From the journal Environmental Science: Atmospheres Peer review history

23-May-2024

Dear Dr Osterwalder:

Manuscript ID: EA-ART-04-2024-000052
TITLE: Spatial and seasonal dynamics of ambient gaseous elemental mercury concentrations over Switzerland

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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Associate Editor, Environmental Sciences: Atmospheres

************

Reviewer 1

The submitted paper Spatial and seasonal dynamics of ambient gaseous elemental mercury concentrations over Switzerland by Osterwalder et al. is definitely worth publishing. It has value not only in terms of the data set included, but also the methods and supplement can serve as detailed inspiration for others. The paper is well written, the chapters are well organized, free of typos and make perfect sense. Figures and tables are instructive and necessary, and I especially appreciate the energy and work put into the Supplementary Information section. The references are up to date, but in the review of field studies (lines 60-66 and later in the discussion lines) using MerPAS samplers, the references included are mostly from overseas. I am missing some more recent European works that benefit from the use of MerPAS samplers from areas near former Hg emission sources, e.g. Nováková et al. (2022) and Navrátil et al. (2023). It would be useful to add these references.
Finally, I have a technical question regarding the processing of nickel boats used for the analysis of sulfur-rich sorbent. How were the nickel boats treated in terms of cycling? Part of the excess sulfur from the decomposing sorbent is captured by the added sodium bicarbonate, but another part causes corrosion of the nickel boats. Have you addressed this issue in any way?
Refs.
Nováková, Tereza, et al. "Reconstructing atmospheric Hg levels near the oldest chemical factory in central Europe using a tree ring archive." Environmental Pollution 304 (2022): 119215.
Navrátil, Tomáš, et al. "Atmospheric mercury and its deposition during the phasing out of an amalgam electrolysis plant: temporal, seasonal, and spatial patterns." Environmental Science and Pollution Research 30.59 (2023): 123586-123602.

Reviewer 2

I found this paper to be an interesting, extensive and worthwhile endeavour by Osterwalder et al. To my knowledge, this may be the first paper I have seen using the MerPAS sampler across a national network, though the idea of doing this appears to be gaining traction in many places. I liked that the paper came about from policy urging within the Minamata Convention context for putting together a network of low-cost and easily maintained monitoring. I found the discussion to be relatively well handled, though somewhat speculative/opportunistic in explanatory parts and does not always seem to consider all possible controls for any one statement. That said, this may likely as good as is possible without further study. I just think it requires some toning down largely in the abstract. My overall recommendation would be that the paper is acceptable following the consideration of some relatively minor revision suggestions.

Suggestions and comments:
• Perhaps my most pertinent comment is that I found the language in parts of the abstract to over-step a little the analysis of the data in the paper itself. This can be resolved by either modifying language or by providing more writing and analysis in the paper itself. Specifically, the ideas of near-urban sources and rural sinks makes sense, but within the paper itself is largely speculative. The abstract states that patterns “imply” this, which I suppose is fairly careful language, but I feel just not quite justice to what is in the paper. Additionally, it is not entirely clear to me that seasonal variations are indeed quantitatively resolvable by the analysis in the paper, or just qualitatively.
• Title: I feel like the network you've established is a major part of and triumph of this paper and I was a bit surprised something about that is not part of the title.
• Line 74-75: This objective is investigated quite indirectly and I think it would be good to be more specific about that. The eventual use of PM and NOx emissions is good comparison, but indirect and lacks control. Same goes for proximity of forest in rural areas. It is somewhat speculative with the data available and again, not controlled in a scientific way. I don’t think this makes it not worthwhile to look into; it’s just about whether the objective as written can really be met.
• Line 110-117: This is all pretty intense for trying to ensure no contamination during storage and transport. Do you know how necessary this all is? I ask because I think this complication may have an impact of some sort on how easy it is for people to possibly replicate or their willingness to do what should be relatively simple.
• I found the end of section 3.2 kind of repetitive to the start of section 3.3. Perhaps move some of that end of section 3.2 stuff down into section 3.3?
• Line 257-263: It's almost like a mass balance calculation is about to happen, but doesn't.
• Paragraph ending line 278: This almost needs a bit more context. NOx and PM10 concentrations are a LOT higher, but Hg only a little? Worth a short explanation.
• Line 288-291: To me, this might be the strongest and most logical evidence for the assertion of a vegetation control on Hg0 concentrations, but I find it a little under-emphasized in the paper. To me, this is considerably stronger evidence than mere urban-rural differences, for example.
• Paragraph ending line 318: This is fine as a qualitative analysis, but it looks like maybe there would not be distinguishable seasonal dynamics in a quantitative analysis? It seems this could use some additional discussion.
• Figure 4: I found this figure is overly busy-looking and the legend in particular very difficult to read. There is just a bit too much going on. Up to the authors, but one recommendation is that SI Figure S6 is much more digestible to me as a reader, though some similar "busy" attributes (e.g., rural <1000 m asl), but more spread out amongst the characteristics.
• Line 370: Some of the other seasonal work and related discussion seems as pertinent as this, but not really commented on in the conclusions. Why not?


Very minor things:
• Line 21: Is “pollution” really the correct word here? Perhaps replace with “anthropogenic emissions”?
• Line 31: "vegetation uptake" maybe a bit too broad as this could possibly mean via the roots to some people. Perhaps specifically stomatal uptake by vegetation?
• Line 35-37: Note that somewhat related to this, but a bit different is that there have been serious, Hg-specific measures especially in the US in the 2000-2010 period that also should not be ignored here.
• Line 47-49: This sentence may be important context about ecosystem and health effects, but is very out of place here in the flow of this writing. Slight bit more explanation and placement of context would go a long way.
• Line 102: When you say they are grouped a certain way (here, this is the second way of grouping), I think it is imperative to not wait for later in the article to say why.
• Figure 2 caption: you highlight meaning of blue and brown bars, but not grey.

Point-by-point response to the comments by the Reviewers on “Spatial and seasonal dynamics of gaseous elemental mercury concentrations over Switzerland observed by a passive air sampler network.” (EA-ART-04-2024-000052)

Please note that line numbers refer to the revised article with changes highlighted.

Reviewer #1:

Comment 1: The submitted paper Spatial and seasonal dynamics of ambient gaseous elemental mercury concentrations over Switzerland by Osterwalder et al. is definitely worth publishing. It has value not only in terms of the data set included, but also the methods and supplement can serve as detailed inspiration for others. The paper is well written, the chapters are well organized, free of typos and make perfect sense. Figures and tables are instructive and necessary, and I especially appreciate the energy and work put into the Supplementary Information section. The references are up to date, but in the review of field studies (lines 60-66 and later in the discussion lines) using MerPAS samplers, the references included are mostly from overseas. I am missing some more recent European works that benefit from the use of MerPAS samplers from areas near former Hg emission sources, e.g. Nováková et al. (2022) and Navrátil et al. (2023). It would be useful to add these references.

Response 1: We have complemented our brief review in field studies about MerPAS deployments with the two suggested references. Please find the following addition in L65: MerPAS® have been installed to monitor Hg0 at remote sites (Hoang et al., 2023) and in proximity of chemical factories that have been emitting Hg to the environment in the past (Nováková et al., 2022, Navrátil et al., 2023).

Comment 2: Finally, I have a technical question regarding the processing of nickel boats used for the analysis of sulfur-rich sorbent. How were the nickel boats treated in terms of cycling? Part of the excess sulfur from the decomposing sorbent is captured by the added sodium bicarbonate, but another part causes corrosion of the nickel boats. Have you addressed this issue in any way?

Response 2: Yes, we used the nickel boats for the analysis but only used them once. After combustion of the sulfur-impregnated activated carbon we discarded the boats. To clarify how the boats were treated in terms of cycling we added the following to the text in L130: “To ensure the nickel boats were free of Hg, they were baked in a muffle furnace at 500°C for two hours prior to analysis” and in L136: “The nickel boats were only used once”. Our nickel boat blank values (method blank) were low 0.08 ± 0.16 ng (n = 200) and the target value (0.3 ng) was well above that. There are other studies such as Szponar et al. (2020), referred to in Sect. 2.1., who have also used nickel boats to analyze AC: https://doi.org/10.1021/acs.est.0c02251.

Reviewer #2:

I found this paper to be an interesting, extensive and worthwhile endeavour by Osterwalder et al. To my knowledge, this may be the first paper I have seen using the MerPAS sampler across a national network, though the idea of doing this appears to be gaining traction in many places. I liked that the paper came about from policy urging within the Minamata Convention context for putting together a network of low-cost and easily maintained monitoring. I found the discussion to be relatively well handled, though somewhat speculative/opportunistic in explanatory parts and does not always seem to consider all possible controls for any one statement. That said, this may likely as good as is possible without further study. I just think it requires some toning down largely in the abstract. My overall recommendation would be that the paper is acceptable following the consideration of some relatively minor revision suggestions.

Comment 1: Perhaps my most pertinent comment is that I found the language in parts of the abstract to over-step a little the analysis of the data in the paper itself. This can be resolved by either modifying language or by providing more writing and analysis in the paper itself.

Response 1: Thanks a lot for the suggestion. We have toned down the language in the abstract. The language was modified as suggested (L11): This concentration difference can be explained by more local Hg emissions at urban sites (e.g., by fuel combustion) throughout the year as well as by more pronounced stomatal Hg0 uptake at rural sites during spring and summer. We recommend continuing the Swiss Atmospheric Mercury Network to support the call from the Minamata Convention to monitor atmospheric Hg0 as a control on whether international efforts are successful in reducing Hg in the environment. Longer term records from such monitoring networks will also help improve the understanding of both regional and global Hg cycles.

Comment 2: Specifically, the ideas of near-urban sources and rural sinks makes sense, but within the paper itself is largely speculative. The abstract states that patterns “imply” this, which I suppose is fairly careful language, but I feel just not quite justice to what is in the paper. Additionally, it is not entirely clear to me that seasonal variations are indeed quantitatively resolvable by the analysis in the paper, or just qualitatively.

Response 2: We have removed the word “imply” and use more careful words now (see response #1). Indeed, the seasonal variations are quantitatively resolvable comparing autumn/winter and spring/summer periods. We have edited the sentences in L339 and L349, accordingly (see response #10). Throughout the manuscript, we are more careful now to not over interpret the seasonal dynamics. We have for example removed the sentence “The newly established Swiss Atmospheric Mercury Network has proven its usefulness in resolving seasonal Hg0 dynamics at urban and rural sites.” from the abstract.

Comment 3: Title: I feel like the network you've established is a major part of and triumph of this paper and I was a bit surprised something about that is not part of the title.

Response 3: That is a good point, thanks a lot. We have changed the title as suggested: Spatial and seasonal dynamics of gaseous elemental mercury concentrations over Switzerland observed by a passive air sampler network.

Comment 4: Line 74-75: This objective is investigated quite indirectly and I think it would be good to be more specific about that. The eventual use of PM and NOx emissions is good comparison, but indirect and lacks control. Same goes for proximity of forest in rural areas. It is somewhat speculative with the data available and again, not controlled in a scientific way. I don’t think this makes it not worthwhile to look into; it’s just about whether the objective as written can really be met.

Response 4: In the last paragraph of the introduction, we now solely state objectives of the study that can be met. Lines 78 – 82 were removed from the introduction. All the below listed objectives were investigated directly: “The objectives of the study were to 1) calculate a mean contemporary atmospheric Hg0 concentration for Switzerland, 2) characterize differences in atmospheric Hg0 concentration between rural and urban sites, and 3) identify the seasonal dynamics of atmospheric Hg0 concentrations at rural and urban sites.”

Comment 5: Line 110-117: This is all pretty intense for trying to ensure no contamination during storage and transport. Do you know how necessary this all is? I ask because I think this complication may have an impact of some sort on how easy it is for people to possibly replicate or their willingness to do what should be relatively simple.

Response 5: We cannot really assess whether it is necessary to 1) transport and store the MerPAS® in glass jars and 2) purge the storage box with Hg-free air. However, the MerPAS® is a diffusive sampler which is not totally airtight. Thus, any Hg trapped in a container or box has the potential to be collected by the MerPAS®. Therefore, our steps 1) and 2) are taken to best minimize the potential of diffusion occurring during transport and storage. To our knowledge, most of the previous studies report low field blanks without applying measures 1) and 2). Our decision to use glass jars to protect the MerPAS® from ambient air contamination during transport and storage was made after discussions with co-authors of Hong et al. (2023). They found that “acceptably low and consistent field blank contamination could only be achieved by storing samplers in sealed glass jars during transport and storage.” Our decision to use a box purged with Hg-free air to store the glass jars was taken, because indoor air can exceed ambient Hg levels, potentially affecting the Hg concentration on the MerPAS®.

Comment 6: I found the end of section 3.2 kind of repetitive to the start of section 3.3. Perhaps move some of that end of section 3.2 stuff down into section 3.3?

Response 6: We followed your suggestion and moved the entire paragraph from L237 to L248 to section 3.3. The transition from paragraph 3.2 to 3.3 hopefully reads smoother now.

Comment 7: Line 257-263: It's almost like a mass balance calculation is about to happen, but doesn't.

Response 7: We prefer to stick with a descriptive explanation for the decrease in Hg in the city of Basel when comparing data from November 5 to December 7, 2018, with data from winter 2022/23. A complete mass balance could likely only be performed with Hg measurements for each point source in each urban region. There were no changes made in L279-285.

Comment 8: Paragraph ending line 278: This almost needs a bit more context. NOx and PM10 concentrations are a LOT higher, but Hg only a little? Worth a short explanation.

Response 8: We have added a short explanation to the text in L290. The smaller difference between urban and rural Hg0 concentrations (factor of 1.1) compared to NOx and PM10 can be explained by the fact that NOx and PM10, unlike Hg0, have significant emission sources in urban environments. In addition, Hg0 has an atmospheric lifetime of about one year (Saiz-Lopez et al., 2018), while NOx and PM10 have an atmospheric lifetime of only a few weeks at most (Seinfeld and Pandis, 2016). The long atmospheric lifetime allows Hg0 to travel long distances before deposition, leading to more uniform global distribution.

Comment 9: Line 288-291: To me, this might be the strongest and most logical evidence for the assertion of a vegetation control on Hg0 concentrations, but I find it a little under-emphasized in the paper. To me, this is considerably stronger evidence than mere urban-rural differences, for example.

Response 9: We have emphasized this relationship more in the paper and added the following paragraph (L318): The positive linear relationship between Hg0 concentration and elevation (R2 = 0.71, Fig. S3b in Sect. S5) is likely driven by higher ecosystem productivity and subsequently increased stomatal Hg0 uptake at lower elevations (e.g., higher daily daytime mean air temperatures) compared to higher elevations (e.g., lower daily daytime mean air temperatures). Such patterns in gross primary production (GPP) have repeatedly been observed at the flux sites within the Swiss FluxNet (Wolf et al., 2013, Gharun et al., 2020), also reflected in elevational gradients in the length of the growing seasons and ecosystem surface conductance values (Gharun et al., 2020). Higher stomatal Hg0 uptake at lower elevations also corresponds to observed higher ratios of intercellular to ambient CO2 concentration (ci/ca) of foliage (Gharun et al., 2021). Accordingly, Chang et al., (2021) reported that GPP of croplands, deciduous forests, coniferous forests and grasslands (major vegetated land use types in Switzerland) peak at daily daytime mean air temperatures of 26.7°C, 22.3°C, 20.0°C and 17.7°C, respectively. Thus, GPP was most likely larger at MAG (283 m a.s.l.) with daily daytime mean air temperatures of 15.4 °C compared to RIG (1031 m a.s.l.) with 9.9 °C.

Comment 10: Paragraph ending line 318: This is fine as a qualitative analysis, but it looks like maybe there would not be distinguishable seasonal dynamics in a quantitative analysis? It seems this could use some additional discussion.

Response 10: We have added the results of the Mann-Whitney U test to prove that the seasonal distributions of data (autumn/winter and spring/summer) at urban (p = 0.043) and rural (p = 0.002) sites differed significantly. Thus, we were able to distinguish seasonal dynamics in a quantitative analysis. We have edited the sentence in L339: The mean Hg0 concentrations at the urban sites are slightly higher in spring and summer compared to autumn and winter (MWU test, p < 0.05). And in L348: At the rural sites, there was a seasonal dynamic in Hg0 concentrations with lower Hg0 in spring and summer compared to autumn and winter (MWU test, p < 0.01).

Comment 11: Figure 4: I found this figure is overly busy-looking and the legend in particular very difficult to read. There is just a bit too much going on. Up to the authors, but one recommendation is that SI Figure S6 is much more digestible to me as a reader, though some similar "busy" attributes (e.g., rural <1000 m asl), but more spread out amongst the characteristics.

Response 11: Thanks for the comment. We have simplified Figure 4 by only showing the boxplot, the seasonal average and the single data points. The lines were deleted, and the legend font size increased.

Comment 12: Line 370: Some of the other seasonal work and related discussion seems as pertinent as this, but not really commented on in the conclusions. Why not?

Response 12: Thanks for picking this up. We have commented on the seasonal work in the conclusions now. See L410-419.

Comment 13: Line 21: Is “pollution” really the correct word here? Perhaps replace with “anthropogenic emissions”?

Response 13: Indeed. We have replaced “pollution” with “anthropogenic emissions.”

Comment 14: Line 31: "vegetation uptake" maybe a bit too broad as this could possibly mean via the roots to some people. Perhaps specifically stomatal uptake by vegetation?

Response 14: That is a good point. We know stick to the term “stomatal Hg0 uptake” throughout the manuscript.

Comment 15: Line 35-37: Note that somewhat related to this, but a bit different is that there have been serious, Hg-specific measures especially in the US in the 2000-2010 period that also should not be ignored here.

Response 15: We have adjusted these lines and added Olson et al., (2020) confirming that Hg concentrations across the United States have been decreasing across all monitoring sites since 1996 (L37): Since then, northern hemispheric background Hg concentrations have been declining (Olson et al., 2020; MacSween et al., 2022) and typically range between 1 and 1.5 ng m-3 today (Sonke et al., 2023). In L69 we have added another reference to AMNet: Air Hg0 concentrations were systematically measured within the Atmospheric Mercury Network (AMNet) across the US from 2008 – 2010 (Lan et al., 2012) and within the Global Mercury Observation System (GMOS) using Tekran® 2537 instruments between 2010 and 2015 (Sprovieri et al., 2016).

Comment 16: Line 47-49: This sentence may be important context about ecosystem and health effects, but is very out of place here in the flow of this writing. Slight bit more explanation and placement of context would go a long way.

Response 16: We have moved the sentence about health effects to the beginning of the paragraph (L24): The threat posed by Hg pollution is mainly related to bioaccumulation of methylmercury in the aquatic food chain (Mergler et al., 2007; Schartup et al., 2019) and has united 147 countries in the UNEP Minamata Convention to curb Hg emissions and reduce human and wildlife exposure to Hg (UNEP, 2013). These actions have increased the reading flow as suggested by the Reviewer.

Comment 17: Line 102: When you say they are grouped a certain way (here, this is the second way of grouping), I think it is imperative to not wait for later in the article to say why.

Response 17: We have added a sentence to explain why the sites were grouped in a certain way L104: The 22 sites have been grouped into seven station types representing typical locations in Switzerland with potentially different air pollution levels (FOEN, 2022): urban traffic, urban center, suburban, rural highway, rural sites below 1000 m a.s.l., rural sites above 1000 m a.s.l., and a high-altitude site (Tab. 1). We further added to L112: “To meet the objectives of the study …”

Comment 18: Figure 2 caption: you highlight meaning of blue and brown bars, but not grey.

Response 18: We have added the following explanation to the caption of Figure 2: The rural highway sites and the high altitude site (gray bars) were not assigned to either group.

Added references:

C. H. Lamborg, C. R. Hammerschmidt, K. L. Bowman, G. J. Swarr, K. M. Munson, D. C. Ohnemus, P. J. Lam, L.-E. Heimbürger, M. J. A. Rijkenberg and M. A. Saito, A global ocean inventory of anthropogenic mercury based on water column measurements. Nature, 2014, 512, 65–68.

C. I. Olson, H. Fakhraei and C. T. Driscoll, Mercury emissions, atmospheric concentrations, and wet deposition across the conterminous United States: Changes over 20 years of monitoring. Environ. Sci. Technol. Lett., 2020, 7, 376–381.

J.H. Seinfeld, and S.N. Pandis, 2016, Atmospheric Chemistry and Physics: From air pollution to climate change, John Wiley & Sons, Hoboken.

M. Gharun, L. Hörtnagl, E. Paul-Limoges, S. Ghiasi, I. Feigenwinter, S. Burri, K. Marquardt, S. Etzold, R. Zweifel, W. Eugster and N. Buchmann, Physiological response of Swiss ecosystems to 2018 drought across plant types and elevation, Phil. Trans. R. Soc., 2020, B37520190521

M. Gharun, S. Klesse, G. Tomlinson, P. Waldner, B. Stocker, B. Rihm, R. Siegwolf and N. Buchmann, Effect of nitrogen deposition on centennial forest water-use efficiency, Environ. Res. Lett., 2021, 16, 114036, https://doi.org/10.1088/1748-9326/ac30f9

Q. Chang, X. Xiao, R. Doughty, X. Wu, W. Jiao and Y. Qin, Assessing variability of optimum air temperature for photosynthesis across site-years, sites and biomes and their effects on photosynthesis estimation, Agric. For., 2021, 298–299, 108277, https://doi.org/10.1016/j.agrformet.2020.108277.

S. Wolf, W. Eugster, C. Ammann, M. Häni, S. Zielis, R. Hiller, J. Stieger, D. Imer, L. Merbold and N. Buchmann. Contrasting Response of Grassland versus Forest Carbon and Water Fluxes to Spring Drought in Switzerland. Environ. Res. Lett., 2013, 8, 035007, https://doi.org/10.1088/1748-9326/8/3/035007

T. Navrátil, J. Rohovec, J. Shanley, Š. Matoušková, T. Nováková, A. H. Šmejkalová and R. Prokeš, Atmospheric mercury and its deposition during the phasing out of an amalgam electrolysis plant: Temporal, seasonal, and spatial Patterns, Environ. Sci. Pollut. Res., 2023, 30, 123586–123602.

T. Nováková, T. Navrátil, M. Schütze, J. Rohovec, Š. Matoušková, M. Hošek and T. M. Grygar, Reconstructing Atmospheric Hg Levels near the Oldest Chemical Factory in Central Europe Using a Tree Ring Archive, Environ. Pollut., 2022, 304, 119215.

X. Lan, R. Talbot, M. Castro, K. Perry and W. Luke, Seasonal and diurnal variations of atmospheric mercury across the US determined from AMNet monitoring data, Atmos. Chem. Phys., 2012, 12, 10569–10582.

22-Jun-2024

Dear Dr Osterwalder:

Manuscript ID: EA-ART-04-2024-000052.R1
TITLE: Spatial and seasonal dynamics of gaseous elemental mercury concentrations over Switzerland observed by a passive air sampler network

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Reviewer 2

Bravo to the authors for quick and well-done minor revisions to this manuscript. I have nothing further to add.