Yang et al. collected size-segregated aerosol particles at an urban site in Hong Kong (China) at four different seasons, measured abundance and solubility of 10 metals, and investigated factors which modulated metal solubility. They found that for a given metal, in general solubility was higher in fine particles than coarse particles. In addition, they concluded that sulfate played a key role in modulating metal solubility in fine particles. Overall, the date presented in this manuscript is very valuable, and the findings are also interesting. I would recommend it for final publication after the following comments are addressed.

Major comments:

Line 174-178: After sonication, how were insoluble materials separated from the aqueous extracts? Was it achieved by high speed vortexing? Filtration is usually used by many studies, with filter pore-size clearly stated. More information should be provided here.

Line 250-259: Can the authors show and discuss correlations between Al and other metals? This may give further insights (in addition to size distribution) to their sources.

Line 352-353: Several previous studies, including our work (Zhang et al., 2022) and references therein, also found that Fe solubility (as also other metals) was higher in fine particles than coarse particles. The authors may consider discussing these studies.

Line 392-405: As this work did not manage to detect oxalate in aerosol particles, I feel this paragraph is tedious and not very relevant. The authors may consider making it more concise.

Minor comments:

Line 67-69: The first sentence only mentioned “many atmospheric processes”, but the second and third sentences mainly discussed ocean biogeochemistry. The authors may need to modify the first sentence to make it more appropriate.

Line 203-204: would it be enough to use only one unit (ng/L) here?

Line 239: It may be better to use particle diameter for x-axis in Figure 1.

The author study the variability in the concentrations of water-soluble Cr, Fe, Co, Cu, Pb, and Mn and their fractional solubilities. They found the metal solubilities were driven by both the aerosol acidity levels and liquid water concentrations. These database is very important to further discuss or study health impact of metal particles in HK. Also, the authors specifically discussed the possible reasons about the solubization. Finally, the authors attributed the major reason to be sulfate. I think that the size-resolved study is very interesting. I think that this work is acceptable.

• L49-52, are there any references to study metal particles in HK or upwind cities such as Guangzhou. Ref: Atmospheric Environment, 41, 2, Pages 432-447; Int. J. Environment and Pollution, Vol. 36, Nos. 1/2/3, 2009; Environ. Sci. Technol., 47(16), 9124-9131;
• Here why did the authors did not consider the nitrates and organic acids. Does nitrates have large contribution for metal solubility? Nitrate is also acidic aerosols in the air.
• If the authors discussed the metal solublization, the author firstly should consider the mixing state of metal particles. Recent, some studies nicely obtain iron associated with sulfate in nanoscale using nanotechniques. Adv., 3(3), e1601749, 1601741-1601746. At this stage, the authors make some discussions and then you turned into your relationship.

General comments

The authors measured the mass concentrations and fractional solubilities of different metals in size-fractionated aerosols collected at an urban site in Hong Kong. The authors confirmed the key roles of sulfate in the acid dissolution of metals in fine aerosols. On the other hand, the observational data indicated high solubilities at low sulfate concentrations, suggesting the other effects. For measurement report, the effect on high solubilities at low sulfate concentrations due to the measurement technique should be clarified. I can recommend this paper for publication in ACP after the following comments are addressed.

Specific comments

l.53: The three references have not provided evidence for associations between water-soluble metals in aerosols and adverse human health outcomes that are significantly greater than for their water-insoluble forms. Please consider rephrasing this with relevant references.

l.174: What are the advantages and disadvantages of this method to measure water-soluble metals, compared to filterable metals which include colloids, nanoparticles, and aqueous species? How did you optimize the sonication time, the vortexing speed, and its duration? Please elucidate the differences of nanoparticle size distribution in the solution extract before and after the process. How do you consider the disaggregation and detachment of nanoparticles from aerosol particles during the process? This might be associated with high solubilities at low sulfate concentrations. It is desirable for measurement report to show the differences of solubilities between the water-soluble and filterable metals using representative samples. Please elucidate the effect of the measurement technique on high solubilities at low sulfate concentrations.

l.224: How did you consider the activity coefficient?

Fig. S3 and Fig. S4: How do you explain the high contributions of dust for Cu in Fig. S3 as opposed to Fig. S4? How do you explain the low contributions of dust for Fe and Mn in Fig. S4 as opposed to Fig. S3? How do you explain the high contributions of industrial 1 and ship emissions for Mn in Fig. S4 as opposed to Fig. S3? How do you interpret the high contribution of dust for Co? How do you interpret the high contribution of sea salt for Fe?

l.473: Fig. 6 does not show strong correlations (R > 0.7) between the fractional solubilities except Cu and sulfate. Please rephrase this. 

How do you explain higher solubilities at lower sulfate and Wi concentrations if acid processing played a major role in enhancing the solubilities of these six metals?

Why don’t you analyze the correlation for each factor? Please discuss the reasons of the stronger correlation between WS metals and sulfate than between the fractional solubility of metals and sulfate.