Comments

This manuscript by Ueda et al., investigated the morphological features, mixing states and water solubility of Fe-containing particles in aged fine aerosol particles over the Indian Ocean. The topic of this study is attractive and interesting, which is very useful for understanding the ageing of particles and the dissolution of Fe-containing particles during the transport of aerosols and the morphology variations of Fe-containing particles. Overall, the manuscript is logical, and the main issues are very well discussed in this paper. I would therefore recommend this manuscript for publication after the authors have addressed the following comments.

Major concerns:

1. The units of horizontal variation of mass concentrations of (a) nss-SO₄^2-, (b) NH₄⁺, and (c) Fe in PM_2.5 in Figure 2 are μg/kg and ng/kg, however, there are μg/m³ and ng/m³ in Table 2, which is confused for me, please explain the differences between the two units and make them uniform.
2. Figure 2c (mass concentrations of Fe) has two more data points compared to figures 2a and 2b. Why?
3. Line 232: “For non-sea-salt components, the relations between the doubled nss-SO₄²⁻ molar concentration and the NH₄⁺ plus nss-K⁺ molar concentration were usually between 1:1 and 2:1 (Fig. S1b), suggesting that nss-SO₄²⁻ originated from ammonium sulfate, ammonium bisulfate, and ammonium potassium rather than from sulfuric acid.” Here, “ammonium potassium” should be “potassium sulfate” ?
4. Line 227-228 and line 232-233: “The values of nss-K⁺, which are regarded as originating mainly from biomass burning……; For non-sea-salt components, the relations between the doubled nss-SO₄²⁻ molar concentration and the NH₄⁺ plus nss-K⁺ molar concentration were usually between 1:1 and 2:1”. The author argued that nss-K⁺ was mainly from biomass burning, please provide more evidence to support this view.
5. In the section: “3.2 Individual particle features and co-existing states with Fe of sulfate and soot”, the author chooses samples #01 and #07 as example to illustrate the individual particle features, however, I fail to get the idea why the author selects these two samples. Is there any special in these two samples, the reasons should be given.
6. Compared with the filed observation results, the CAM5-ATRAS model underestimates Fe by nearly 1/3 (from Figure 9a), why? Are there other unknown sources of Fe or is there a large uncertainty in the Fe emission inventory used by your model?

Some other minor issues:

1. Line 132: Cl^−-, NO₃⁻, SO₄²⁻, Na⁺, NH⁺……., here, “Cl^−- andNH⁺” should be revised to “Cl⁻ andNH₄⁺”. Please check the similar issues in whole text.
2. The numbers and letters are so small that they can't be read clearly in Figure 4 and Figure 7, please adjust the font size.

Comments to the muanuscript egusphere-2023-389

Ueda et al. Morphological features and water solubility of iron in aged fine aerosol particles over the Indian Ocean.

Reviewer's comments

The authors investigate aerosol particles' iron (Fe) properties over the Indian Ocean aboard a research vessel. The article presents important information on these particles' morphology, concentration, and degree of solubility. In general terms, the article is well-written. However, some minor observations need to be addressed. The article can be accepted after answering the following questions.

Abstract

line 15. the authors mention that they analyzed particles in the size range 0.3-0.8, using a cascade impactor. As written, it appears that they studied a continuous range of particle sizes. However, the methodology mentions that the sampler only has three nominal sizes (1.6, 0.8, and 0.3 μm). Please mention these three sizes in the abstract.

Methodology

The water dialysis process was important to study the water's mixing state and elucidate the Fe's solubility. Could you expand the description of this technique and mention the instrument used for it?

Indicate the bibliographic source from which equations 1 and 2 were obtained.

Please revise the wording in the first line of the first paragraph of section 2.1. It is not easy to understand the name of the cruise ship where the study was conducted.

Line 200: Check the spelling in this sentence.

It is recommended to use the same concentration units in tables and figures. Tables 2 and 3 report the concentrations in μm/m3 and ng/m3, respectively. While in Figure 2, the units are reported as μg/kg.

Results

It is suggested that labels (a), (b), (c), and (d) have to be added to each panel in Figure 8. The figure caption should also be improved to make it easier to read.

Figure 10 has many elements. It is suggested to separate them and present some results individually. In addition, the figure caption is very long and difficult to read.