General Assessment

This is a high-quality manuscript that is well suited for publication in Earth System Science Data. It presents an up-to-date monthly CO₂ emission dataset for Japan and provides a comprehensive description of the underlying data sources, accounting methodology, and the differences among alternative estimation approaches. The authors have made commendable efforts to explain the possible origins of discrepancies between different methods, and the entire workflow—from data collection to emission estimates—is presented in a transparent and reproducible manner. I particularly appreciate the authors' careful treatment of the underlying data and the thorough documentation of the inventory development process.

Overall, I recommend publication in Earth System Science Data after only minor revisions.

Comments

1. Discussion and presentation of uncertainties

The authors discuss the sources of uncertainty and report that “The uncertainty for total emissions in each month is estimated to be ±3% at 2 standard deviations (95% confidence interval), given the uncertainties in the NIR are also given at 2 standard deviations.” From my perspective, this uncertainty appears remarkably small, particularly at the monthly timescale.

I encourage the authors to provide a more detailed discussion to justify why such a low uncertainty is reasonable. In addition, it would be valuable to assess the robustness of the uncertainty estimates using approaches other than the conventional root-sum-of-squares method, if possible, so that the results can be cross-validated.

Furthermore, have the authors considered adding uncertainty ranges to some of the emission time series? Displaying the uncertainty ranges directly on the figures would provide readers with a more intuitive understanding of the confidence range and make it easier to judge whether differences among various estimates are statistically meaningful. In my opinion, strengthening the uncertainty analysis and presentation would represent the most important improvement to this manuscript.

2. Comparison with other inventories

In Figure 12, I suggest that the authors first present the absolute emissions from all inventories before comparing their relative differences. Showing only the relative differences makes it difficult for readers to appreciate the baseline magnitude of each inventory.

In addition, the recently released MEIC-global-CO₂ inventory also provides monthly CO₂ emissions with approximately a two-year reporting lag. The authors may consider including this inventory in the comparison.

3. Manuscript organization

Although the detailed methodological descriptions and comprehensive comparisons are valuable, the manuscript is currently rather lengthy. As a reader, it is sometimes difficult to quickly locate the most relevant information.

I therefore suggest moving part of the detailed methodological descriptions and some of the extended discussions to the Supplementary Information or Appendix, allowing the main text to place greater emphasis on the key findings and conclusions. This would substantially improve the readability of the paper.

4. Treatment of international aviation

I would appreciate further clarification on how fuel consumption associated with international aviation is treated in the inventory. Since this can influence the definition of national emissions, a brief explanation of the accounting approach would be helpful.

This manuscript develops a low-lag monthly CO₂ emission dataset for Japan using primarily official monthly activity data. The data sources, estimation methods, data processing, and uncertainty assessment are described in detail. The resulting dataset is valuable for timely emission monitoring, policymaking, public awareness, and independent verification, and generally meets the quality requirements of Earth System Science Data. However, several minor issues should be addressed to improve the clarity and readability of the manuscript. Therefore, I offer the following comments and suggestions.

1. The manuscript is somewhat lengthy and could be streamlined to improve readability. In particular, the detailed descriptions of other datasets in Sect. 3.6 partly overlap with the Introduction, and some of these methodological details could be shortened or moved to the Appendix. The discussion of potential applications could also be condensed, allowing the main text to focus more clearly on the construction, evaluation, and key findings of the dataset.
2. Table 1 lists refinery feedstocks among the secondary fuels with available trade, stock, and consumption data, whereas Sect. 3.5.1 states that refinery feedstocks are excluded from this study due to the lack of reliable monthly data. Please clarify this apparent inconsistency and revise Table 1 or the corresponding text accordingly.
3. Before presenting the relative differences in Figure 12, I suggest first showing the absolute monthly emissions from the different datasets. This would provide readers with a clearer view of their overall emission levels, trends, and seasonal patterns before interpreting the relative discrepancies.
4. Although some differences in emission coverage are discussed in Sect. 3.6, I suggest adding a brief table, perhaps in the Appendix, summarizing the emission scope of all datasets compared. This would help clarify whether the total emissions compared across datasets are directly comparable.
5. Please carefully check the manuscript for formatting, cross-reference, and reference-list errors. For example, “see 50Appendix F” appears in the Introduction, “Figure 8Figure 8” appears in Sect. 3.1, and several incomplete references such as “Figure )” remain. The reference list also contains several formatting inconsistencies, including duplicated punctuation, a duplicated DOI prefix for Min and Choi (2024), and incomplete page ranges for some journal articles. Please carefully proofread and standardize the manuscript and reference list according to the journal format.
6. The presentation of several figures should be improved for clarity and consistency. In particular, the labels and text in some figures are too small to read clearly (e.g., Figures 10 and 12), and Figure C1 uses different line colors without a corresponding legend. Please check the figures throughout the manuscript and improve their readability and presentation where necessary.
7. For Table 2, please briefly clarify how “Trend” was calculated and how its statistical significance was assessed.
8. Footnote 3 in Sect. 2.4.3 appears to contain an arithmetic inconsistency: 0.428/(1−0.428) is approximately 0.748 rather than 0.728. Please verify the conversion factor and clarify whether this is only a typographical error or whether it affects the lime-emission estimates and the released dataset.