Optical Analysis Based on UV Absorption Spectrum for Monitoring Total Organic Carbon and Nitrate Nitrogen in River Water

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

• The study uses: pure DI + TOC standards (ii) pure DI + NO₃–N standards (iii) mixed solutions with turbidity (e.g., 20–20–20 mg/L).No natural water samples (rivers, lakes, wastewater) with real organic/inorganic matrix complexity were analysed. This is a major limitation, because UV-absorbance for TOC and NO₃ is highly matrix-dependent, and turbidity is not the only interfering factor. Include real river or lake water validation data.
• The manuscript recognizes that the 10 mm optical path causes problems in turbid samples (p. 14–15), but the study does not test alternative pathlengths. Provide comparison with standard laboratory methods (TOC analyser, IC).
• The Beer–Lambert law holds only under: (a) monochromatic light, (b) absence of stray light (c) linear absorbance region, (d) homogeneous sample.
• The HASM-4000 uses: (a) broadband Xenon flash lamp, (b) 1D array detector, (c) diffuse optical path.Yet the manuscript assumes perfect Beer–Lambert behaviour. Discuss limitations of Beer–Lambert in broadband, short-path UV sensors.

• Quantify noise reduction and algorithm performance. Add quantitative noise metrics (SNR, RSD, Allan variance).
• ‘On page 14, Figures 13 and 14 show: Perfect linearity (R² = 0.999) after removing outliers.Justify or remove outlier elimination in R² = 0.999 results.

• Address overlapping absorption peaks using improved chemometrics. Figure 8 (p. 10) and Figure 12 (p. 13) show that noise remains high even after OPC, MAvg, BinInterp. Noise reduction is still insufficient for low-concentration environmental samples (<2 mg/L).
• Add full uncertainty analysis.
• In conclusions: Reduce overstated claims on field applicability. Authors may remove or soften claims of "successful measurement" and "field applicability" until real deployment data are included.
• Author should reduce figure count by ~20%, improve label clarity, and adjust font size.
• Key optical sensor studies missing (e.g., Huber, Christo filogiannis, Pellerin). Many references are Korean local documents; add international sensor literature.

Author Response

Dear Reviewer 1,

We truly appreciate your insightful comments, particularly regarding the theoretical background and field applicability. Your feedback has significantly strengthened the scientific depth of our study.

Please find our detailed point-by-point responses in the attached file. Please note that we have submitted a single integrated response letter covering comments from all reviewers to provide a complete overview of the revisions. We have also conducted professional English proofreading and extensive revisions to figures and tables to ensure clarity.

Sincerely, Prof. ChangKyoo Yoo (On behalf of all authors)

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

General Comments:

 

The paper presents the development of a new device for measuring TOC and NO₃-N in river waters. The title of the paper should clearly indicate that the device is still in its initial development phase and that it has only been tested through laboratory experiments, as no river water samples have been analyzed.

 

The authors state that the aim of the study is "contributing to the technological advancement of Korea's automated water quality monitoring networks." This objective should be better explained and justified. Specifically, the authors should clarify whether the proposed technology has relevance only within the local context or can be applied internationally, and if not, provide an explanation.

 

The manuscript requires substantial improvement to reach the scientific and technical standards necessary for publication. Many sections appear to have been written using AI language tools, resulting in repetition and stylistic features not aligned with the expected style of a scientific paper.

 

The statement "Particularly in river and lake environments, the majority of nitrogen compounds exist in nitrate nitrogen form, making NO₃-N measurement function as an effective proxy indicator for TN" requires clarification. In river sections near significant wastewater discharges, this assumption may not be valid.

 

Minor Changes:

 

Line 102: The term "indigenous" should be reconsidered in the context of technological equipment.

Line 125: Confirm the values "10 (15) mg/L."

Lines 144–150: Improve the presentation of the mathematical formulations and avoid repetition in the text referring to T.

Line 156: All section titles should be numbered according to the journal's formatting rules.

Line 164: All equations should be numbered.

Lines 165–169: Bullet points are unnecessary and should be removed.

Line 178: All section titles should be numbered in accordance with journal rules.

Lines 189–193: Number the mathematical expressions and remove bullet formatting.

Line 208: Remove the bullet point and appropriately number (or delete) the section title.

Figure 1: The figure presents different equipment and miscellaneous items. Improve both the figure layout and caption.

Lines 218–221: Avoid repetition; this information was already provided.

Table 1: The table content serves mainly as a legend for Figure 2. It could be incorporated into the figure or included in the caption.

Figure 3: Review the title and units on the y-axis of both graphs.

Lines 243–267: Revise the text for clarity regarding the numerical ranges ("abruptly at 210–230 nm"). The widening of the spectrum appears more evident only for increases from TOC-10 to TOC-20 and from NO₃-N 5 to NO₃-N 10.

Figures 6 and 7: The curve fitting tables are too small to read. Consider separating the numerical results from the graphs if they are relevant.

Lines 313–320: Consider deleting this paragraph, as it repeats previously presented content.

Figures 9 and 10: Figure 9 shows results for two wavelengths, whereas Figure 10 shows results for three. This discrepancy should be explained and justified.

Line 373: The expression "number divided by 'bin(2.14 nm, 6)' (2500/6)" is unclear and should be presented more clearly.

Lines 410–421: Bullet points should be avoided; equations should be numbered, and this methodological content might be more appropriate in an earlier section. The constant values could be summarised in a table.

Lines 423–424: The authors must explain the criteria used to identify outliers, as this may influence the results. Additionally, they should demonstrate, using sample data, how adjusting the "TOC ratio and NO₃ ratio values in the concentration calculation equations" improves the results.

Comments on the Quality of English Language

-

Author Response

Dear Reviewer 2,

We are grateful for your constructive criticism regarding the manuscript's presentation and writing style. We have carefully addressed all your points to meet the high standards of the journal.

Specifically, we have conducted professional English proofreading and extensively revised all figures and tables as requested. Please find our detailed responses in the attached file. (Note: This is the same integrated response letter uploaded for Reviewer 1, containing the full details of our revisions and the editing certificate.)

Sincerely, Prof. ChangKyoo Yoo (On behalf of all authors)

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Version 2 represents a substantial and genuine improvement over Version 1. The authors have clearly responded to core reviewer concerns by strengthening methodological clarity, tightening interpretation, and clarifying limitations.

The very high R² values are credible for standard solutions, but the authors correctly avoid overclaiming field performance.

While some issues remain, v2 has progressed from a technically promising but weakly contextualized manuscript to a coherent and defensible applied research article suitable for publication. 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have adequately addressed the comments of the first round of revision and incorporated the suggested revisions clearly and consistently. The revised manuscript exhibits an improvement in clarity and scientific rigor, and the remaining issues have been satisfactorily addressed. Therefore, the manuscript can be considered suitable for acceptance.