A Critical Review on Soil Gas Analysis: Modern Technologies and Problems

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I do not see this paper as a critical review. The introduction does not say much, except that portioning occurs. It does not lay out the objective of the paper. When I think about soil, there are field studies and sampling for in-lab measurements. The instrumentation cons and pros apply differently. The paper reads like a thesis introduction and not really a review paper.

 

For IR, I am surprised that it is not called NDIR for the first method. Path length and multiple reflection cells are needed. Methane, for example, high sensitivity is hard with a simple device at environmental concentrations. Path lengths are mentioned, one for open-air. For spectroscopy (all the ones including MS), you are dealing with the gases above the soil because light will not pass through. Not too much is said about  thermal desorption methods of soil sample. Only Figure 11 and associated text seem to touch on this topic.  There is also very little comparing accuracy, precision (LOD, LOQ)  and other critical factors.

 

Section 3 is gas emissions. However, all the other methods discussed also deal with gas emissions. Of course, Section 3 is about more complex measurements, but is different and feels unconnected the first part talking about method.

 

Sections 4 and 5 do not address where the field is going. For example, “Important problems that need to be addressed to improve the quality of gas pattern analysis have been compared.”  There is no such sentence, such as x, y and x need to be addressed as discussed in this article.

Author Response

I do not see this paper as a critical review. The introduction does not say much, except that portioning occurs. It does not lay out the objective of the paper. When I think about soil, there are field studies and sampling for in-lab measurements. The instrumentation cons and pros apply differently. The paper reads like a thesis introduction and not really a review paper.

Comment:

Thank you for remark. The Introduction section was significantly rewritten. The additional information was added.

 

For IR, I am surprised that it is not called NDIR for the first method. Path length and multiple reflection cells are needed. Methane, for example, high sensitivity is hard with a simple device at environmental concentrations. Path lengths are mentioned, one for open-air.

Comment:

Thank you for remark. Actually, IR spectroscopy was mentioned as a general method. Further we have added some explanations on the application of NDIR and FTIR spectroscopy.

Concerning the path lengths. Some researches omitted this value in the papers. We have added the values we have found in the papers.

 

For spectroscopy (all the ones including MS), you are dealing with the gases above the soil because light will not pass through.

Comment:

Thank you for suggestion. This comment was added in the very beginning of Section 2.

 

Not too much is said about thermal desorption methods of soil sample. Only Figure 11 and associated text seem to touch on this topic.  

Comment:

Thank you for suggestion. There methods are used mainly for polluted soils. This paper is mainly directed to determination of gases forming as a result of presence of microorganisms, which do not allow using high temperatures. Anyway, some short discussion was added at the end of Section 5.  

 

There is also very little comparing accuracy, precision (LOD, LOQ) and other critical factors.

Comment:

We have found some information and added some (there are not too much values reported in the papers devoted to soil gas analysis) LoDs (which are presented more frequently in the papers).

 

Section 3 is gas emissions. However, all the other methods discussed also deal with gas emissions. Of course, Section 3 is about more complex measurements, but is different and feels unconnected the first part talking about method.

Comment:

Thank you for remark. We have changed the sections significantly. Now, section 3 is different and mainly devoted to MS/GC compared to optical spectroscopy in Section 2. Section 4 is mainly devoted to sensors, and section is about the sampling.

 

Sections 4 and 5 do not address where the field is going. For example, “Important problems that need to be addressed to improve the quality of gas pattern analysis have been compared.”  There is no such sentence, such as x, y and x need to be addressed as discussed in this article.

Comment:

Thank you for remark. These sections were rewritten and organized in a new way.

Reviewer 2 Report

Comments and Suggestions for Authors

Overall, the review paper appears to be a well-organized and informative piece that covers a wide range of relevant topics in soil gas analysis. It addresses important technical and methodological aspects while presenting challenges and potential solutions, which is essential for a review paper. However, there are several areas where it could be improved to enhance its clarity, depth, and overall impact. Here’s a summary of the strengths and areas for improvement:

1-    I recommend that the corresponding author use the academic email.

2-    Your introduction does not meet the requirements for a review paper introduction.  In the introduction of a review paper, it's essential to provide a clear background on the topic, explaining its significance and the broader context within the field. The scope of the review should be well-defined, specifying what aspects will be covered and highlighting the relevance of the review at this moment. It's also important to identify any existing research gaps or unresolved issues that the review aims to address, which underscores the necessity of the paper. The introduction should clearly state the objective, whether it's to summarize current knowledge, identify trends, or suggest future directions. Additionally, mentioning the practical or theoretical significance of the topic adds value, particularly if it has important applications. Finally, a brief outline of the paper’s structure helps guide the reader through the review.

3-    The general road map of the structure of the article should be stated at the end of the introduction.

4-    Section 2 (at the beginning) could benefit from better clarity in explaining the distinctions between the methods and their relative advantages or limitations.

5-    Consider expanding the role of laser technology advancements in enhancing the resolution and sensitivity of infrared spectroscopy for soil gases.

6-    While the Raman Spectroscopy method allows for the detection of non-polar molecules (e.g., O₂, N₂), it suffers from low signal-to-noise ratio (SNR) in certain environments. How can this limitation be mitigated?

7-    It would be helpful to discuss the technological challenges in implementing cavity ring-down spectroscopy in soil gas analysis, such as the high-quality mirror requirement.

8-    How does TDLAS compare with CRD in terms of sensitivity and practicality for soil gas monitoring?

9-    The discussion of how these spectroscopic methods can be applied in the context of environmental monitoring and precision agriculture could be expanded.

10- The glass-based chambers are said to be less common. Could you discuss more about why glass chambers, despite being inert, are not widely used (e.g., fragility, cost)? Are there any innovations in this area?

11- The description of static and dynamic chambers is informative, but further clarification on when each method is most appropriate would be helpful.

12- You mention the use of an oscillating fan and a dish-type structure to mitigate pressure build-up. It would be beneficial to discuss potential alternatives or recent innovations in pressure management during long-term measurements.

13- The impact of groundwater pressure and precipitation on gas exchange is briefly mentioned.

14- The third section could be enhanced with more case studies or examples of successful gas emission sampling in different ecosystems (e.g., agricultural fields, forested areas, urban settings).

15- Expanding on how challenges manifest in real-world applications would strengthen the section 4.

16- The mention of chemiresistive gas sensors is interesting, but you should expand on their limitations as well. Also its better to refer to room temperature chemiresistive gas sensors (DOI: 10.1016/j.snb.2022.131418)

17- The suggestion to use AI approaches for selecting sampling points is an innovative idea. However, it would be useful to explain how this would be implemented practically. Consider discussing the potential limitations of AI in soil gas analysis, such as the need for large datasets for training models and the difficulty of obtaining comprehensive soil gas data across different environments.

18- Since the presence of biological agents is also mentioned, exploring how microbial activity affects gas emissions and what methods are used to account for these influences could be informative.

19- What are the specific gases that commonly interfere with each other during soil gas analysis?

20- Add further elaboration on how PCA works to reduce signal interference.

21- Consider ending the Challenges section with a forward-looking statement about potential solutions to these challenges.

Author Response

Overall, the review paper appears to be a well-organized and informative piece that covers a wide range of relevant topics in soil gas analysis. It addresses important technical and methodological aspects while presenting challenges and potential solutions, which is essential for a review paper. However, there are several areas where it could be improved to enhance its clarity, depth, and overall impact. Here’s a summary of the strengths and areas for improvement:

1-    I recommend that the corresponding author use the academic email.

Comment: Thank you for remark. The academic e-mail was added.

2-    Your introduction does not meet the requirements for a review paper introduction.  In the introduction of a review paper, it's essential to provide a clear background on the topic, explaining its significance and the broader context within the field. The scope of the review should be well-defined, specifying what aspects will be covered and highlighting the relevance of the review at this moment. It's also important to identify any existing research gaps or unresolved issues that the review aims to address, which underscores the necessity of the paper. The introduction should clearly state the objective, whether it's to summarize current knowledge, identify trends, or suggest future directions. Additionally, mentioning the practical or theoretical significance of the topic adds value, particularly if it has important applications. Finally, a brief outline of the paper’s structure helps guide the reader through the review.

Comment: Thank you for remark. The Introduction was rewritten significantly. The necessary information was added. The problem of soil analysis has been written.

3-    The general road map of the structure of the article should be stated at the end of the introduction.

Comment: Thank you for remark. The structure of the article was presented at the end of Introduction.

4-    Section 2 (at the beginning) could benefit from better clarity in explaining the distinctions between the methods and their relative advantages or limitations.

Comment: Thank you for remark.  The description of features and limitations of each method was added at the very beginning of Section 2.

“…..There are some limitations associated with each spectroscopic technique, which determine the ability to detect certain gases in soils. For example, infrared spectroscopy is limited in its ability to detect gases with non-polarizable chemical bonds (such as N₂, H₂, O₂, etc.), while gases with polarized bonds can be detected (e.g., CO₂). Some optical methods (e.g., Raman spectroscopy) do not have this limitation. Optical methods must have their own specific optical path length, which can affect the dimensions of the analyzers, potentially making them relatively large. The issue of the size of analyzers can be addressed by utilizing a set of gas sensors that are small enough, such as chemiresistive sensors, optodes, non-dispersive infrared sensors, etc. Some methods enable the detection of gases produced by various isotopes (e.g., mass spectrometry), which is useful not only for soil analysis but also for geophysics and environmental studies…..”

5-    Consider expanding the role of laser technology advancements in enhancing the resolution and sensitivity of infrared spectroscopy for soil gases.

6-    While the Raman Spectroscopy method allows for the detection of non-polar molecules (e.g., O₂, N₂), it suffers from low signal-to-noise ratio (SNR) in certain environments. How can this limitation be mitigated?

Comment: Thank you for remark. The additional explanation was added at the end of corresponding subsection.

“….Even though Raman spectroscopy makes it possible to detect non-polar molecules (as mentioned above), the challenge of achieving a high signal-to-noise ratio (SNR) in certain environments remains critical. One of the most available and long-established methods of SNR improvement is the use of matched filters, but their use requires knowledge of the shape of the substance peak in the spectrum to match the filter's impulse response. But there are other techniques for mitigating Raman spectrometry. Fan et al. [81] used automatic denoising method of convolutional denoising autoencoder for advancement of the SNR in Raman spectra without manual intervention (Raman spectrometer with 785 nm laser and Ne/Ar source). For example, the increase of SNR from 2.2 to 4.4 for classical spectrometer, and from 3.2 to 7.2 for optimized spectrometer was achieved. There are other approaches, such as spatially compressed illumination reported in [82]; deep learning-based approach in [83]; feature extraction method [84], etc. However, the majority of these methods were not tested for Raman spectra of gases, even in the field of soil science…..”

7-    It would be helpful to discuss the technological challenges in implementing cavity ring-down spectroscopy in soil gas analysis, such as the high-quality mirror requirement.

Comment: Thank you for remark. Some comments were added to this requirement.

“For different types of gases, laser parameter adjustments are necessary, which can be complicated by its technical characteristics. The requirement to use the high-reflectivity mirrors (in order to achieve the longer path lengths) is a feature of CRD spectroscopy and cavity enhanced absorption spectroscopy, compared to chemical ionization mass spectrometry (CIMS) [109]. For example, the mirrors with reflectivity of R ≥99.9985% with an effective path length of 20 km were used in [110].”

8-    How does TDLAS compare with CRD in terms of sensitivity and practicality for soil gas monitoring?

Comment: The comment was added.

Comparing TDLAS and CRDS technologies, it is important to note that CRDS allows for an increase in the optical path of absorption to thousands of meters using an optical mirror, which ensures extremely high detection accuracy. However, this method imposes quite high requirements on the experimental setup and complicates the conduct of online measurements on-site. TDLAS, on the other hand, is characterized by high adaptability to challenging conditions, excellent selectivity, affordability, and its measurement results meet most practical needs. The applied significance of this method has always attracted the attention of researchers. With the advancement of technologies, scientists are increasingly focusing on improving the detection sensitivity of the TDLAS system using various approaches.

9-    The discussion of how these spectroscopic methods can be applied in the context of environmental monitoring and precision agriculture could be expanded.

Comment: The application of spectroscopy methods for these fields is in the paper (Introduction section, subsections 2.2/2.3, 2.4.

10- The glass-based chambers are said to be less common. Could you discuss more about why glass chambers, despite being inert, are not widely used (e.g., fragility, cost)? Are there any innovations in this area?

Comment: Thank you for remark. Unfortunately, there are no innovations in this field. It is hard to avoid the fragility of glass, that limiting the applications field-based analyzers.

11- The description of static and dynamic chambers is informative, but further clarification on when each method is most appropriate would be helpful.

Comment: Thank you for remark. It is hard to extend this issue, since the chambers and their design could deserve the separate paper. However, the reasonable comments were added in the Section no. 5.

12- You mention the use of an oscillating fan and a dish-type structure to mitigate pressure build-up. It would be beneficial to discuss potential alternatives or recent innovations in pressure management during long-term measurements.

Comment: Thank you for suggestion. Unfortunately, there are not so much data reported on the pressure control during long-term measurements. Only few papers we found were added for discussion.

13- The impact of groundwater pressure and precipitation on gas exchange is briefly mentioned.

Comment: It is hard to estimate the impact of such factors on gas exchange, especially, when microorganisms are in the soil. The sentences on the factors, impacting the gas exchanges were added.

14- The third section could be enhanced with more case studies or examples of successful gas emission sampling in different ecosystems (e.g., agricultural fields, forested areas, urban settings).

Comment: The problem of sampling in different ecosystem deserves the separate review article, even in terms of different ecosystems. Unfortunately, this paper is devoted to methods of analysis of gas emission from soils and mainly directed to gas formed by microorganisms (we have highlighted this in the Introduction).  

15- Expanding on how challenges manifest in real-world applications would strengthen the section 4.

Comment: We have tried to reorganize the paper structure and add some explanations.

16- The mention of chemiresistive gas sensors is interesting, but you should expand on their limitations as well. Also its better to refer to room temperature chemiresistive gas sensors (DOI: 10.1016/j.snb.2022.131418)

Comment: Thank you for remark. The paper mentioned was used for discussion. The limitations were added. 

17- The suggestion to use AI approaches for selecting sampling points is an innovative idea. However, it would be useful to explain how this would be implemented practically. Consider discussing the potential limitations of AI in soil gas analysis, such as the need for large datasets for training models and the difficulty of obtaining comprehensive soil gas data across different environments.

Comment: Thank you for suggestion. Some additional comments on the application of AI for soil gas analysis were added, but unfortunately there are no articles on the application of AI for soil gas analysis and we are not IT specialists to formulate the features in AI field. Therefore, only limited comments were added. 

18- Since the presence of biological agents is also mentioned, exploring how microbial activity affects gas emissions and what methods are used to account for these influences could be informative.

Comment: The Introduction section was accompanied with the review of gases emitted as a result of activity of microorganisms. Here, the gas analysis is a method to detect such effects, namely. The analysis of solid phase of soil is time-consuming and it is hard to carry out the field measurements. 

19- What are the specific gases that commonly interfere with each other during soil gas analysis?

Comment: Thank you for remark. Actually, there is no specific research on the gas interference during soil gas analysis. There are some feature of determination of hydrocarbons, for example, sych as in the aliphatic fractions, BTEX (benzene, toluene, ethylbenzene, xylenes), MTBE (methyl-ter-butyl-ether), ETBE (ethyl-ter-butyl-ether), halogenated VOCs (volatile organic compounds, such as tetrachloroethylene, trichloroethylene, and so on), which may interfere. But this paper is not devoted to such a problem.

The additional comment was added.

20- Add further elaboration on how PCA works to reduce signal interference.

Comment: The additional explanation was added to Section 4. Additionally the references on PCA-based approach for SNR enhancement were added.

“Some articles have reported on advanced approaches in the treatment of data from chemiresistive gas sensors using calculation algorithms, which could benefit soil gas analysis. For example, Shooshtari [186] successfully used an electronic nose based on carbon nanotube-TiO₂ nanostructures for the detection of VOCs, along with support vector machine algorithms and principal component analysis (PCA) for data reduction. There are a lot of cases of PCA application for treatment the data of gas sensors [179]. It makes it possible to create the score plot of various characteristics of sensors (response, response time, recovery time, etc.) to various gases and find the distinguishable areas by various types of sensors for searching the most appropriate one [187]. The application of PCA-based approaches is also useful to improve the SNR values, but in other fields [188,189]. It can be suggested that such techniques may find its application in soil gas analysis.”

21- Consider ending the Challenges section with a forward-looking statement about potential solutions to these challenges.

Comment:

Thank you for remark. The solutions were added into subsequent section.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

• It's better to export your file to PDF or save it as a Word document in Simple Markup. It's difficult to review the paper in All Markup version.

• What was the reason for adding an author with another affiliation to your manuscript in the new version? Your paper is a review paper, and you may not have needed to conduct any experiments or simulations for adding a new author!

• The quality of Figure 3 is very low.

• The mention of modern automated flux chambers and their design solutions is valuable. However, more details could be provided on the specific technologies that minimize gas losses and the impact of external factors like temperature and infrared radiation. A comparison of the performance of different models (e.g., Li-Cor vs. Picarro) could add depth.

Author Response

• It's better to export your file to PDF or save it as a Word document in Simple Markup. It's difficult to review the paper in All Markup version.

Comment: Thank you for remark. We used the recommendation of the editorial office, to use the track changes regime in Word. Now, we have accepted all the changes and carry out the revision, showing changes with color.

• What was the reason for adding an author with another affiliation to your manuscript in the new version? Your paper is a review paper, and you may not have needed to conduct any experiments or simulations for adding a new author!

Comment: Thank you for remark. The journal gave us a short time to make a revision. Since, the authors were limited with their ability to write the revision fast, the additional author was engaged to carry out some critical comments for the paper according to reviewers’ remarks, e.g. the Challenges and Introduction sections, as well as Section no.5 on the chambers.

• The quality of Figure 3 is very low.

Comment: Thank you for remark. The Figure 3 with higher resolution was added.

• The mention of modern automated flux chambers and their design solutions is valuable. However, more details could be provided on the specific technologies that minimize gas losses and the impact of external factors like temperature and infrared radiation. A comparison of the performance of different models (e.g., Li-Cor vs. Picarro) could add depth.

Comment: Thank you for remark. We have added the table with a comparison of various models. Some comments were added to the design of chambers with negotiation effects of solar radiation (however, there are not so much information provided by the producers of analyzers).