The processes which influence changing aerosol iron fractional solubility during atmospheric transport remain a crucial gap in our understanding. Additionally, ship-based collections of size-fractionated aerosols over the open ocean are rare. This work reports results from a field study which captures these sorts of samples and attempts to use a combination of spectroscopic and microscopic analytical methods to examine aerosol iron speciation along with chemical modeling to explore these processes. The authors find that most of the total aerosol iron was found on coarse particles of >1 µm but that iron solubility was higher on smaller particles. This observation is attributed to atmospheric processes which expose the smaller particles to environmental acidity levels beyond the inherent buffering capacity of the aerosol. Further, the authors suggest that Fe(III) complexes with humic-like substances (Fe(III)-HULIS) stabilize solubilized iron during transport prior to deposition on the ocean.

This study is fundamentally sound and uses methods that are well established in the field. The novelty lies in the application of these methods on samples collected across a meridional section of the Pacific Ocean between the temperate north and south. This paper lies firmly within the scope of ACP and will be of interest to the aerosol and ocean communities following revision.

The study finds that most of the total aerosol Fe is found in the coarse fraction defined as >1.3 µm but that more labile Fe was in the fine faction. Statistical tests show correlation aerosol Fe solubility and the abundance of Fe(III)-HULIS. Using models, they suggest that aerosol Fe is solubilized at pH<3 and these complexes keep the Fe stabile as pH increases as ferrihydrite precipitation is suppressed. These results are all interesting and will contribute to our collective understanding of the processing of aerosols in the atmosphere.

The text is not without weaknesses which can be improved by careful revision in some cases but perhaps not in others. Generally, the text requires careful editing for clarity and grammar. A fundamental concern of mine is that few samples were collected for this study. In total, only five marine samples and one land-based sample encompass all measurements. My concern is that results might be overinterpreted if the samples are not wholly representative. For instance, the SPO sample covers approximately 17 degrees of latitude over four days. Are there caveats to any of the conclusions because of this? Are the SPO and CPO truly different?

Some of the conclusions hinge on inferences from the SXTM single particle analysis.  More generally on this point, the comparisons of the five different samples was given far less emphasis than the geochemical modeling which I think is the reverse of the proper strategy.

The authors rely to heavily on references to the supplemental material. For example, on page 11 the authors make eight references to figures and tables in the supplement. This is too much. I encourage a careful consideration of what data and figures are required and these materials be included in the main paper. As an example, I am surprised that Table S1 which includes crucial information about the aerosol size fractionation scheme, filter type, and sample description scheme is relegated to the supplement. Indeed, Equation 3 in the Supplement defines the term [H+]_mineral which is used throughout the paper. This equation needs to be in the primary article.

Specific Comments:

Line 76: “...decreases a saturation index of Fe…” Revise for clarity

Line 114-115: I don’t understand why the label PM_1.3-10.2 is used for coarse particles but PM_1.3 is used for fine. Both represent a range of size classes but only the former indicates as such. PM_coarse and PM_fine would be appropriate or, if the desire is to maintain the current form, perhaps PM_1.3-10.2 and PM_<1.3.

Line 117: How were blank concentrations in ng/cm2 converted to pg/m3?

Line 136: How long were the aerosol samples stored prior to labile metal extraction? Were the samples frozen?

Line 148: More details are needed on the ion analysis. What were the levels of detection? Were there issues with the stability of ammonium and/or oxalate?

Line 156: [H+]_mineral must be defined in the main article.

Line 171: What is LCF?

Equations 8 and 9: What do the constants 1.76 and 0.76 represent?

Line 219: What is pH (optional)?

Line 299: WPO1<WPO2<WPO3< SPO≈CPO Does this ranking indicated aerosol abundance, particle diameter, distance, something else?

Line 349: What is OD_c-pre?

Line 402: The text states that the pH of cloud water decreases by 0.03 units but the next sentence states that there is an increase in aerosol pH by cloud processes. Please clarify.

Line 435: I do not think that changing PM_1.3-2.3 to PM_1.0-2.5 is simplifying.

Figure 1: Suggest removing the gray arrows as these do not represent samples included in this paper.

Figure 2: Air mass back trajectories could be moved to the supplement.

Figure 6: I am not sure that the panes b, c, and e are necessary for the main paper and could be moved to the supplement as many of these images already are. 6(e) is not described in the caption.

Figure 7: Much more detail is required in the caption. Clarify that S(n) refers to specific size classes. What are the significance of Particle 1, Particle 2, and Particle 3? What is SRFA?

Figure 8 is not necessary.

General comments

Organic ligands have been postulated to enhance aerosol iron solubility, but the chemical speciation of Fe complexes in size-resolved aerosols is not characterized well. The authors analyzed Fe species in size-fractionated aerosol particles over the Pacific. The X-ray spectrum analysis using reference materials indicated that fine particles contained ferric organic complexes with humic-like substances. The Fe(III)-HULIS was suggested to be formed during transport to the Pacific. The results presented in this paper contribute to better understanding of Fe cycles. I have some comments and questions to improve this paper.

Specific comments

P2., l.30: Even though EF of Fe is close to one, L-Fe in fine particles can be derived from anthropogenic sources due to much higher solubility, as is indicated by Fe stable isotope ratios and the model estimates over the northwestern Pacific. This lower Fe solubility for mineral dust is partly because the high dust/liquid ratio due to low water content in mineral dust could suppress the Fe dissolution even in acidic condition over polluted regions, in addition to the buffering capacity of calcite. Moreover, the Fe dissolution rate for mineral dust is much slower than fly ash. Please see below and consider rephrasing L-Fe for mineral dust in PM1.3 throughout the paper.

p.3, l.72: Please define the average pH.

p.4, l.81: Please specify previous studies for HULIS on mineral dust or other types. How does it form on mineral dust or other types?

p.4, l.83: Please specify previous studies for siderophore on mineral dust or other types. How does it form on mineral dust or other types?

p.6, l.137: Please specify the filter. Sub-micron particles for Stage 6 samples might penetrate the filter regardless its chemical form including nano particulate form. How do you consider this?

p.6, l.157: How do you consider the external mixing of Fe-bearing particles with the main component of the marine aerosols mentioned in introduction?

Figure 6: Fig. 6 appears before Fig. 2. Please also correct the caption (d) and check the consistency of Fe species with (a).

p.7, l.176: Please also describe the distinction of Fe(III)-HULIS from ferrihydrite and goethite.

p.8, l.202: These equations are not valid at the high dust/liquid ratio due to low water content in mineral dust, which could suppress the Fe dissolution even in acidic solution. How do you consider the degree of the suppression?

p.8, l.210 and l.211: The cloud cycle and dissolution time depend on the transport pathway of the particles. Please specify the method and references to justify the 12 hours and half of the transportation time.

p.8, l.214: The dissolution curve is fitted to the mass concentration. Please specify the dissolution rates of biotite and illite after normalizing the dissolution rates to the mineral mass.

p.8, l.225: How do you consider calcite? How do you also consider the external mixing of Fe-bearing particles with the main component of the marine aerosols mentioned in introduction?

p.9, l.251: Please show the enrichment factors for Beijing aerosol and NOTOGRO.

p.10, l.276: How do you reconcile non-anthropogenic Fe source with the positive correlation between Fe solubility and EF of Pb?

p.10, l.280: How do you reconcile the positive correlation between Fe solubility and non-sea-salt sulfate with non-anthropogenic Fe source?

p.10, l.284: Please show Fe solubilities for Beijing aerosol and NOTOGRO.

p.10, l.287: How do you confirm this before the 7-day backward trajectories?

p.10, l.290: How do you reconcile this with the positive correlation between Cd or Pb and non-sea-salt sulfate? How do you explain low non-sea-salt sulfate concentrations over SPO? Please show non-sea-salt sulfate concentration which can be attributed to biogenic S emission alone.

p.11, l.315: Please show biotite fraction for Beijing dust quantitatively.

p.11, l.319: The biotite fraction in S6-WPO2 is higher than that in S6-NOTOGRO. The biotite fraction in the S5-WPO3 is also higher than that in S5-NOTOGRO. These results rather suggest that biotite in fine particles is relatively insoluble. How do you explain higher biotite fraction over the oceans than NOTOGRO?

p.11, l.335: Please show the fraction of Al species quantitatively.

p.12, l.360: Non-spherical dust particles can be converted to spherical particles when they are intensely altered. Thus, the irregular shapes rather suggest that phyllosilicate particles in S6-WPO2 are relatively unaltered. How do you reconcile this intensely altered particles with irregular shapes?

p.13, l.375 and Fig. 9: Why don’t you show the dissolution curve from the Beijing dust? In the laboratory experiments, Fe solubility for mineral dust has not reached more than 15% at pH 1 for the proton-promoted dissolution time up to 120 (h), in contrast to fly ash. Moreover, Eq. 7 is not valid at the high dust/liquid ratio due to low water content in mineral dust, which could suppress the Fe dissolution even in acidic condition over polluted regions. Thus, it is extremely hard to accept such high Fe solubility for the samples with no evidence from the laboratory experiments and field observations for mineral dust near the source regions. The results presented in this paper rather suggest that L-Fe in fine particle is mainly derived from anthropogenic source.

p.14, l.412: How do you reconcile this with the acidifications of mineral dust by sulfate derived from biogenic S mentioned in this paper?