency distribution. Both would, e. g. for the used Bruker TStar spectrometer, clearly reveal that other than the authors implicitly assume even within the “5 mm area” the excitation and detection would have radial dependencies, i.e., decrease with distance to the center.
- In line 120 authors describe a Reynolds number of ~3000 in the nozzles as a design criterion. This is in my understanding for cylindrical pipes the transition regime between turbulent (>3000) and laminar (<=2300) flow. In line 125 to 130 authors describe that the flow profile in the nozzles should be “plug-shaped”, i. e. turbulent, rather than “parabolic” aka laminar. This is in contradiction to line 137, where a Reynolds number of < 3000 is described as desirable. The relation between number and diameter of nozzles and mean velocity could be described more precisely and as a design challenge. It is desirable to have the finally applied ratios of nozzle lengths to diameters presented as a result of theoretical calculations or CFD modelling.
- Attrition (section 3.3.1). To me, sample contamination by metallic attrition appears a bit far-fetched. This section could possibly be eliminated. Authors could e. g. have made a point in discussing attrition by solid, abrasive particles. The authors also did not consider the possible contamination of a quartz disc by unintended mechanical contact with metallic parts (e. g. impactor structure, tweezers…) or other materials which have had mechanical contact to metals. This deems to me a more likely possible source of contamination. The presentation of related results in section 4, Table 2 has some deficiencies as will be discussed below.
- Line 265: The statement on the “effective area” is vague. As pointed out above, the efficiency will most probably have a radial dependency. Authors do not present an argument why this could be ignored. Line 269: “Calibration samples as external standards”. This is a crucial point in quantitative TXRF analysis, and the paper is much too vague here. The authors should explain the calibration samples in detail. I assume that these were prepared by drying a small drop of an aqueous solution with known element quantity (probably Yttrium?) in the center of the carrier. I have the impression that the authors implicitly assume for the whole deposition pattern area the same detection efficiency as for a punctiform centrical deposition of an internal standard and they should give a proper justification for this assumption. As the commercial Bruker software for the TStar offers several options for quantification the chosen method (internal fundamental parameters, internal standard…) should be mentioned.
- Chapter 4, Table 2: “…mean blank values of impactor stages 1 to 5 …”: I guess that this are the means over repeated measurements. Lines 291 to 295. I would expect a proper measure of variance, at least an estimation of the uncertainty. Each single measurement should come with an uncertainty provided by the analyzing software. The absolute lower detection limits (LLOD) should be quantified. Line 299 to 301: The significance of the statement should be reevaluated considering realistic uncertainties in the measurements. I am reluctant to accept conclusions without proper consideration of uncertainties, the more so as the data are close to the LLOD.
- Chapter 4 from line 345 on and Figure 5 and Supplement Table S.2: “Figure 5 shows the mass concentrations of the trace metals Zn, Cu, Mn, Pb and Ni collected during these three sampling periods in three size fractions, i. e. PM1 (top of blue bar), PM2.5 (top of green bar) and PM10 (top of red bar).”. Authors should describe how they calculated the PMx mass fractions from the masses determined by TXRF on the five cascade impactor stages. Were for the 0.11 to 1 µm fraction the masses from stages 3 to 5 just summed up, and accordingly for the other PM fractions in Figure 5? If the coarse (10 µm) stage was included into the calculations at least all values labeled as PM10 (top of red bar) do contain a priori unknown quantities of particles larger than 10 µm as no preseparator was reportedly used and the separation curve steepness according to DIN 481 or US EPA was not determined. However, the photo (Fig. 3) probably shows an inlet separator which is not described at all.
- Conclusions and Outlook seems not well-balanced: One half of the text can be considered merely as outlook. The following three points can in essence be regarded original results from this work:
- “Due to the compact arrangement of the impactor 375 nozzles, the high detection sensitivity of the TXRF analysis method can be fully exploited.” A convincing proof of this statement is however lacking. The statement in lines 375 to 379 is valid.
- The statement on the “new spin-coating method” is valid.
- No contamination issues during mounting/dismounting of carriers.
Minor points / editorial suggestions:
Line 76 to 77: The citation refers to rather broad outdoor aerosol size distributions, e. g. accumulation or coarse mode. There might be outdoor scenarios where the Aitken mode is interesting to observe even with high time resolution, or indoor scenarios were the < 0.1 µm fraction is dominant. Authors should comment on that.
Line 82: “slm” (standard litre per minute) refers to us-american nomenclature with standard conditions 0 deg Celsius (32 deg F) and 1.0125 bar (14.69 psia). European nomenclature uses subscript “s” for standard flow at 20 C and 1.0125 bar, e.g., m³s /h. Standard flow at 0 C and 1.0125 bar is in Europe expressed as e.g., m³n / h with subscript “n”
Line 165: “in the low single-digit percentage range” ?? “few percent relative deviation” sounds better.
Statement summary of the reviewer:
Pro: The three design targets described in line 62 were reached. The overall functionality of a newly designed impactor could be demonstrated in the field; the high time resolution as well. The stages design with respect to flow, nozzle numbers, diameters, lengths, and lateral arrangement was comprehensively and in detail described and supported by theoretical considerations. Application of an adhesive on carriers by spin coating was successfully demonstrated and seems to be an interesting alternative to spraying.
Contra: Unfortunately, the paper does not present convincing proof of the expected advantage of the new design over commercial impactors such as increased efficiency of TXRF quantification of deposited elements. It would be desirable if the authors discuss shape and intensity profile of the detection area of the TStar in more detail to underpin their point of view. The discussion on cross contaminations and contamination by attrition lacks proper consideration of measurement uncertainties and LLODs. The quantitative evaluation of outdoor data in terms of element concentrations in PM fractions is not described sufficiently.
Citation: https://doi.org/10.5194/amt-2023-182-RC1
Claudio Crazzolara and Andreas Balthasar Held
Claudio Crazzolara and Andreas Balthasar Held
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