Using Moss Walls for Air Quality Monitoring: Extending Their Utility Beyond Traditional Green Infrastructure

Round 1

Reviewer 1 Report

This "pioneering study" as authors write in the Abstract contradicts to  Occam's principle - "Don't invent entities beyond what is necessary". There is an "active" moss biomonitoing method based on using moss bags exposed to air pollution studies. Why to invent so called "walls" (wall panels) which are not walls in the literal sense of the word, but  panels of quite small size to be called walls! The size of these panals is not even specified, which is very strange - we can judge about their size  only by the Figure  4 (photo).

The concept of experimentation seems far-fetched and has no practical application.

The use of mosses as biomonitors of air pollution is well known and does not need further confirmation, as the authors state in the Conclusion:

"...In the coming decades, moss walls with H. cupressiforme, and mosses in general, will certainly be a sustainable solution for urban environment and will represent attractive source for air biomonitoring".

One should remember that in many European countries collection of moss is under strict control or even forbidden!

Author Response

Thank you for your valuable feedback. We appreciate the opportunity to clarify our study's objectives and its potential applications. Below you can find our answers to your comments.

“The concept of experimentation seems far-fetched and has no practical application. The use of mosses as biomonitors of air pollution is well known and does not need further confirmation, as the authors state in the Conclusion: "...In the coming decades, moss walls with H. cupressiforme, and mosses in general, will certainly be a sustainable solution for urban environment and will represent attractive source for air biomonitoring". One should remember that in many European countries collection of moss is under strict control or even forbidden!”

On the Concept of Experimentation and Practical Application: While the use of mosses as biomonitors is well established, our study aimed to explore their potential in a controlled experimental setup to enhance their application in urban environments. We acknowledge that moss biomonitoring has been widely studied – as pointed in the manuscript, e.g. L70-73, but our work contributes to refining methodologies and understanding the efficacy of Hypnum cupressiforme in a specific setting. Furthermore, our findings support the practical implementation of moss walls as both biomonitoring tools and sustainable urban greening solutions, which align with the growing interest in nature-based strategies for air quality management.

On the Novelty of Using Mosses for Biomonitoring: We agree that moss biomonitoring has been extensively studied; however, our research does not aim to merely reconfirm existing knowledge. Instead, it explores the feasibility of integrating moss walls in urban environments as both passive air quality indicators and potential remediation tools. The conclusions drawn reflect the increasing relevance of such nature-based solutions, particularly in densely populated areas.

On Moss Collection Regulations in Europe: We acknowledge the concerns regarding moss collection restrictions in certain European countries. However, our study does not advocate for large-scale harvesting of mosses but rather emphasizes the potential of Hypnum cupressiforme, which is a widespread and abundant species found across various habitats, and present on all continents except Antarctica. According to the International Union for Conservation of Nature (IUCN) Red List, H. cupressiforme is classified as a species of "Least Concern" (LC), indicating it is not currently at significant risk of extinction. Regarding European conservation directives, Hypnum cupressiforme is not listed in the annexes of the European Union's Habitats Directive, which focuses on the conservation of natural habitats and wild fauna and flora within the EU.

 

“The title: "External moss walls: from the inside out" sounds pretentious and does not reflect the essence of the article.”

Based on your feedback, that of Reviewer #2, and the suggestion of Reviewer #3, we have revised the title to: ' Using moss walls for air quality monitoring: extending their utility beyond traditional green infrastructure.' We believe this revised title more accurately reflects the scope of our study.

 

“The introduction is a mixture of representations of biomonitoring with mosses and unsubstantiated claims that mosses can be used for air purification due to their sorption capacity. References to the UN Convention on Long-Range Transboundary Air Pollution (CLRTAP) show that the authors do not understand the essence of this Сonvention which deals mainly with passive biomonitoring using mosses over large areas.”

We acknowledge the reviewer's comment regarding the distinction between mosses as biomonitors and their potential role in air purification. Our intention was to highlight both aspects, as they are interrelated and relevant to the broader context of our research. While we recognize that the CLRTAP primarily focuses on passive biomonitoring of air pollution using mosses over large areas, specifically concerning long-range transport and deposition of pollutants like heavy metals, which are a focus of our study, the CLRTAP was mentioned to provide context for the broader field of air quality assessment. Furthermore, one of the authors participates in ICP-Vegetation, where H. cupressiforme is routinely used for passive biomonitoring; thus, we aimed to briefly connect our work to this established methodology. We appreciate that Reviewer #2 and Reviewer #3 found the introduction comprehensive and concise. Given the relevance of both biomonitoring and air purification to our research area, we have chosen to retain this section, while ensuring that the distinction between these two aspects and the specific focus of the CLRTAP are clearly articulated.

The methodology is fundamentally flawed. Is it possible to fix moss on a metal base? According to the Instruction (Moss Manual of the UNECE ICP Vegetation), when collecting the moss one should remove the rings from fingers and watch from your hands to avoid contamination of the moss with metals. What can we say about the long-term exposure of the moss near / on metal tubes and a metal stand, as we can see in Figure 4?

Thank you for your critical observation. We acknowledge the concerns regarding potential contamination from metal surfaces. To clarify, the image in Figure 4(a) depicts a galvanized steel thin net positioned above the moss.

In our study, we carefully considered the potential metal impact on moss samples, particularly concerning Zn content. We believe that this influence should be minimal because zinc in alloys, such as galvanized steel, exists in a solid, metallic form (Chatterjee, 2014). Plants cannot directly absorb zinc in this metallic state (Mertens & Smolders, 2013).

 

“Is it worth talking about results if the methodology is wrong from the start? One of the shortcomings of the paper is the lack of quality control of the analysis - no analysis data of the M2 and M3 Reference Materials are given. These materials, by the way, are very old, I am not sure that they are not expired. Even assuming everything is done correctly, the conclusion about the environmental risk caused by cadmium still requires some assumptions about its source. This is not in the Conclusion either.”

Thank you for your critical feedback. We acknowledge the importance of robust quality control in analytical procedures. In our study, we ensured accuracy, e.g. dilution by an additional factor of 5 to minimize matrix effects and ensure low quantification limits for trace elements; application of ¹⁰³Rh as an internal standard to correct for non-spectral matrix effects and instrument drift (L175-178 of the manuscript); blank samples (L181-182).

However, we recognize the concern regarding the lack of explicit data for the M2 and M3 Reference Materials. We have now included below the relevant analysis data; if needed this table could be incorporated in the revised manuscript to enhance transparency and methodological rigor.

 

Table 1. Measured and certified values for the element concentrations (mg kg ⁻¹) with corresponding combined uncertainties (k = 2) for the certified elements in reference materials M2, M3 and NCS DC73348—bush branches and leaves.

Element	CRM M2				CRM M3				CRM Bush branches and leaves (NCS DC 73348)
	Measured		Certified		Measured		Certified		Measured		Certified
	mg/kg	U (k=2)	mg/kg	U (k=2)	mg/kg	U (k=2)	mg/kg	U (k=2)	mg/kg	U (k=2)	mg/kg	U (k=2)
Al	180	19	178	15	176	17	169	10	1040	90	-	-
As	0.89	0.16	0.98	0.07	< 0.1	-	0.105	0.007	0.89	0.12	0.95	0.12
Cd	0.44	0.04	0.45	0.019	0.10	0.01	0.11	0.005	0.19	0.02	0.14	0.06
Co	0.99	0.09	0.98	0.06	0.13	0.02	0.12	0.006	0.44	0.02	0.39	0.05
Cr	1.03	0.10	0.97	0.17	0.65	0.06	0.67	0.19	1.92	0.08	2.3	0.3
Cu	62.8	4.9	68.7	2.5	3.77	0.47	3.76	0.23	5.67	0.6	5.2	0.5
Fe	282	21	262	35	147	13	138	12	973	53	1020	67
Mn	299	31	342	17	500	48	535	30	60.5	7.0	58.0	6.0
Ni	15.5	1.0	16.3	0.9	0.99	0.12	0.95	0.08	1.72	0.21	1.7	0.4
Pb	6.76	0.53	6.37	0.43	3.56	0.19	3.33	0.25	7.35	0.40	7.10	1.1
Zn	35.4	1.2	36.1	1.2	26.5	0.6	25.4	1.1	20.7	0.7	20.6	2.2

 

We verified reference materials validity by observing a good agreement between the measured and certified values in all three cases. In our opinion, this proves that the analytical method we proposed is applicable for the determination of the studied elements in moss samples.

On the issue of cadmium and its environmental risk, we agree that determining its source is crucial for assessing potential impacts. This is why the second sentence in the Conclusion states that the industrially influenced site was identified as polluted. Furthermore, one of the moss walls (MW3) was intentionally placed at that specific location, near the KCM 2000 industrial complex, Bulgaria's largest non-ferrous metal production company, to assess its impact (see Table 4).

 

Incorrect methodology does not make this article relevant to the discussion of biomonitoring or air purification.

Based on the explanation to your comments above and given that both Reviewer #2 and Reviewer #3 acknowledged that the research design is appropriate and the methods are adequately described, we hope that this addresses your concerns regarding the methodology. Additionally, we have carefully reviewed and revised the manuscript to further clarify our approach and ensure transparency in our methods. We believe that our findings contribute valuable insights to the discussion of biomonitoring and air purification, and we appreciate your critical assessment, which has helped us strengthen our work.

The composition of Figure 3 looks unrepresentative and it is not immediately clear what is what.

Thank you for your comment. We have revised the figure caption to make it more informative.

 

This "pioneering study" as authors write in the Abstract contradicts to  Occam's principle - "Don't invent entities beyond what is necessary". There is an "active" moss biomonitoing method based on using moss bags exposed to air pollution studies. Why to invent so called "walls" (wall panels) which are not walls in the literal sense of the word, but  panels of quite small size to be called walls! The size of these panals is not even specified, which is very strange - we can judge about their size  only by the Figure  4 (photo).

Thank you for your feedback. We acknowledge the existence of the active moss biomonitoring method using moss bags and appreciate your perspective on Occam's principle. However, our study explores an alternative approach by using moss in a structured way, which we refer to as "pilot moss walls." This terminology aligns with previous studies and green infrastructure concepts that describe vertical moss-based air quality monitoring systems.

To address your concern regarding the size of the "pilot moss walls" in this study, we have now explicitly specified their dimensions in the revised manuscript to ensure clarity (L106).

We hope this clarification resolves any ambiguity regarding their scale and purpose.

Author Response File: Author Response.pdf

Reviewer 2 Report

The aim of the research is to evaluate the potential of life in the open air of the moss walls H. cupressiforme, as tools for monitoring air quality in urban environments appears to have been achieved. the results demonstrate the feasibility of using live moss walls for monitoring air quality: it would be important to extend the research to other urban areas and intensify the time intervals over a longer period in order to obtain more predictive results on the effectiveness of the study methods. The research appears well organised: the introduction effectively explains the aims of the study.
The materials and methods are clearly exposed and the results are in line with the intended objectives.
The figures are largely explanatory of the methodology and results achieved. the bibliography is complete.
It is hoped that the authors can extend the research to other urban areas and with long-term time intervals in order to
have more effective results.

Author Response

Thank you for your positive and constructive feedback. We appreciate your recognition of our study’s objectives, methodology, and findings. Your suggestion to extend the research to other urban areas and longer time intervals is valuable, and we will consider it for future studies.

The aim of the research is to evaluate the potential of life in the open air of the moss walls H. cupressiforme, as tools for monitoring air quality in urban environments appears to have been achieved. the results demonstrate the feasibility of using live moss walls for monitoring air quality: it would be important to extend the research to other urban areas and intensify the time intervals over a longer period in order to obtain more predictive results on the effectiveness of the study methods.

The research appears well organised: the introduction effectively explains the aims of the study.
The materials and methods are clearly exposed and the results are in line with the intended objectives.
The figures are largely explanatory of the methodology and results achieved. the bibliography is complete.It is hoped that the authors can extend the research to other urban areas and with long-term time intervals in order to have more effective results.

Title: Explain the aims of the work better.

Based on your feedback, that of Reviewer #1, and the suggestion of Reviewer #3, we have revised the title to: Using moss walls for air quality monitoring: extending their utility beyond traditional green infrastructure. We believe this revised title more accurately reflects the scope of our study.

Author Response File: Author Response.pdf

Reviewer 3 Report

The study investigates an interesting topic that can serve as a basis for future monitoring of air quality in urban areas while at the same time refining the space. Therefore, I gladly recommend this work to be published after a few minor changes that will improve the understanding of the text and the clarity of the presented results. 

Small changes are necessary to make all Figures and Tables clearer, and few additional improvements (explanations) in the M&М Section is needed. Please see the Detail comments for this.

Line 46: Sentence "The natural fiber used..." is not necessary here.

Lines 92-94: Sentence "Our findings demonstrate..." is good for Conclusions section. Or even title!

Line 96: 4.1 Moss collection and moss wall location - Numbering is wrong, it should be 2.1. Please correct this further in the whole manuscript and all chapters

Line 99: Figure 3 should be Figure 1. and Table 1. Please correct all the following figures and tables in the text accordingly.

Line 102: Figure 1.

Line 103: Table 1.

Line 105: Wrong numbering.

Line 121: Figure 2.

Line 122: Wrong numbering.

Line 126: Please check and correct order and numbers of the references in the text.

Line 149: Later in the manuscript you use "second exposure period" hence it should be defined here, for example "... at the end of the first month (first exposure period) and fifth month (second exposure period) of exposure".

Lines 151/152/182/193/198/208: Wrong numbering.

Lines 218/223/224: Instead of RI should be ERI?

Line 228: Wrong numbering.

Line 233: Increase of what? It should be made clear, like "The most notable increase of toxic elements "

Line 234/239: Table 2.

In general, Table 2 is not shown clearly enough. Are these cumulative measures for all locations/walls? I think that the column "background" should be named "first exposure" while column "maximum" should be "second exposure period". Or use different terms, but use it uniformly in the whole text.

Line 244: In design of the experiment it is not defined what is the background sample? It is not clear enough whether in this case this locality is MW2. If so, use these abbreviation uniformly throughout the text, and not its descriptions.

Lines 242/251: Figure 3.

Lines 255/263: Table 3

Line 265: Wrong numbering.

Line 269: Sentence "Tocopherol synthesis..." is more appropriate for the Discussion section.

Line 276: Wrong numbering.

Lines 277-280: This part is more appropriate for the Discussion section.

Line 282: Аt the initiation of their installation or after one month like it is written in M&M Section?

Line 294: MW3-5?

Author Response

Thank you for your positive evaluation and recommendation for publication. We appreciate your constructive feedback and have made the suggested minor revisions to improve the clarity of the figures, tables, and methodology section.

The study investigates an interesting topic that can serve as a basis for future monitoring of air quality in urban areas while at the same time refining the space. Therefore, I gladly recommend this work to be published after a few minor changes that will improve the understanding of the text and the clarity of the presented results. Small changes are necessary to make all Figures and Tables clearer, and few additional improvements (explanations) in the M&М Section is needed. Please see the Detail comments for this.

We sincerely appreciate your insightful comments, listed below, which have helped us refine our manuscript.

 

Line 46: Sentence "The natural fiber used..." is not necessary here.
The sentence was deleted.

 

Lines 92-94: Sentence "Our findings demonstrate..." is good for Conclusions section. Or even title!

Thanks to your valuable suggestion, the sentence has been moved to the Conclusion and used as the basis for the new title.

 

Line 96: 4.1 Moss collection and moss wall location - Numbering is wrong, it should be 2.1. Please correct this further in the whole manuscript and all chapters

Implemented.

Line 99: Figure 3 should be Figure 1. and Table 1. Please correct all the following figures and tables in the text accordingly.

Line 102: Figure 1.

Line 103: Table 1.

Line 105: Wrong numbering.

Line 121: Figure 2.

Line 122: Wrong numbering.

Lines 151/152/182/193/198/208: Wrong numbering.

Line 228: Wrong numbering

Line 234/239: Table 2.

Lines 242/251: Figure 3.

Lines 255/263: Table 3

Line 265: Wrong numbering.

Line 276: Wrong numbering.

Implemented.

 

Line 126: Please check and correct order and numbers of the references in the text.
Thank you for your comment. We have checked all references in the manuscript and corrected the numbering accordingly.

Line 149: Later in the manuscript you use "second exposure period" hence it should be defined here, for example "... at the end of the first month (first exposure period) and fifth month (second exposure period) of exposure".

Implemented.

 

Lines 218/223/224: Instead of RI should be ERI?

Implemented.

 

Line 233: Increase of what? It should be made clear, like "The most notable increase of toxic elements "

Implemented.

 

In general, Table 2 is not shown clearly enough. Are these cumulative measures for all locations/walls? I think that the column "background" should be named "first exposure" while column "maximum" should be "second exposure period". Or use different terms, but use it uniformly in the whole text.

Thank you for your comment. We have added clearer descriptions within Table 2 and revised the first sentence of the paragraph introducing the table to clarify which moss wall these maximum values refer to.

 

Line 244: In design of the experiment it is not defined what is the background sample? It is not clear enough whether in this case this locality is MW2. If so, use these abbreviation uniformly throughout the text, and not its descriptions.

Thank you! We have added a clarification on the sample referred to as the background, as suggested in Section 2.1, Moss Collection and Moss Wall Location (L96–97). Additionally, we have more precisely introduced MW2 as a low-impact urban site. We hope that, thanks to your recommendation, uniform terminology regarding locations and samples has been achieved.

 

Line 269: Sentence "Tocopherol synthesis..." is more appropriate for the Discussion section.

The sentence was moved to Discussion.

Lines 277-280: This part is more appropriate for the Discussion section.

The paragraph was moved to Discussion.

Line 282: Аt the initiation of their installation or after one month like it is written in M&M Section?

Thank you – corrected to one month.

Line 294: MW3-5?

Corrected to MW3-2.

 

Please consider changing the title of the paper in order to describe content more precisely. One sentence from the manuscript is good starting point that can be taken into account - for example: Using moss walls for air quality monitoring: extending their utility beyond traditional aesthetic and environmental roles.

Thank you for your valuable suggestion. Based on your feedback, we have revised the title to: 'Using moss walls for air quality monitoring: extending their utility beyond traditional green infrastructure'. We believe this new title better reflects the scope of our study and appreciate your insight.

 

The study investigates an interesting topic that can serve as a basis for future monitoring of air quality in urban areas while at the same time refining the space. Therefore, I gladly recommend this work to be published after a few minor changes that will improve the understanding of the text and the clarity of the presented results. Small changes are necessary to make all Figures and Tables clearer, and few additional improvements (explanations) in the M&М Section is needed. Please see the Detail comments for this.

We appreciate your insightful comments, which have been addressed in the revised manuscript and helped us refine it.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The design of experiment is interesting.
I think  table with the results of the quality control need to be included in the manuscript.
In my opinion manuscript can be accepted fro publication. 

The design of experiment is interesting.
I think  table with the results of the quality control need to be included in the manuscript.
In my opinion manuscript can be accepted fro publication.