Characterization of Secondary Aerosol Formation via HONO and HNO3 Reactions and Source Apportionment in Daejeon and Iksan, Republic of Korea
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors study the formation and removal processes of HONO and HNO3 and their contribution to SOA formation using observations and PMF analysis. Two typical sites, urban Daejeon and suburban Iksan were used. Different characteristics of all main chemicals in SOA formations are investigated and compared for the two sites. The results provide useful information for regional air pollution control. My comments and suggestions are as follows.
L70 and 76, ”Error! Reference source not found..”, please make a check.
Sections 2.2-2.5, it is suggested to introduce the main specifications (accuracy, detection limit, reproducibility, uncertainty, calibration, and etc.) of the key instruments, as they are important in the measurements and this study.
L102, Figure 1 should use high resolutions for the right part.
L124-126,’ A total of 76 VOC species were analyzed at the Daejeon site, and 67 species at the Iksan site (Table S2)” should be rewritten.
L202, The accessed data about https://mcm.leeds.ac.uk/MCM/ should be given.
L210-212,” The top 25 VOC compounds, for which the product of average concentration and MIR exceeded 1, were included in the model (Tables S4 and S5)”, please give explanations the reasons for average concentration and MIR exceeded 1 were included in the model.
L250-252, “OC contributions were higher in Iksan, suggesting a stronger influence from biomass burning”, more supports are needed.
L253-257, more supports are suggested.
L269,” In Daejeon, secondary aerosols were the dominant contributors”, secondary aerosols were the dominant contributors to what? It should be explained in more detail.
L273,” In Iksan, both secondary sulfate and nitrate contributed 17.3% each”, similar as that in L269.
Section 3.3 and all text, RMSE values using the percent (%) is recommended and added.
L291-293, “RMSE values were lower than the corresponding standard deviations, indicating that the model effectively reproduced the observed variability”, it is right for the model effectively reproduced the observed variability, more explanations are suggested. Tables S9, some compositions with large errors, e.g., STD/AVG, more than 50%, what are the reasons?
L324, what is the unknown loss pathway (Lunknown) in Daejeon? Why it is not shown in the text and Figure 4?
Figure 6. Please make explanations for the lines.
Talking about SOA chemistry, it is suggested to discuss the anthropogenic and biogenic VOCs (AVOCs, BVOCs) roles in the urban (Daejeon) and suburban (Iksan) regions. Can the authors make specific simulations of AVOCs and BVOCs to SOA? More studies and discussions are suggested.
Author Response
We sincerely thank the reviewer for the thorough and constructive comments, which helped us improve the quality and clarity of our manuscript. Below we provide point-by-point responses to each comment.
Comment (L70 and L76): “Error! Reference source not found..” appears.
Response: We checked and corrected this error in the revised manuscript.
Comment (Sections 2.2–2.5): Please introduce the main specifications (accuracy, detection limit, reproducibility, uncertainty, calibration, etc.) of the key instruments.
Response: Calibration results and performance evaluation of the instruments are already described in Fig. S2 of the Supplementary Materials. We believe this provides sufficient information to demonstrate the accuracy and reliability of the key instruments used.
Comment (L102): Figure 1 (right part) should use high resolution.
Response: While a lower-resolution version was embedded in the draft, a high-resolution figure has been submitted to the editorial office.
Comment (L124–126): “A total of 76 VOC species were analyzed at the Daejeon site, and 67 species at the Iksan site (Table S2)” should be rewritten.
Response: We revised this sentence for improved readability in the revised manuscript.
Comment (L202): The accessed data about https://mcm.leeds.ac.uk/MCM/ should be given.
Response: It is not entirely clear which specific information about data access the reviewer refers to. If more detail is provided, we will make every effort to incorporate it.
Comment (L210–212): Please explain why the top 25 VOCs (average concentration × MIR > 1) were included in the model.
Response: We used the product of average concentration and MIR as an indicator to select VOCs that significantly affect photochemical reactivity. Both factors are important: species with high MIR but extremely low atmospheric concentrations, or species with high concentrations but very low MIR, are unlikely to play a meaningful role. Therefore, we included only those compounds where the product exceeded 1 under the studied environmental conditions.
Comment (L250–252): More support is needed for the statement that higher OC contributions in Iksan indicate stronger biomass burning influence.
Response: We agree that the statement was not well supported. Therefore, the sentence has been deleted.
Comment (L253–257): More supports are suggested.
Response: We found the statement to be ambiguous and inaccurate, and thus it has been removed from the revised manuscript.
Comment (L269): Please specify “secondary aerosols were the dominant contributors”—contributors to what?
Response: We revised the sentence to explicitly state that secondary aerosols were the dominant contributors to PM₂.₅, and we placed the supporting reference at the end of the sentence.
Comment (L273): “In Iksan, both secondary sulfate and nitrate contributed 17.3% each”—similar clarification needed as in L269.
Response: We revised the sentence and added the appropriate references to clarify the statement.
Comment (Section 3.3 and text): RMSE values should be expressed in percent (%).
Response: We respectfully believe that RMSE should retain the unit of the evaluated variable, which is the conventional practice for this type of model assessment.
Comment (L291–293, Table S9): Please provide more detailed explanation for the RMSE evaluation and why some species have large errors (STD/AVG > 50%).
Response: The F0AM box model we used estimates concentration changes from basic homogeneous gas-phase reactions. Model accuracy improves with the inclusion of a larger number of VOC species. While we minimized uncertainties as much as possible, for some compounds, the lack of data for relevant reactants resulted in relatively larger uncertainties. We added this explanation in the revised manuscript.
Comment (L324): What is the unknown loss pathway (Lunknown) in Daejeon? Why is it not shown in the text and Figure 4?
Response: The amount of unknown loss in Daejeon was calculated to be negligible (zero) during the sampling period based on the applied formula. Therefore, it was not shown in the text or Figure 4.
Comment (Figure 6): Please explain the lines.
Response: We added explanations to the figure caption. The error bars represent standard deviations.
Comment (SOA chemistry): Please discuss the roles of AVOCs and BVOCs in SOA formation in urban (Daejeon) and suburban (Iksan) regions. Can the authors simulate their contributions?
Response: This study primarily focused on inorganic aerosols, particularly HONO and HNO₃. Although SOA chemistry was outside the scope of the present analysis, we fully agree with the reviewer that SOA plays an important role in secondary aerosol formation. We are currently conducting a separate study on SOA, which is under review. In future work, we plan to integrate the findings from both studies for a more comprehensive analysis.
We again thank the reviewer for these insightful and constructive comments. We believe that the revisions and clarifications have improved the overall clarity and rigor of our manuscript.
Reviewer 2 Report
Comments and Suggestions for AuthorsThis manuscript provides a comprehensive investigation of air pollutant compositions, including major ions, carbonaceous species, trace metals, and gaseous pollutants (HONO, HNO₃, and VOCs) in Daejeon and Iksan, South Korea. The study offers valuable insights into secondary aerosol formation and source apportionment. While the research presents an extensive dataset, several aspects require improvement to meet the journal's publication standards.
- The current abstract lacks clear logical progression. I recommend restructuring it to: First present the concentration characteristics and spatial differences of pollutants at both sites; Then discuss source apportionment results; Finally analyze secondary formation pathways. The abstract should be more concise and focused on these key elements. The authors can also reorganize in different logical flow.
- In the section of 3.1 Temporal and spatial characteristics of pollutant concentrations. This section should more clearly delineate: The temporal variations in PM₂.₅ and related species concentrations; Distinct spatial patterns between the two sites. Particular emphasis should be placed on differences in key atmospheric species relevant to source apportionment and secondary aerosol formation. A comparative analysis of these characteristics would strengthen this section.
- PMF Source Apportionment Methodology: The current analysis presents a methodological inconsistency: Daejeon data only includes summer measurements while Iksan data incorporates all seasons. To enable valid comparisons, I recommend: (a) First comparing summer data between both sites; (b) Then analyzing seasonal variations in Iksan separately. Additionally, please address any potential biases in comparing single-season data (Daejeon) with annual averages (Iksan).
- “Section 3.7 review of previous studies on the external secondary aerosols” would be more appropriately placed in the Introduction. This reorganization would: Better establish the scientific context; More clearly identify knowledge gaps; More effectively highlight the significance of the current research.
Author Response
We sincerely thank the reviewer for the constructive and insightful comments. We carefully considered all points, and our responses are provided below.
Comment (Abstract): The abstract lacks clear logical progression. It should first present the concentration characteristics and spatial differences, then discuss source apportionment, and finally analyze secondary formation pathways.
Response: We appreciate this suggestion. While the main elements requested by the reviewer (concentration characteristics, source apportionment, and secondary formation pathways) are already included in the abstract, we slightly revised the text to improve clarity and logical flow without altering its overall structure. We believe the revised version better emphasizes these three key aspects while maintaining conciseness.
Comment (Section 3.1): This section should more clearly delineate the temporal variations and spatial patterns of pollutants.
Response: We agree that clearer delineation improves readability. Section 3.1 already discusses seasonal and diurnal variations (temporal variability) and inter-site differences (spatial variability). To address the reviewer’s suggestion, we revised the introductory sentences of Section 3.1 to explicitly separate temporal variability from spatial contrasts, while keeping the original content intact. This revision highlights the differences between temporal and spatial characteristics more clearly.
Comment (PMF Source Apportionment Methodology): There is a methodological inconsistency—Daejeon includes only summer data, while Iksan incorporates all seasons. To enable valid comparisons, first compare summer data, then analyze seasonal variations in Iksan separately.
Response: We appreciate this important point. As described in Section 2.4, the dataset limitation for Daejeon restricted our PMF analysis to the summer campaign only, whereas Iksan included three seasons. Accordingly, inter-site comparisons were primarily based on the summer data, and the additional seasonal results for Iksan were presented to highlight intra-site variability rather than for direct comparison with Daejeon.
Comment (Section 3.7): The review of previous studies on external secondary aerosols would be more appropriately placed in the Introduction.
Response: We appreciate this comment. Since Section 3.7 was originally designed to complement our findings in the Discussion, we retained it in its current position. However, we added linking sentences to the Introduction to provide earlier context on external aerosol influences. We believe this approach preserves the intended flow of the manuscript while addressing the reviewer’s concern about contextual framing.
We are grateful to the reviewer for these valuable suggestions. The revisions and clarifications have improved the manuscript’s organization and readability, while keeping the original data analysis framework consistent
Reviewer 3 Report
Comments and Suggestions for AuthorsThe manuscript "Characterization of Secondary Aerosol Formation via HONO and HNO₃ Reactions and Source Apportionment in Daejeon and Iksan, South Korea" by Kyoung-Chan Kim and Jin-Seok Han assesses the atmospheric formation and removal of HONO and HNO₃ and their contribution to PM2.5 formation via field observations and atmospheric modelling.
The article is well written and the information follows a logical flow through the manuscript. A detailed methodology is provided for the field campaigns performed in both sites, data evaluation and scrutiny, and for the atmospheric modeling. Discussions and conclusions are backed by sufficient field data. Additionally, the SM adds value to the overall study, the supplementary information included there highlights a good data quality management. The use of different atmospheric tools, even though they are common, such as ISOROPIA II, PMF analysis, MCM, 0-dimention box modeling and other relevant tools, helps to an efficient exploitation the acquired field data for the investigated pollutants.
I am in favor that this manuscript to be published in Atmosphere Journal, however, some minor observations should be addressed before publication.
Reviewer`s opinion: minor observation.
Comments for author File: 
Comments.pdf
Author Response
We sincerely thank the reviewer for the positive and encouraging feedback on our manuscript. We are pleased that the reviewer found the methodology, data quality, and discussion to be logical and well supported. We also appreciate the recognition of the value added by the supplementary materials and the use of multiple atmospheric tools.
As the reviewer noted only minor observations, we carefully checked the manuscript and made small editorial adjustments to improve clarity and consistency. These revisions did not alter the scientific content but enhanced readability.
We greatly appreciate the reviewer’s supportive comments and recommendation for publication.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsI recommend that this paper be accepted for publication in Atmosphere
