Module Figures: Peer Review in Scientific Publishing • Visionlearning

27 May 2007
Dear Authors:

I have received two reviews of your manuscript entitled "Quantifying the effect of humic matter on the suppression of mercury emissions from artificial soil surfaces" submitted for publication in Applied Geochemistry. In addition, I have read your paper and have some additional comments below. All reviewers, including myself, agree the paper after revisions is acceptable for publication.

I have attached both reviewers' comments to this email. Both reviewers raise some important issues that need to be clearly addressed in your revised paper. I agree with their concerns and below have added a few others that need to be addressed.

Sincerely,
Editor for Special Issue of Applied Geochemistry

Additional detailed comments from the Editor:

The mass balance needs to be considered [as detailed by reviewer 2]. My guess is your flow rate is producing an artificially high flux. The way to deal with this would be to use the actual concentration difference between the inlet and outlet instead of the flux to calculate the amount lost. Plot the difference between the outlet and inlet concentrations rather than flux.

Excerpts of comments from Reviewer 1:

I have received two reviews of your manuscript entitled "Quantifying the effect of humic matter on the suppression of mercury emissions from artificial soil surfaces" submitted for publication in Applied Geochemistry. In addition I have read your paper and have some additional comments that are below. All reviewers including myself agree the paper after revisions is acceptable for publication.

The "suppression of mercury emission" [in the title] is an interpretation of the experimental observations, rather than an unequivocal conclusion. It might be better to use a [more] conservative title like "Quantifying the Effect of Humic Matter on Mercury Emissions from Artificial Soil Surfaces". I'd think the reader might come up with some different interpretations other than "suppression".

Would [additional experiments with] controls of humic matter plus Hg(II) salt only (without any sand) offer any more [information]?

Excerpts of comments from Reviewer 2:

In response to direct comments requested by the editor:

Originality: This paper systematically tests the combined impact of humic matter content and light in synthetic soils. Very few studies have reported similar work.

Importance: This work's main conclusion is that organic matter content alters Hg emissions from soils. This conclusion is of significant interest to mercury biogeochemists and may promote related field-based research, and help in the interpretation of current sets.

Manner of presentation: The paper is short, clear and to the point. More discussion of possible mechanisms and more details on related field studies (where fluxes and organic matter have been correlated) could be added.

Quality of figures and tables: I do not think that the authors have reached an optimal design for the graphical presentation of their data. Figure 1, 2 & 3 could be easily combined, which would help the reader to compare the results taken at different intervals for the same experiments. In fact, these graphs could even be transformed in time series line graphs (instead of histograms). I am not sure of the most attractive final design, but the present design can be improved.

Serious flaws or can [the paper] be improved by condensation or deletion of information: I have not found any serious flaws. I can say that I am not totally at ease with a study that reports only results from synthetic soils. It would have been nice to complement this data with some «real» soils. But I think that such a systematic, laboratory study is useful and pertinent.

Does the title and abstract correspond to the content of the manuscript: Yes

Would you be willing to re-review this paper after submission with revisions: Not necessary

Specific comments regarding the manuscript:

Can the authors comment on the realism of their approach? What are the limits of using synthetic soils and mixtures of inorganic Hg + humic acids? The fact that they tried different kinds of humic matter is comforting, but I would have like to see more info on potential limitations.

Page 9. Please clarify the design for the long term monitoring section. For instance, were the lights on for 14 days in the "light treatment"? Was this continuous flux sufficient to decrease the pool of Hg in the sample? The following back [of] the envelope calculations left me worried by the results presented here:

If we take an average flux of 2000 ng/m2/h for the light + sand treatment (see figures 1, 2 and 3), then we get over 14 days [and] 44 µg lost by evasion, whereas only 25 µg were added!!

I suspect that the lamps were only ON during the readings, once every week, but this should be more obvious. More info on the impact of the flux on the mass balance of the samples should be added. If the lights were turned OFF between weekly readings, how long were they ON for the readings?

Responses to specific comments raised by the Editor:

While turnover rate is a significant issue, our work represents the relative comparison of samples that were all measured at a constant turnover rate, thus the effect of chamber turnover rate on our conclusions is negated. A discussion of this has been added to the manuscript. In an effort to guide future research, we have added mention of more recent personal communication regarding chamber turnover rate, to our knowledge new data regarding turnover rate is not published. A suggestion was made to report the difference between chamber inlet and outlet Hg concentrations rather than fluxes. As described above, the mass balance of Hg in the samples is not problematic. Further, because all Co-Ci differences are multiplied by a constant turnover rate in the flux equation, this would simply have the effect of changing the magnitude of the numbers (and graph axes) reported, not the relative difference between numbers - which is the basis of all conclusions of the work. Also, because the majority of researchers report results as fluxes, we feel that reporting our results as concentration differences would make this work inaccessible to mercury researchers. Our methodology and flux measurements are all based on peer-reviewed, published literature (Lindberg et al., 2002) and follow standard protocols. We believe discussing the limitations of the method is therefore sufficient in this context.

July 10, 2007
Dear Editors:

Enclosed is our revised manuscript. We have addressed all of the comments returned to us in the reviews of our paper. In addition, at the suggestion of reviewer 1, we have conducted additional experiments with 100% humic acid and have added the results of this experiment to our paper to assure that we have adequately addressed the experimental design comments. A detailed list of all individual changes is included below.

All of the listed authors have read the revisions and agree with their conclusions.

Sincerely,
Authors

Detailed list of manuscript revisions.

Responses to comments raised by Reviewer 1:

As directed, we have revised the title of the manuscript to "Quantifying the Effect of Humic Matter on the Emission of Mercury from Artificial Soil Surfaces."

The reviewer raises an interesting question regarding the use of Hg-humic controls (without sand). These controls were not examined at the time of our study. However, to satisfy this question, we have since conducted additional experiments with a 100% humic sample using 1g humic and HgCl2 sample (no sand). The results from this sample were consistent with those presented for our 5% humic sample, confirming that the effect we saw was due to humics, and not the interaction of humics with the sand. We have added this data to the paper and to Figure 1.

Responses to specific comments raised by Reviewer 2:

We have condensed the presentation of data in the Figures as suggested so that only one pair of graphs is now used (new Figure 1) instead of the three pairs that were used in the previous version of the manuscript.

Regarding the manner in which samples were stored between measurements, our Methods section states: "All samples were stored in the dark at constant temperature (~23°C) between measurements and monitored in both dark and light for mercury flux at regular intervals." We have emphasized this statement in the results section and clarified that all flux measurements were taken over a 1.5-2 hr sampling period.

Responses to specific comments raised by the Editor:

While turnover rate is a significant issue, our work represents the relative comparison of samples measured at a constant turnover rate, thus negating its effect on our conclusions. We have added a discussion of this to the manuscript. Additionally, we have included a mention of more recent personal communication regarding chamber turnover rate. A suggestion was made to report the difference between chamber inlet and outlet Hg concentrations rather than fluxes, but since all Co-Ci differences are multiplied by a constant turnover rate in the flux equation, this would only affect the magnitude of the numbers reported, not the relative differences on which our conclusions are based. Since most researchers report results as fluxes, changing our reporting method could make our work inaccessible to mercury researchers. Our methodology follows standard peer-reviewed protocols.