Near Real-Time &amp; Benchtop XRF Intercomparison for PM Elemental Analysis on Quartz and Teflon Filters: A Case Study Across Three European Cities

Papagiannis, Stefanos; Manousakas, Manousos I.; Anagnostopoulos, Dimitrios F.; Pipkridas, Michael; Baalbaki, Rima; Sciare, Jean; O'Sullivan, Niall; Hellebust, Stig; Wenger, John; Fossum, Kirsten N.; Ovadnevaite, Jurgita; Tremper, Anja; Green, David; Eleftheriadis, Konstantinos; Diapouli, Evangelia

doi:10.5194/egusphere-2025-5977

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https://doi.org/10.5194/egusphere-2025-5977

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06 Jan 2026

| 06 Jan 2026

Near Real-Time & Benchtop XRF Intercomparison for PM Elemental Analysis on Quartz and Teflon Filters: A Case Study Across Three European Cities

Stefanos Papagiannis, Manousos I. Manousakas, Dimitrios F. Anagnostopoulos, Michael Pipkridas, Rima Baalbaki, Jean Sciare, Niall O'Sullivan, Stig Hellebust, John Wenger, Kirsten N. Fossum, Jurgita Ovadnevaite, Anja Tremper, David Green, Konstantinos Eleftheriadis, and Evangelia Diapouli

Abstract. This study presents an extensive intercomparison between a benchtop X-ray fluorescence (XRF) system and near real-time XRF monitors (Xact 625 and 625i) for measuring elemental concentrations in ambient aerosols. The measurements were conducted across three locations: Athens (Greece, March 2024), Nicosia (Cyprus, March–January 2023), and Dublin (Ireland, December 2022–February 2023). The focus was on evaluating the chemical composition of particulate matter (PM) and the impact of filter substrate choice on measurement consistency. The study specifically examines the elements Si, S, Cl, K, Ca, V, Ti, Mn, Fe, Cu, Ni, Zn, Sr, and Pb. The results highlight that filter type plays a crucial role in ensuring accurate measurements when utilizing the benchtop XRF system. At the Athens site, where PTFE filters were used, the agreement between the Xact 625i and the benchtop XRF system was stronger, with slopes generally closer to unity for most elements. In contrast, quartz fiber filters at the Dublin and Nicosia sites led to systematic deviations, especially for light elements such as S, Cl, and K, even after applying correction factors. For heavier elements like Fe, Mn, and Cu, the filter effect was less pronounced, though some variation across sites remained. Zn consistently showed good agreement, while Pb exhibited weaker correlation, possibly due to differences in the calibration curves of the two systems. Overall, this study not only evaluates instrument performance across multiple environments but also provides practical guidance for mitigating filter-related artifacts to enhance consistency in elemental aerosol measurements.

How to cite. Papagiannis, S., Manousakas, M. I., Anagnostopoulos, D. F., Pipkridas, M., Baalbaki, R., Sciare, J., O'Sullivan, N., Hellebust, S., Wenger, J., Fossum, K. N., Ovadnevaite, J., Tremper, A., Green, D., Eleftheriadis, K., and Diapouli, E.: Near Real-Time & Benchtop XRF Intercomparison for PM Elemental Analysis on Quartz and Teflon Filters: A Case Study Across Three European Cities, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-5977, 2026.

Received: 01 Dec 2025 – Discussion started: 06 Jan 2026

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RC1: 'Comment on egusphere-2025-5977', Anonymous Referee #1, 13 Feb 2026

General comment

The paper reports a comparison of Xact measurements with offline detection of using laboratory XRF. This was done in three sites with short-term campaigns using Teflon and quartz substrates. The topic is interesting even if authors should try to discuss more the reliability of Xact. Moreover, the description of the methods and the discussion/interpretation of results should be revised in a number of different ways, see my specific comment, before considering the paper for publication.

Specific comments

Lines 25-26. Here it is reported only 14 elements even if in the methodology it is reported that both the benchtop XEF and the Xact were calibrated with a significantly larger number of elements. In addition, there is not even a case in which all 14 elements are compared. The choice of the elements investigated should be discussed in more details and explained the reasons for certain choices. For example, Al is never considered, not even for Teflon substrates, even if this an element measurable with XRF and one of the forces of XRF compared to ICP for example. It is not reported because not reliably measurable by the Xact?

Line 28. “for most elements”, actually it is reported only for 8 elements.

Lines 33-34. Not really clear if this can be achieved form this paper because this may be done comparing measurements on the two substrates with the same instrument and this was not really done systematically.

Lines 48-54. I would suggest a revision of these sentences mentioning that coal burning, especially in power plants, has been shown to have distinctive Al and Si content (Cesari et al., 2016. Atmospheric Research 182, 282-293) rather than Cl and Br that are often related to sea spray especially near the coasts.

Line 61. Greater than what? It does not seem that Xact have greater precision or identify a larger number of elements. Probably you are referring to the time resolution.

Lines 69-72. This is interesting but from this work it is not clear if this replacement is actually possible. A comment on this in the conclusions is needed.

Section 2.2.1. The LODs of several elements, especially when using quartz, are not actually due to the measured of a blank rather, to their variability (i.e. three times standard deviation of concentrations measured on blanks of similar). My question here is if the measurements shown are reported correcting for blanks or not. Furthermore, it is reported a LOD also for Si on quartz. Do you believe that this element can be measured on such substrate if concentrations are larger than LOD? I am not convinced at all of this and results are not reported for this element so why to report a LOD?

Lines 215-219. Yes, light elements are less easily measurable on quartz. However, some benchtop XRF can do this al least for Na, Al, and Mg like, for example, in Potì et al. (2025, Science of the Total Environment 976, 179283). In my opinion, the part should be explained better because it could be a limitation related to the specific instruments used. Even if XAct should be able to measure Al.

Table 1 and Fig.3. It should be better to show in the figure all 8 elements. In addition, if the fits are done forcing through the origin, the R2 loose its usual meaning and it could be better to report the Pearson coefficient or the square of the Pearson coefficients.

The analysis of the effects of thickness is interesting. However, no comparison with the slopes mentioned before is reported. How these values agree with your intercomp quartz-teflon?

It is mentioned that on quartz only three elements were corrected because the other slopes were similar to one. What about the slopes of the elements not checked but reported in table 2?

Lines 338. Actually not most elements but for some that have been compared.

Conclusions. Some conclusions on the reliability of XAct should be reported as a summary comment to help in understand if it can replace benchtop instruments in the future.

Citation: https://doi.org/10.5194/egusphere-2025-5977-RC1
RC2:
'Comment on egusphere-2025-5977', Anonymous Referee #2, 18 Mar 2026
Referee’s comments on the manuscript “Near Real-Time & Benchtop XRF Intercomparison for PM Elemental Analysis on Quartz and Teflon Filters: A Case Study Across Three European Cities” by the authors Stefanos Papagiannis, Manousos I. Manousakas, Dimitrios F. Anagnostopoulos, Michael Pipkridas, Rima Baalbaki, Jean Sciare, Niall O'Sullivan, Stig Hellebust, John Wenger, Kirsten N. Fossum, Jurgita Ovadnevaite, Anja Tremper, David Green, Konstantinos Eleftheriadis, and Evangelia Diapouli.
The present study focuses on the intercomparison between near real-time XRF monitors (Xact 625 and Xact 625i) and a benchtop XRF spectrometer. The study includes measurements across three locations (Athens, Nicosia, and Dublin) in the PM2.5 size fraction.
The study also investigates the effect of different filter substrates used for XRF analyses (Teflon vs Quartz). For this purpose, a dedicated measurement campaign was conducted using daily PM10 samples in Athens. The resulting data were used to derive correction factors, which enabled the comparison between Xact measurements and daily measurements obtained from quartz filters in Nicosia and Dublin.
Given the increasing use of NRT-XRF spectrometers, studies aimed at characterizing their response compared to more traditional benchtop systems are particularly important. In the Referee’s opinion, the manuscript can be considered for publication only after the authors address the following major revisions.
General comments
The introduction would benefit from a clearer contextualization of ED-XRF among the techniques used to determine the elemental composition of particulate matter, briefly mentioning other commonly used methods such as ICP-MS and PIXE. In addition, the differences between offline (benchtop) and online (in situ) ED-XRF measurements should be better explained. For example, the higher temporal resolution of online measurements often results in lower collected mass and concentrations closer to the detection limits.

Different Xact models (625 and 625i) are used in this study. In a dedicated paragraph, the authors should provide a clear description of these instruments, underlining the differences between the two models.

In Section 2.1, the sampling campaigns are described only focusing on the offline filter samples which were subsequently analysed with the benchtop XRF system. However, additional information (which are now briefly explained in Section 2.2.2) should be provided regarding the sampling campaigns with the Xact systems. For each location, the authors should specify the measurement period with the Xact, the sampled size fraction, the total number of hourly samples collected, and the Xact model used.

In Section 3, the authors should clearly explain, for each site, the criteria used to select the elements included in each intercomparison analysis. In addition, it is not clear why a larger number of elements are considered for the CAO–NIC station compared to the other sites.

The authors discuss the limits of detection and quantification in detail, for both benchtop and NRT XRF systems, but it is not clear how the measured concentrations at each site relate to these values. It would be useful to include a table reporting, for each site and each element, the number of samples with concentrations above the limit of quantification relative to the total number of samples collected. The authors should also comment on the content of this table in a dedicated paragraph. This would help the reader better assess the robustness and representativeness of the dataset.

Specific comments
Page 1, lines 24-25. The authors state that the focus of the manuscript is the evaluation of the chemical composition of particulate matter. However, the study concerns the elemental composition, and the main emphasis seems to be the comparison between near real-time and benchtop XRF systems and the effect of filter substrate choice. The sentence should be revised accordingly.

Page 2, line 60. When discussing NRT-XRF instruments, the authors mention only the Xact 625 and Xact 625i. However, other systems are available and should be mentioned, such as the Horiba PX-375 monitor.

Page 2, lines 62-63. The authors state that the Xact systems allow for an “automated sampling of both PM10 and PM2.5”. According to the cited reference, the measurement of different size fractions (e.g., PM10 and PM2.5) with the Xact monitor is achieved using a switching inlet system. However, this configuration is not a standard feature of all Xact 625 instruments. The authors should clarify this aspect.

Page 3, line 68. The authors mention that an “earlier” version of the Xact625i was used in the cited study. The authors should clarify what relevant differences exist between that version and the instrument considered in the manuscript.

Page 3, lines 72-76, 87. The authors should specify that, while different substrates can be selected for PM sampling prior to benchtop ED-XRF analysis, Xact monitors only operate with Teflon filter tape. The authors should clarify that the choice of the filter substrate only concerns the comparison between Xact and offline systems, and not the Xact measurements themselves.

Page 4, line 120. The sentence appears to be incomplete and should be revised.

Page 5, Section 2.2.1. It is not clear whether the same benchtop ED-XRF system was used to analyse all the offline filters collected at all three sites. If so, the authors should specify in which laboratory the instrument is located.

Page 6, lines 166-167. The authors state that daily checks with PTFE filters are performed on the benchtop XRF system for QA/QC checks. What about checks with quartz blanks? Were the blanks of the quartz filters measured? If so, were blank values subtracted when calculating the concentrations from the filter analyses in Section 3?

Page 6, lines 175-176. The authors should provide a clearer description of the Xact operating cycle, particularly the sequence between the sampling and XRF-analysis.

Page 6, lines 180-181. It appears that two different Xact 625i instruments were used in Athens and Nicosia. If so, this should be more clearly stated, and the authors should clarify whether there are any relevant differences between the two instruments.

Page 7, lines 182-185. It is not clear whether the limits of detection of the Xact monitors were determined by the authors or if the values reported by the manufacturer were used.

Page 8, Section 3.1. The authors should clarify why, in the intercomparison campaign dedicated to PTFE and quartz filters, the only elements considered were S, Cl, K, Ca, Ti, Fe, Ni, Cu, and Zn. Why elements such as V, Mn, Sr, Pb (which are listed in Table 2) were excluded?

Page 8, lines 226-227. The authors state that the fine mode of K, linked to biomass burning, remains the dominant one. This statement may be valid during the winter season and at sites where biomass burning is the main source of K but may not hold true during the summer season or at sites affected by local dust resuspension or by Saharan dust transport. Therefore, the campaign conducted in Cyprus (CAO-NIC) may also be influenced by these effects and the correction factors derived afterwards may not be fully appropriate for this site. Do the authors have any references regarding the typical modes of K at the measurement sites?

Page 8, line 228. The authors state that linear regressions through the origin were performed. The authors should report whether the intercepts were statistically compatible with zero before forcing the regression through the origin and report their values. Otherwise, constraining the regression to pass through zero may not be justified.

Page 9, Table 1. R² values should be added to the table. The table caption would benefit from a more detailed description, including the number of samples considered, the size fraction, and location of the sampling campaign.

Page 10, lines 260-262. Were the results obtained from the code simulations subsequently used to derive the correction factors in the following part of the manuscript?

Page 10, Figure 3. Looking at the K plot, most of the points appear to lie outside the confidence band around the regression line. The authors should perform an appropriate statistical test to evaluate whether the data are actually well described by the fitted regression line. Moreover, the figure caption would benefit from a more detailed description, including the measurement campaign to which the data refer.

Page 11, line 281. For clarity, the authors should specify to which sampling campaign and location the data refer.

Page 12, Table 2. The table would benefit from an additional line specifying the filter substrates being compared.

Page 12, lines 291-292. The authors refer to the Si regression line as a “strong agreement between the instruments”. However, the regression line is based on only four data points, which is not statistically robust. Therefore, the authors should interpret this result with greater caution and consider rephrasing their statement.

Page 12, lines 295-296. The authors claim that Cl shows a “strong correlation”. However, looking at the Cl scatterplot in Figure S1, most of the data points at very low concentrations do not appear to lie on the regression line. In addition, the regression itself seems to be strongly influenced by the presence of an outlier. Since it is well known that outliers can significantly affect correlation analyses, the authors are encouraged to repeat the regression excluding the outlier and discuss its impact on the results.

Page 4 Supplementary Information, Figure S1. Also considering the Ca scatterplot, the regression appears to be influenced by the presence of an outlier. It would be helpful if the authors could repeat the regression analysis without this point and discuss its impact on the results.

Page 12, lines 298-299. Since the Fe slope is 0.66, indicating a substantial deviation from unity, the use of the term “strong agreement” to describe Fe appears inappropriate. The authors are encouraged to rephrase this statement.

Page 14, lines 329-333. The authors state that the applied corrections have substantially improved the correlations. While this appears to be the case for K, it is less evident for S (R² = 0.49) and especially for Cl. The authors should therefore acknowledge that significant discrepancies still remain and moderate their statement accordingly.

Page 8 Supplementary Information, Figure S5. Considering Cu and Sr scatterplots, the regressions appear to be strongly influenced by the presence of an outlier. The authors should repeat the regression analysis without this point and discuss its impact on the results.

Page 16, lines 360- 367. The authors should better highlight and discuss those elements for which the correlation coefficients are low, such as Fe, Ca, and K. Possible reasons for these discrepancies should be explored and commented on. In addition, Figure S4 suggests the presence of a clear bias in the K measurements, which should be explicitly discussed by the authors.

Page 16, line 363. The authors should avoid using the term “excellent”, as the slope is not compatible with unity.

Page 18, line 388. Please add range (min-max) for R²

Page 18, lines 396-400. The cited study (Cadeo et al., 2025) used mixed cellulose ester membrane filters, not Teflon filters. While these are also membrane filters, similar to Teflon, the authors should clarify this distinction.

Page 18, lines 397-398. In this campaign, only 8 elements were considered, whereas the cited article included 16. What about the other elements? The authors should explain the rationale behind their exclusion.

Page 18, line 399. Xact works only with Teflon filter tape as substrate. The authors should specify that is the intercomparison between Xact measurements and offline XRF measurements which performs better when Teflon filters are used for offline sampling.

Page 19, lines 405-409. A possible explanation for the observed differences could be related to blank contribution in quartz filters, which can vary significantly from filter to filter. The authors are encouraged to clarify whether field and/or laboratory blanks were measured, whether blank subtraction was performed, and whether the variability of the blanks was evaluated.A more thorough discussion of these aspects is needed to better support the interpretation of the results.

Page 21, lines 457-458. Have the authors considered potential line interferences between Pb and As as a possible explanation for the suboptimal quantification of Pb?

Technical corrections
Page 1, line 23: please revise “March-January 2023” to “March 2022-January 2023” to be consistent with what is stated on page 4, line 123.

Page 2, line 55: “clearly” should be changed to “consequently”

Page 5, line 135: please revise “ -18°C. until” to “ -18°C until”

Supplementary Information, Figures S6-S7. The authors are encouraged to change the colour used for the Xact data in these supplementary plots, as the white markers are difficult to see.
Citation: https://doi.org/10.5194/egusphere-2025-5977-RC2

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Short summary

This study compared continuous and benchtop XRF elemental monitors across three European cities to validate air quality measurement consistency. PTFE filters yield strong agreement, but common quartz filters introduce systematic deviations, especially for light elements, due to attenuation effects. We provide practical methodologies to harmonize data from diverse filter types and instruments, which is essential for pollution source tracking and reliable data sharing across monitoring networks.


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