This manuscript investigated the components of total and soluble iron in PM2.5 in both daytime and nighttime in a coastal city, and tried to evaluate the effects of aqueous-phase and photochemical reactions on that. The topic is interesting and the discussion is comprehensive.

However, the language needs to be extensively improved throughout the manuscript, and the content could be more concise. Moreover, the references need to be double-checked. The original source should be cited as much as possible. For example, the original source of ISORROPIA II should be acknowledged as well.

There are several other issues that need to be considered before the publication in ACP:

1. In Table 1, aerosol pH values are with high standard deviations. Does it make the mean pH values less compatible? Are the data points actually quite overlapped?
2. Figure 3, the data points are for both clean and slightly-polluted periods. They are mixed together, not separately marked like in Figure 5, why?
3. Figure 6, color bar is missing.

The manuscript investigated the daytime and nighttime %FeS in PM_2.5 under clean and SP conditions in a coastal city of China. They found that there was a significant difference between daytime and nighttime FeS% under clean and SP conditions, respectively. Also, they explored the main factors influencing the dissolution of iron, such as aqueous-phase reactions vs. photochemical processes. Although the authors have explored the mechanism of iron dissolution in many ways, the paper suffered from many flaws. For the acidity of the aerosol, this study lacked both constraints of semivolatile gases and a scientific method to verify the accuracy of the simulation results, and thus the authors were very hasty in concluding that aerosols are more acidic, which is very uncritical. In addition, the authors reconstructed PM_2.5 based on aerosol chemical compositions, which was much higher than PM_2.5 at nearby station during the same period, is puzzling and suggested that the data in this study may be inaccurate. The authors characterized the relative strength of aerosol acidity, and the use of the (2[SO₄^2-]+[NO₃^-])/PM_2.5R is incorrect, indicating that the author lacked basic knowledge. The language used in the text is still rough, and it is recommended that the authors strengthen the coherence of the language and simplify redundant expressions. In summary, I do not recommend this article for publication in EGUsphere at present form and it should be returned to the authors for major revision.

Major concerns:

Page 2, line 31-34: Add references including more recent works. For example:

[1] Li, W., Xu, L., Liu, X., Zhang, J., Lin, Y., Yao, X., Gao, H., Zhang, D., Chen, J., and Wang, W., 2017. Air pollution–aerosol interactions produce more bioavailable iron for ocean ecosystems, Sci. Adv., 3, e1601749.

[2] Toner, B.M., 2023. An improved model of the ocean iron cycle. Nature, 620, 41-42.

Page 2, line 31: It’s not open oceans, exactly, but the HNLC regions.

Page 5, line 114: NH₃ is a major alkaline gas in the atmosphere that neutralizes the acidity of aerosols, and the lack of NH₃ will overestimate the acidity of aerosols. How does the author deal with this issue?

Line 119: Equation 1: here, the authors only calculated aerosol acidity for inorganic ions? What about organic acids? The authors measured organic acids, so how was the contribution of organic acids calculated? The contribution of organic matter should be added.

Line 120-122: For ISORROPIA simulations, good results after two iterations of validation do not imply that ISORROPIA is able to accurately calculate the pH of the aerosol, which depends on the concentration of semivolatile gases and ionic components. In addition, the goodness of model simulation results is usually assessed by minimizing the difference between simulated ions and gases and measured values, as detailed in Guo et al. 2017 and Song et al., 2018.

References:

[1] Guo, H., Liu, J., Froyd, K. D., Roberts, J. M., Veres, P. R., Hayes, P. L., Jimenez, J. L., Nenes, A., and Weber, R. J.: Fine particle pH and gas–particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign, Atmos. Chem. Phys., 17, 5703–5719, https://doi.org/10.5194/acp-17-5703-2017.

[2] Song, S., Gao, M., Xu, W., Shao, J., Shi, G., Wang, S., Wang, Y., Sun, Y., and McElroy, M. B.: Fine-particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models, Atmos. Chem. Phys., 18, 7423–7438, https://doi.org/10.5194/acp-18-7423-2018.

Line 131: equation 2, The authors performed a mass reconstruction of PM_2.5, which needs to be compared with PM_2.5 from nearby monitoring stations to demonstrate the validity of this approach, at least in terms of trends. In addition, as can be seen in Table 1, the sum of the components of PM_2.5 at each stage exceeds the PM_2.5 concentration observed at the nearby ambient monitoring station, so why is there such a big difference? According to the data provided by the authors, the reconstructed PM_2.5 = 33.47 was calculated, which is almost twice as much as that of the nearby monitoring station. The difference is so obvious at such a close distance that the authors firstly need to ensure the accuracy of the data before the next analysis.

Line 158-160: Table 1, the calculated pHs are very low, indicating that the aerosols are strongly acidic. However, according to Wang et al., 2022, aerosols in the northern offshore are weakly acidic, and the pH of the aerosols in this study is much higher than that in the literature, which I think is most likely caused by the lack of data on NH₃, and therefore the pH calculated by the authors here lacks credibility. I suggest that the authors include data on NH₃ from the same period, or find data on atmospheric NH₃ concentration in Qingdao from previous literature.

References:

[1]   Wang, G., Tao, Y., Chen, J., Liu, C., Qin, X., Li, H., Yun, L., Zhang, M., Zheng, H., Gui, H., Liu, J., Huo, J., Fu, Q., Deng, C., Huang, K*., 2022. Quantitative Decomposition of Influencing Factors to Aerosol pH Variation over the Coasts of the South China Sea, East China Sea, and Bohai Sea. Environmental Science & Technology Letters, 9(10), 815–821, https://doi.org/10.1021/acs.estlett.2c00527.

Line 163-164, line 182: Here the authors are advised to use the reconstructed PM_2.5 mass.

Line 165: What are degraded air conditions? In general, atmospheric boundary layer heights are higher during the day and lower at night, and here the authors' assertion that the higher FeT and Fes during the day are due to degraded atmospheric diffusion conditions lacks conclusive evidence. It is also possible that this is facilitated by stronger photochemical processes during the day.

Line 187-190: The use of (2[SO₄^2-]+[NO₃^-])/PM_2.5R to characterize the level of acidity in a unit of PM_2.5 is erroneous; the acidity of the aerosol, however, is determined by the amount of H⁺ in the aqueous system, which is determined by the combination of acids and bases in the aerosol system.

Line 199-203: Why are the authors' calculated daytime and nighttime aerosol pH values so large for different pollution scenarios when the SNA percentages are close? During the cleaning period, daytime pH is lower than nighttime, and during the pollution period is nighttime lower than daytime?

Line 255-256: “A decrease of 10% in RH resulted in a notable reduction of 7.6% in SOR and 7.2% in NOR (Figure 5).” How can the authors come to such a solid conclusion when the correlation here is not very high? What is the level of significance?

Line 297: Figure 6 confused me. What are the different colors in the figure represent?

Line 316: For Figure 7, I believe the effect of oxalic acid on iron dissolution is important enough to suggest that the authors devote a chapter to the mechanism of oxalic acid and iron dissolution. In the current version, the authors did not do a good job of exploring the facilitating effect of oxalic acid on iron dissolution.