Evolutionary Characteristics of Sulphate Ions in Condensable Particulate Matter Following Ultra-Low Emissions from Coal-Fired Power Plants During Low Winter Temperatures
Round 1
Reviewer 1 Report (Previous Reviewer 1)
Comments and Suggestions for AuthorsBased on the results of the second review, it can be seen that the authors have supplemented and made changes to the article. However, despite the edits, there are a few comments:
In Figure 1 it is necessary to increase the font of captions, it is very difficult to read;
It is necessary to indicate the measurement errors for the values given in Table 1. As some values may be within the margin of error and therefore not representative.
Were additional tests performed beyond scanning microscopy? For example, using FT-IR spectrometer, X-ray diffraction analysis, or other equipment. Thus, the results presented do not allow for a comprehensive study.
In Figure 3, arrange all the graphs in order. There are displacements and reading is difficult.
The paper should emphasize the practical thesis in the conclusions, as well as detail and present in a different form the conclusions about the influence of CO condensation temperatures on flue gases of URS.
Author Response
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Reviewer 2 Report (New Reviewer)
Comments and Suggestions for AuthorsThis paper presents experimental investigations into emissions generated by coal-fired power plants. Utilizing a custom-designed laboratory setup developed as a key methodological contribution of the research, the authors conducted systematic tests. While the study demonstrates notable advancements in emission analysis methodology, the findings reveal unresolved issues that warrant further examination.
Comment 1: Insufficient Experimental Repetition and Statistical Rigor
While the paper mentions that each test point was repeated three times (Section 3.1.1), it lacks quantitative measures of reproducibility, such as standard deviations, confidence intervals, or error bars in figures. Furthermore, the analysis relies on visual trend comparisons without statistical validation , weakening the robustness of the conclusions.
Suggested Revision:
Comment 2: Limited Discussion of Real-World Environmental Relevance
The study simulates low-temperature condensation (-20–10°C) but does not address how laboratory conditions (e.g., controlled gas flow rates, absence of wind, or simplified gas mixtures) differ from real-world atmospheric dynamics. Factors like turbulence, competing pollutants, or seasonal humidity variations could alter sulfate formation pathways, limiting the extrapolation of results to actual haze events.
Comment 3: Incomplete Characterization of Aerosol Toxicity
Although the Introduction briefly mentions aerosols’ harm to flora and fauna , the study does not quantify these impacts or correlate sulfate concentrations with biological endpoints (e.g., plant growth inhibition, cytotoxicity assays). This omission reduces the practical significance of the findings for environmental health risk assessments.
Comment 4: Lack of Clarity in Terminology and Abbreviations
Key terms such as “Stefan flow” (Section 3.2) and “non-homogeneous reaction” are introduced without sufficient explanation, potentially confusing readers outside the field. Additionally, abbreviations like CPM and EDX are defined late or inconsistently (e.g., EDX is first mentioned in Section 2.3 without expansion).
Comment 5: Underutilized Data Visualization
Figures 2–5 present raw concentration trends but lack auxiliary visualizations (e.g., phase diagrams, reaction pathway schematics, or size distribution plots) to contextualize sulfate formation mechanisms. For instance, Figure 5 (SEM/EDS) could benefit from annotated micrographs highlighting particle morphology or elemental mapping overlays.
Comment 6: Ambiguity in "Ultra-Low Emission" Standards
The paper frequently references "ultra-low emission" standards (e.g., Sections 1 and 2.2) but does not explicitly define these standards or compare them to international regulations (e.g., EU or U.S. EPA guidelines). This ambiguity obscures the practical implications of the findings for policy alignment.
Comment 7: Inadequate Exploration of Alternative Sulfate Formation Pathways
The focus on SO2/SO3 conversion and NH3/NOx interactions overlooks other potential sulfate sources, such as sea salt aerosols, mineral dust, or organic sulfur compounds. This narrow scope may underestimate the complexity of sulfate dynamics in real-world haze events.
Author Response
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Reviewer 3 Report (New Reviewer)
Comments and Suggestions for AuthorsWhy study -20 degrees? Most flue gases do not cool to this temperature.
Figure 3 seems to have been copied twice, please make the necessary modifications.
Suggest the author to provide a more detailed explanation of the mechanism by which Stefan flow promotes the increase of SO42- as the water content ratio increases.
There is no clear correlation between the content and title of section 3.3.1. Please carefully verify. Additionally, from the results in Figure 4 (a), it can be seen that the concentration of SO42- is higher when only NOx is added. In the textual description, the author describes that NOx and NH3 can synergistically promote this conversion process.
The legend in Figure 4 (b) should be SO3 instead of SO2
The article needs to be carefully checked and proofread, as there are numerous minor errors.For example, conclusion(2), there is a error in this sentence"promote non-homogeneous reactions which in turn can also promote SO42- production"
Author Response
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Round 2
Reviewer 3 Report (New Reviewer)
Comments and Suggestions for AuthorsThe author has answered all the questions I am concerned about and recommends publication.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this paper, experimental studies of emissions from coal-fired power plants are carried out. Tests were performed based on the developed laboratory setup, which indicates a significant contribution from the authors, but there are several questions.:
1. It is not clear from the work whether the tests were performed in the work area, without the use of gases, in order to determine the initial characteristics? It is suggested to show these results in more detail, if any.
2. How was the experiment planned? What mathematical method was used? For example, is a statistical method possible? If so, it is necessary to present this and show the results of correlation and regression analysis.
3. When performing the tests, the results of which are shown in Figures 2 and 3, additional clarifications are needed. How many times were the tests performed for each point? What is the confidence error and convergence of measurements?
4. A more precise conclusion should be given in the conclusion. Namely, the aerosols formed at low temperatures are harmful to flora and fauna. If so, how negative is it?
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Reviewer 2 Report
Comments and Suggestions for AuthorsComments for sustainability-3409415
This study investigates the formation characteristics of sulfate ions (SO₄²⁻) in condensed particulate matter (CPM) after ultra-low emission treatment in coal-fired power plants under low-temperature conditions. The research experimentally simulates the effects of different condensation temperatures, water vapor, NOx, and NH₃ on SO₄²⁻ formation, revealing its formation mechanisms under rapid condensation at low temperatures. While the findings under low-temperature conditions are valuable for understanding atmospheric particulate pollution during winter haze events, the core innovation of the study is not particularly significant. In particular, the study's conclusions are similar to previously published work, yet the results exhibit substantial discrepancies. Major revisions are recommended, with specific comments as follows:
- The introduction states that " However, most of the current studies are limited to ambient temperatures, with little mention of studies on the effect of NOx on the conversion of SO₃ to SO₄²⁻ at low temperatures." It is suggested to supplement the discussion with previous research under low-temperature conditions to highlight the innovation of this study.
- Compared to authors’ previous studies, why was the initial SO₂ concentration in the experimental conditions changed from 110 mg/Nm³ to 100 mg/Nm³? Please clarify the differences between this study and prior work in the experimental conditions and justify the selection of parameters.
- Compared to authors’ previous studies, why do the analytical results of SO₄²⁻ exhibit large discrepancies (include error bar) even under identical initial conditions (5°C) and other similar parameters? Please provide an uncertainty analysis to enhance the credibility of the data.
- In the analysis of the effect of H₂O molar fraction, the experimental conditions include 9.14%, whereas Figure 3 describes 8.14%. Please carefully verify the data and conduct a thorough data consistency check. Additionally, it is recommended to include a quantitative analysis of the relationship between H₂O molar fraction and the SO₄²⁻ formation rate.
- The conclusion section should be rewritten, particularly to emphasize the study's novelty. The effect of temperature on SO₄²⁻ formation under low-temperature conditions appears to be significantly different from previous findings at higher temperature.
Clear description.
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Reviewer 3 Report
Comments and Suggestions for AuthorsThe manuscript “Evolutionary Characteristics of Sulphate Ions in Condensable Particulate Matter Following Ultra-Low Emissions from Coal-Fired Power Plants During Low Winter Temperatures” discusses how sulfate ions (SO42-) in condensable particulate matter (CPM) contribute to hazy weather during low winter temperatures. The formation of sulfate primarily results from transformations of gaseous precursors like SO2, NOx, and NH3 through homogeneous and non-homogeneous reactions.
The authors investigated the influence of condensation temperature, water vapor, NOx, and NH3 affect SO42- generation under rapid low-temperature conditions. They used a flue gas generation system with gas cylinders and SO3 generators to conduct their experiments. Gaseous pollutants were introduced into the reaction device in controlled amounts using mass flow meters, while a humidity generator produced H2O vapor from ultrapure water, with N2 flow adjusted to manage humidity levels. The reaction unit comprised a premixing furnace and a liquid evaporator. All gaseous pollutants were collected in the flue gas generation system, where they reacted, were sampled through a condensation unit, and underwent exhaust gas treatment. The authors also analyzed various parameters including influence of condensation temperature, H2O, NOx, and NH3 also the characteristics and the morphology of various gases including SO2, SO3 and the combination of SO2 and SO3.
The authors found that the concentration of sulfate ions (SO42-) in flue gas changes with the condensation temperature, ranging from 16.41 mg/m3 to 26.79 mg/m3, and then down to 19.83 mg/m3. This variation is influenced by gas-phase reactions during the condensation and freezing of water vapor, along with H2SO4 and HNO3. Notably, the combined effect of SO2 and SO3 can elevate the SO42- concentration to 36.18 mg/m3, which exceeds the concentrations from each gas alone. Additionally, increased water vapor enhances these reactions, leading to more SO42- production. Ammonia (NH3) helps in forming atmospheric H2SO4 aerosols and sulfate particles, sometimes larger than 5 µm. In contrast, nitrogen oxides (NOx) may inhibit SO42- production. At temperatures below freezing, NH3 and NOx can quickly react with SO2 and SO3 to create small sulfate, nitrate, and ammonium particles, which are often less than 2 µm in size. These fine particles are hard to capture and contribute to atmospheric aerosols, making coal burning in winter a significant factor in hazy weather conditions. I recommend publishing this manuscript in the Sustainability Journal.
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