The manuscript investigated total Fe, dissolved Fe and Fe solubility for coarse (>1 μm) and fine (<1 μm) particles in four different seasons at Xi’an, China impacted by anthropogenic emissions and desert dust, combing with the relative humidity and aerosol pH. This work is very useful to realize the importance of RH and aerosol acidity in regulating Fe solubility in atmospheric particles. I would therefore consider the publication of this article once the authors have addressed the following comments.

Major concerns:

1. I think in introduction there are wrong mentions. For example, the organic complexations in L. 74 are not analyzed in following results and discussions. The seasonal variation of Fe solubility, on another hand, has only been explored by a few previous studies as shown in the introduction. Again, you investigated seasonal variations of total Fe, dissolved Fe and Fe solubility in this article. So is it a repetitive work? Please go through the words logically.
2. Combining with the Fig. 4, 8 and 9, as well as table A1, the aerosol Fe solubility in this field observation is lower than some previous studies based on field data on regions influenced by anthropogenic emission and pyrogenic iron source. Please give the reasons for the relative low Fe solubility in the article. If possible, the authors should cite more literatures and field data at sampling sites, summarizing in forms of table to show total Fe and Fe solubility in next revised manuscript.

Minor concerns:

1. 49: “primary production” to “primary productions”.
2. 50: The authors need to clarify the chemical mechanisms as dissolved iron contributing to ROS formations in aerosols. It is not enough only listing the references. I think Fenton reaction is a good standpoint.
3. 59: “a number of studies have been conducted in the last 2-3 decades.” But authors only cite some references in recent years (2018 to 2021) and should replenish more studies in former years than 2010.
4. 66: “contribution” to “contributions”.
5. 74: Irrelevant statement in introduction. From view of this manuscript, the authors aim to study the effect of aerosol acidification on aerosol Fe solubility, rather than organic complexation, as yet it is not shown in entire paper. Please revise it.
6. 112: “W41 filter used for aerosol sampling were acid-washed to reduce background levels.” I am little confused by the acid-wash and you should explain the pretreatment. Or sampling cut-offs and sampler were acid-washed?
7. 119, This sentence could be revised as “Each filter was equally halved.”
8. 122, this again confusing. Why did the authors fill Teflon jar with 20 mL HNO₃ after acid digestion? Whether the results were same if replacement with ultrapure waters?
9. Figure 3 and Figure 7: These plots missed the color legends labeling as the coarse or fine particles.
10. 369: Misspelling, please revise “Ass shown in Figure S11” to “As shown”.
11. 405: “Both Shi et al. (2020) and we suggested that high RH could promote Fe dissolution via acid processing.” Only relationship between RH and Fe fractional solubility is not persuasively in favour of it. If possible, the writer can add the soluble ion balance (I = 2[SO₄] + [NO₃]+ [Cl] − 2[Ca] − [NH₄] − [Na] − 2[Mg] − [K]), a proxy of the acidification of the aerosol aqueous phase, in Fig. 8 to see if they correlate. When I >0, excess H⁺ is required in the associated aqueous phase to neutralize the excess anions.
12. 426: Did the lower aerosol pH exist in summer and autumn? Several studies have shown the formations of secondary inorganic aerosols during heavy haze episodes frequently in winter dominate the higher aerosol acidity in cities, so is conflicting with your findings? Can you give an interpretation for it?
13. “the observed higher Fe solubility in summer and autumn” at L. 429 is completely contrary to the wordings at L. 443-444 as “dissolved Fe concentrations were lowest in spring and summer”. Please revise this.

This study by Zhang et al., investigated the seasonal variation of aerosol Fe solubility for coarse and fine particles at Xi’an. Overall, the manuscript is well organized and the results are clearly presented with comprehensive discussion. The topic of this study is of great interest in the community. The results of this study are interesting and important for the understanding the role of Fe in atmosphere, especially for the biogeochemical cycle of Fe. I am very pleased to recommend this manuscript for publication after a minor revision. My comments are shown as below.

1. Line 140: Gaseous compounds were not considered when calculating aerosol acidity. Clarify if this will affect the trend of aerosol acidity. This is important for the discussion on aerosol acidity. Generally, omission of gaseous compounds will lead systematically underestimation of pH (over estimation of acidity). I believe that the trend of aerosol acidity would not be affected significantly; therefore, the discussion on aerosol acidity is still valid. But a clarification is needed here.
2. Line 146: If the arrival time is 8 am of each day, the number of trajectories should be same as the samples. But the number of trajectories is much more than that of samples. Please verify the accuracy of the description.
3. It is stated that “…suggesting desert dust always as the dominant source of total aerosol Fe at Xi’an, regardless of particle size range and seasons.” (Line 206-207). But in line 202-204, the authors also mentioned anthropogenic emissions as an important factor. The authors need to clarify how they are consistent.
4. Line 229-230: Authors indicated that dissolved Fe concentration in winter for Xi’an is higher than that for Qingdao. It may be resulted from the differences of total Fe concentrations in the two cities. Authors can add one sentence here to make an explanation.
5. Line 255-268: When investigating the source of soluble Fe, the authors talked about the correlation of soluble Fe with elements like K+, Pb and Al, which is valid. However, the authors started the discussion from K+ without any justifications. The correlation with all elements has been actually listed in Table S3. I suggest the authors to have an overall description of the correlation with all elements before mentioned K+ (with the highest correlation).
6. Line 261-262: Any literature to support that biomass burning emission is important for autumn and winter in Xi’an?
7. Line 264: “anthropogenic emission” is a vague description. Please specify it or list some possible anthropogenic sources.
8. Section 4.1: Authors may need to compare the Fe solubilities in winter between Qingdao and Xi’an, as the comparisons for total and dissolved Fe between these two cities.
9. Line 308-310: This sentence does not provide sufficient information. To my understanding, the reverse relationship actually reflects the different source (or affecting factor) of total Fe and water-soluble Fe. The authors can try to explain the mechanism or just state that the mechanism needs to be further investigated.
10. Line 383: I am not sure if the description of “Secondary formation of dissolved aerosol Fe” is accurate. I understand that secondary process may promote the dissolve of Fe but this description may be misleading.
11. Line 409-410: Authors stated that Fe solubility continuously decreased with increasing aerosol pH (from <2 to >5) for fine particles, this trend is generally right. But if we see Figure 9, Fe solubility slightly increased with pH from <3 to >3 for fine particles, so please make your description be more conservative.
12. Authors indicated that desert dust was not the main source of dissolved Fe, and chemical aging showed a small impact on Fe solubility for fine particles. It may imply that anthropogenic emission is the dominant source of dissolved Fe in fine particles. Authors can add some sentences in section 6 to illustrate it and make your conclusions be more specific.

Some other minor issues as listed below:

1) The title of section 3.1 may be more suitable with “Meteorological conditions and particulate matter concentrations” since particulate matter concentrations are also discussed in this section.

2) Line 109: Please delete the second word of “and” in “Coarse (>1 μm) and fine (<1 μm) and aerosol particles”.

3) Line 110: Specify the time is am or pm.

4) Line 124: Specify the model of the ICP-MS and the MDL.

5) Line 144: Verify the time resolution is 3 or 6 hours.

6) Line 185 and line 229: Please replace the Chinese character in parentheses with “and”.

7) Line 213: Correct the typo “deust”.

8) Line 356: Space should be added between the parentheses and word.

9) Line 405: Change “we” to “the current study”.

10) Line 425: The values of Fe solubility for coarse particles in parentheses were wrong. Please follow Table A1 and revise them.

General comments

The study reports seasonal variation of total and dissolved Fe concentrations and fractional solubility in coarse and fine particles collected in Xi'an. More than 120 samples collected over a year are a valuable data set on Fe solubility in East Asia, which can contribute to a better understanding of the factors controlling Fe solubility. However, these valuable data sets were not fully utilized in the paper, as the discussion in this paper is mainly about averages (or medians) for concentrations and Fe solubility. There was no discussion of total and dissolved Fe or its solubility during the two Asian dust events, which have the potential to be important Fe supply events to the ocean surface. Furthermore, Fe solubility in some of the winter samples exceeded 5%, and the reasons for their high solubility should be mentioned. It would be desirable to discuss the daily variation of Fe concentration and Fe solubility in each season to solve these problems.

The importance of anthropogenic Fe on enhancement of Fe solubility in fine aerosol particles were reported by this study. However, the discussions on the emission sources of anthropogenic Fe and its tracer elements are a little bit broad. For instance, the dominant source of anthropogenic Fe in coarse and fine aerosol particles was biomass burning due to a good correlation between dissolved Fe and K⁺ concentrations. It is suspicious that K⁺ in coarse particles is from biomass burning because aerosols from high-temperature combustion, including biomass burning, are abundant in fine aerosol particles. As the authors said in the introduction, emission sources and physicochemical properties are different between coarse and fine aerosol particles. Therefore, discussions on the emission source of anthropogenic Fe and its tracer elements in each size fraction is required. I think that Table S3 is supported to evaluate the emission source of anthropogenic Fe.

The relationship between aerosol pH and iron solubility is the most important topic in this paper. Aerosol pH in coarse aerosol particles is usually 1 to 4 units higher than that in fine aerosol particles because non-volatile cation (e.g., Na, K, Mg, and Ca) is mainly present in coarse aerosol particles. However, the median aerosol pH in coarse aerosol particles was almost the same or lower than those in fine aerosol particles. If the low pH of the coarse particles is due to a bias in the thermodynamic calculations, it may overestimate the importance of acidic processes in the coarse particles. Please explain the reason for lower pH in coarse aerosol particles than in fine aerosol particles.

I am also concerned that the Fe solubility reported by this study was much lower than those in aerosol particles collected in China by previous studies. High Fe solubility (>10%) has been observed in aerosol particles collected in China, including Xi'an, but such high Fe solubilities were not found in this study. It is necessary to explain why Fe solubility differs significantly from previous studies, even though the aerosols were collected in the same city. Since there are several concerns, I cannot recommend the publication of this study in ACP in its current state. However, this manuscript would represent a valuable contribution to the field of aerosol research after major revisions because there are few studies on seasonal variation of Fe solubility and its controlling factors.

Specific comments

L81–83: Please provide specific examples of how seasonal variations of emission sources and chemical processes of Fe affect its solubility.

L95: Many readers are not familiar with Xi'an and its surrounding topography. Please provide a map showing the sampling site and the surrounding area.

L121: Please clarify acid compositions.

L124–126: How were recoveries of target elements determined? If you measure certificated reference material, please specify it.

L135–136: If gas species is not employed as an input parameter, the reverse mode would be the appropriate calculatio method.

L161–163: I guess that the standard deviation of aerosol mass concentration in spring is too small for the concentration range of PM10. Please confirm it.

L182–186: Why were the Fe concentrations of aerosol particles collected in Xi'an higher than those collected in Qingdao? Is Qingdao classified as an industrial, residential, or background area?

L:185: There is a garbled text between 1831±866 and 2058±1037 ng/m³. The same garbled text was found in line 229.

L190–195: Iron in mineral dust is mainly present in coarse aerosol particles. Indeed, the contribution of aerosol mass concentration of coarse aerosol particles in spring and Asian dust events was larger than in other seasons due to the low PM2.5/PM10 ratio. However, the contribution of Fe in coarse aerosol particles to total Fe in TSP was lower than in other seasons. This result is inconsistent with a mass ratio of PM₅/PM₁₀. Please explain the reason for the lowest contribution of Fe in coarse aerosol particles to that in spring samples. In addition, nss-Ca, Al, and Ti are good tracers of Asian dust. Are these elements in the aerosol collected in the spring also present mainly in the fine particles?

L199: Aerosol particles collected in North America and the Atlantic Ocean are unlikely appropriate comparison targets of Fe solubility in aerosol particles collected in China. I would suggest a comparison with dissolved iron in aerosols collected in Xi'an (e.g., He et al., 2021: https://doi.org/10.1016/j.apr.2021.101090,  Lei et al., 2023: https://doi.org/10.1016/j.atmosenv.2022.119507).

L208: In general, [Fe]/[Al] means molar concentration ratio rather than mass concentration ratio. Please removed the bracket.

L258–261: Coal combustion is also a dominant source of K⁺ in fine aerosol particles. Indeed, dissolved Fe concentration correlated with Pb as a tracer element of coal combustions. In addition, fuel oil (e.g., heavy oil, gasoline, and diesel) combustions emit dissolved Fe. Furthermore, the steel industry is the dominant source of anthropogenic Fe-oxides. A more detailed discussion of the emission sources of dissolved Fe in aerosol particles is needed.

L262–265: Please discuss in detail regarding emission sources of Pb and dissolved Fe in coarse and fine aerosol particles. Lead in fine aerosol particles is derived from high-temperature combustions (e.g., coal combustions and municipal solid waste incineration). In contrast, Pb in coarse aerosol particles is derived from the resuspension of road dust (e.g., the pigment of road paint). Therefore, emission sources of dissolved Fe are likely different between coarse and fine aerosol particles.

L305–306: Please explain why the inverse relationship between Fe solubility and total Fe in autumn samples could not be observed. In addition, it seems that the constant of a and the power of b in equation 1 has seasonal dependence. Please discuss the reason for the seasonal variation of the fitting equation.

L347–349: Why not evaluate the relationship of Fe solubility with nitrate and sulfate separately? Identifying the acid species that increase iron solubility is one of the important topics.

L369: Ass rephrased by as.

L414–416: Please explain why Fe solubility was low (<1%) in some aerosol particles despite low pH and high RH. Also, were the aerosol samples with lower Fe solubility at low pH and high RH found in a particular season or in any season?

L416-419: Specific surface area is one of the factors controlling fractional Fe solubility in aerosol particles (Baker and Jickells, 2006; McDaniel et al., 2019). The specific surface area of fine aerosol particles is usually higher than that of coarse aerosol particles, indicating that fine aerosol particles are more reactive than coarse aerosol particles. Therefore, the fact that the Fe solubility of fine aerosol particles is higher than that of coarse aerosol particles does not guarantee that the contribution of anthropogenic Fe to Fe solubility is greater.

L425–429: Indeed, aerosol pH in spring was higher than those in autumn, but median temperature and RH were almost the same between spring and autumn. Therefore, temperature and RH are not the reason for the higher pH of spring than autumn. One possible reason for high aerosol pH in spring is the large abundance of CaCO3 in Asian dust that acts as buffer species. Please discuss the seasonal variability of aerosol pH with accurate descriptions of the relationship of aerosol pH with temperature, RH, non-volatile cation concentration, etc.

L427: Please rephrase Table S5 by Table S4.

Figure 1: Does the solid line represent the median and the closed square the mean? Please provide legends for the median and average (as well as other box plots).

Figure 3: Several plots in panels (c) and (d) are shown with cross symbol. What does the cross symbol represent? Cross symbols can be found in Figures 7, S6 and S11.

Table S3. More detailed discussion on correlation of dissolved Fe concentration with trace elements are required to estimate anthropogenic Fe source. Are correlation coefficients higher than 0.5 listed in bold? If so, please specify that. Also, please bold the correlation coefficient for Pb in fine grains collected in winter.

Figure S7. This figure is not cited in the main text.