1. Writing and Readability

The manuscript is generally well-structured encompassing the conventional format of scientific articles. The language is clear and concise, facilitating comprehension. However, there are occasional grammatical errors and awkward phrasings that could benefit from careful proofreading. For instance, the sentence:

"Further fossil fuel combustion contributed more significantly at CB than at BN." could be rephrased for clarity as:

"Furthermore, fossil fuel combustion contributed more significantly at CB than at BN."

Overall, the manuscript is accessible to readers with a background in atmospheric sciences or environmental chemistry.

1. Scientific Content

2.1. Experimental Design

The study investigates the chemical and stable carbon isotopic compositions of PM₂.₅ collected from two forest sites in China—Changbai Mountain (CB) in the north and Xishuangbanna (BN) in the south—during summer and winter periods. The sampling strategy includes day and night collections, providing temporal resolution. The analysis encompasses carbonaceous and nitrogenous components, water-soluble inorganic ions (WSIIs), and δ¹³C of total carbon (δ¹³CTC).

While the study design is comprehensive, there are concerns regarding certain methodological choices:

Filter Material: The use of quartz filters for collecting PM₂.₅ samples intended for WSII analysis is questionable. Quartz filters are known to have high blank values for certain ions, which can interfere with accurate quantification. Although the authors mention using blanks to correct for background levels, the inherent high background of quartz filters, especially for cations like Na⁺, Ca²⁺, and Mg²⁺, can compromise the reliability of the measurements. Alternative filter materials, such as Teflon or PCT, are more suitable for WSII analysis due to their lower blank values.

Trajectory Analysis: The study employs backward trajectory analysis at a fixed altitude of 500 meters to infer the potential sources of air masses. However, this approach may not accurately represent the transport pathways of surface-level aerosols, especially considering the diurnal variation of the planetary boundary layer (PBL). During nighttime, the PBL can be shallow, and air masses at 500 meters may reside in the residual layer, not interacting with surface emissions. Therefore, trajectory analyses should consider the dynamic nature of the PBL and possibly include multiple altitudes to capture a more representative range of transport pathways.

2.2. Data Presentation and Interpretation

The results indicate seasonal and diurnal variations in PM₂.₅ composition, with higher concentrations of carbonaceous and nitrogenous components in winter. The dominance of SO₄²⁻, NO₃⁻, and NH₄⁺ at CB, and SO₄²⁻, NH₄⁺, and Na⁺ at BN, is reported. The δ¹³CTC values suggest contributions from biomass burning and fossil fuel combustion.

While the data presentation is generally clear, there are areas where further clarification is needed:

Ion Balance: The study does not discuss the ion balance between measured cations and anions. An imbalance could indicate missing species or analytical errors. Given the use of quartz filters, which can introduce artifacts, a discussion on ion balance would strengthen the reliability of the WSII data.

Source Apportionment: The authors rely on δ¹³CTC values and the relative abundance of chemical species to infer sources. However, more robust source apportionment techniques, such as Positive Matrix Factorization (PMF) or Chemical Mass Balance (CMB) modeling, could provide quantitative estimates of source contributions and reduce uncertainty.

2.3. Novelty and Contribution

The study contributes to the limited data on PM₂.₅ composition in forested regions of China, particularly regarding δ¹³CTC measurements. However, the findings largely corroborate existing knowledge about the sources and seasonal variations of PM₂.₅. The use of δ¹³CTC as a tracer is valuable, but its application here does not yield novel insights into source apportionment beyond what is already known.

1. Specific Comments

Line 117: The authors state that the contribution of CaCO₃ to aerosols is negligible, yet later identify soil as a significant source of PM₂.₅. This appears contradictory, as soil dust typically contains substantial amounts of calcium carbonate. Clarification is needed to reconcile these statements.

Table 1: The table1 appears to have two different legends, which may cause confusion. Ensuring consistency in table legends is essential for clarity.

1. Conclusion

The manuscript provides valuable data on PM₂.₅ composition in two forested regions of China, with a focus on seasonal and diurnal variations. However, methodological concerns, particularly regarding filter selection for WSII analysis and the trajectory analysis approach, need to be addressed. The study's findings align with existing literature, and while the inclusion of δ¹³CTC measurements is commendable, it does not substantially advance the understanding of PM₂.₅ sources.

Recommendation: Major revisions are necessary to address the methodological issues and enhance the robustness of the source apportionment analysis. Incorporating more suitable filter materials, refining trajectory analyses, and employing quantitative source apportionment models would significantly strengthen the study.

This article presents a measurement report of the concentration of main inorganic ions, of EC/ OC, and carbon isotope in fine particulate matter (PM2.5) collected at two sites in China. The introduction section is clear and I sincerely appreciate the references up to date. Nevertheless, references are lacking in the other sections to assess the methodology and the discussion of the results. The dataset is interesting and deserves publication. Nevertheless, I have some major concerns regarding the methodology, the results and the discussion.

• The two sites presented should be better described. Many research stations are available in China. Why is it interesting to compare these two in particular?
• Lines 74-78: please add a reference for the methodology used for EC/OC measurement
• Lines 92-95: the description of the analytical methodology is not sufficient.
• Lines 98-99: repetition of the description of the filtration step
• Lines 101-102: the analytical methodology for the detection of NO2- is not clear: NO2- reacts with the acid to form “new nitrogen compounds”, which are detected at 540 nm… which compounds?
• Line 103: “aggregating” means “sum”?
• Line 117: The authors assume that carbonates are negligible since Ca2+ concentrations are low. But carbonates can be also as Na+ or Mg2+ salts. A measurement of inorganic carbon would be more appropriate.
• Figure 2: why one point par day instead of a continuous temporal variation?
• Paragraph 3 (results and discussion): The two sites are at low altitude, respectively CB 740 m asl and BN 872 m asl. I assume that they are in the boundary layer in summer, but CB could be in the free troposphere in winter. A discussion should be added on the environmental conditions encountered at these sites.
• Page 7, Table 1: Maybe illustrate the table with Figure(s) and report the table in the supplementary material file.
• Lines 149-150: Could you please give a reference of the equations used? Equivalent concentration is obtained by dividing the molar concentration by the charge and not by the ion mass. I have never seen this formula and I assume that the following discussion needs to be revised.
• Lines 163: It would be interesting to treat separately anions and cations.
• Line 166: the authors compare with urban sites… What about the comparison with rural sites?
• Line 172-179: The authors explain in deep that DMS emitted from phytoplankton can explain the high SO42- concentration. If there such a huge marine influence, why Cl- is so low? Please, reconsider the explanation.
• Lines 172-179: There is a long discussion on SO42-, but nothing on NO3-, which is 73 times higher in winter than in summer at CB. This need an explanation.
• Lines 184-185 and following: I wonder if the two observatories are equipped with NOx, SO2 and O3 analysers. In that case, the discussion would be better supported by experimental measurements of these compounds during aerosol sampling.
• Figure 4: A concentration of NO3- equal to zero during all the summer for both sites is surprising. I wonder if something went wrong with the analysis. Have you compared this result to those obtained for sites with similar environmental conditions? Pathak et al. (10.5194/acp-9-1711-2009) found, for example, that nitrate is underestimates when the particles collected are deliquescent.
• Line 251: A correlation with R² = 0.51 is not so strong
• Line 258: please explain how you calculated nss-SO42- and nssK+ or report a reference
• Lines 266-267: measurements of SO2, NOx and NH3 would greatly strengthen the discussion.
• Line 271: I have some concern about the concentration of NO3- around zero during summer at both sites, thus I’m not confident in the results and discussion about nitrate/sulphate ratios.
• Line 284: The air masses arriving at CB and BN during summer show a strong marine influence. How do you explain such a low concentration of Cl- (not discussed anywhere else in the article)?
• Lines 308-311: references are missing about the studies cited.
• General remark: The title promises chemical and stable carbon isotopic compositions, but it is mainly focused on physicochemical characterisation, which is not such a novelty. Only short paragraphs are devoted to stable isotopes, which could really improve the novelty of the work.