Review of Rubén Soussé-Villa et al.: “A Comprehensive Global Modelling Assessment of Nitrate Heterogeneous Formation on Desert Dust”

This paper investigates the processes driving nitrate formation on fine and coarse particles on a global scale, using the MONARCH global atmospheric chemistry transport model. It specifically focuses on the key processes involved in nitrate formation over dust and evaluates their representation within the model. The study integrates varying levels of complexity in dust heterogeneous chemistry into the MONARCH model. Three main mechanisms for particulate nitrate formation were implemented: fTEQ, HYB, and DBCLL. The methodologies incorporate various assumptions, including uptake coefficients, reversible partitioning, and the influence of dust and sea-salt alkalinity. The study further indicates that the formation of coarse nitrate through the irreversible uptake of HNO3(g) on coarse particles is highly sensitive to whether it occurs solely on dust or on both dust and sea-salt particles. The analysis emphasizes the implications of nitrate formation on burdens and the role of alkalinity. The findings show differences based on the selected methodology, with a broad range of burdens for the particulate nitrate and the correlations with observations. Overall, the authors highlight the importance of incorporating dust and sea-salt alkalinity into global nitrate simulations along with thermodynamic processes, which were found to be more aligned with observational data.

General comments:

The manuscript provides a clear description of its objectives and is well written. I acknowledge the authors' effort in presenting the numerous sensitivity simulations; however, the lengthy discussion on the differences in these sensitivity simulations used to incorporate nitrate particles into the model may be challenging for the reader (e.g., many abbreviations concerning the various subcases, etc.) to grasp the significance of the results. Some simulations’ analysis could be included in the supplementary material and only briefly discussed in the main text to better emphasize the primary findings of this study. Aside from this minor concern, the discussion of the results is generally very well organized, though some repetition is evident at the beginning of some sections. I don’t have any major comments, but there are a few minor issues regarding the modeling method that can be discussed to support the results and conclusions of this study. Therefore, I recommend a revision to address these issues before the acceptance of the submission.



Minor comments:

Line 335: It is not clear why the 50% fraction of H2SO4 is applied. Can the authors provide some evidence for this fraction?

Section 2.4: Does the model track separately the different SO4-SS, SO4-DU, NO3-SS, NO3-DU, NH4-SS, and NH4-DU species calculated by ISORROPIA? How many (additional) species does the model use for the different sensitivity simulations? How much does the computational cost increase depending on the simulation setup?

Section 2.4: According to Table 1, the model does not consider any heterogeneous chemistry that can promote NO2 to HNO3 conversion on the surface of dust particles. How might this impact the findings of this study?

Lines 438-444 and lines 449-451: They both seem like repetitions of Sect. 2.4. The same is also happening in other parts, especially in the introduction paragraphs. I don’t think this is 100% necessary, but in general, I would suggest that the authors consider ways to simplify the text of the paper. This would reduce the density of the article’s information in the main body of the text.

Lines 392-407: This part might be better moved to Sect. 2.1 or have another section added, as it disrupts the discussion of the different assumptions applied.

Line 667: According to the text, Karydis et al. (2016) did not apply the metastable assumption as in this work. How might this impact the difference in biases?

Technical Comments

Line 461: arosol → aerosol

Line 868: hydrolisis → hydrolysis

Nitrogen is important in several perspectives, and it can affect air quality mainly through oxidized and reduced nitrogen. This study focuses on the nitrate formation processes, and designed a series of numerical experiments to elucidate the main governing mechanisms. The overall study is important and interesting, but I do feel the manuscript needs a substantial revision to be easily followed.

1. Abstract: regarding the first sentence. I am not sure the main focus is nitrate or the desert dust. I feel the authors try to elucidate the issues of nitrate, and dust is one of the factors affecting concentrations of nitrate. Line 5-6: “This study investigates key processes in nitrate formation over dust and evaluates their representation in models.” By reading the manuscript, I think the authors not only examine nitrate formation over dust, but also on sea salt and others. Does the abstract correctly deliver the message?
2. Introduction: the authors have tried to discuss the potential issues related to heterogeneous reactions (e.g., HNO3 to dust). Similar as the previous question: I am not sure the main focus of this study is dust or nitrate. I feel it is nitrate instead of desert dust. At the third last paragraph, the authors pointed out a general inaccuracy of current models in reproducing nitrate and misrepresentation of nitrogen heterogenous chemistry processes on dust and sea salt. Following this, the authors mentioned Following that, the authors try to systematically investigate the underlying processes governing the issues. However, I feel it is not clear what issues are in the current models. Is it due to the DMT or TEQ? It does not look like the authors have discussed the problems clearly. In this way, the readers would not know how the authors can disentangle the problems.
3. Line 154: assuming complete nucleation

              I am not sure what complete nucleation means. Does this mean that all gas H2SO4 is nucleated? This does not seem reasonable.

        4. Line 181 In this study, we investigate the primary chemical pathways responsible for NO₃^-formation on coarse particles by integrating

                Why do the authors only investigate the pathways of nitrate formation on coarse particles? How about the fine mode?

         5. Line 188 hydrophilic

            Why hydrophilic? Some explanation is needed.

         6. Table 1: Not sure why in this table, there is no uptake of HNO3(g) on dust.

               Indeed, Line 214: the authors mentioned that “For the uptake of HNO3(g) on dust (R4-5 in Table 1)”. I don’t understand why R5 is related to dust (it apparently says sea salt). The caption of Table 1 says DU represents Dust, why specifically using CaCO3?

         7. Line 216-218: “However, literature reports varying values for Sc based on dust alkalinity assumptions, ranging from Sc= 1/30 for the industrially-standardized Arizona Test Dust (Möhler et al., 2006; Herich et al., 2009; Suman et al., 2024) to Sc=0.018 for samples from the China Loess (with 39% CaCO3 content) (Krueger et al., 2004; Wei, 2010).”

               What is the setting in this study considering the large range of Sc?

         8. For RH above 90%, γ(SO2) remains constant at γ(SO2) = Sc·5.0·10−4

              What is the value of Sc in this equation

         9. Line 301: The dust NVC global average content result in: 5.17% Ca2+, 0.79% Na+, 2.37% K+, 1.32% Mg2+ for the Journet et al. (2014) dataset, and 3.68% Ca2+, 0.87% Na+, 3.15% K+, 1.75% Mg2+ for Claquin et al. (1999).

            Which one does the author use? Please add some descriptions besides of laying out the two different values.

       10. Table 4: there is no unit

        11. Descriptions in section 3: I feel it is not easy to follow. Too many numbers. Too many abbreviations make it very hard to follow. If possible, please elaborate a bit more to emphasize the main focus. More references can be added to enhance the readability, and readers might know what is important.

        12. The authors mentioned many times about dust and sea salt. Did the authors do any evaluation on these two species? Can the model reasonably reproduce dust and sea salt?

        13. For the reduced and oxidized nitrogen, did the authors compare the results with observations? I think comparison with observations are useful to understand the nitrogen budget.

        14. The conclusions: It seems not clear what is the best option to take for the nitrate reactions. The reversible or irreversible? Please summarize to make it clear whether there is an optimal option, or multiple options to derive reasonable simulations of nitrate.