Review of “Fossil-Dominated SOA Formation in Coastal China: Size-Divergent Pathways of Aqueous Fenton Reactions versus Gas-phase VOC Autoxidation” by Wang et al.

This manuscript presents analysis of size-segregated water-soluble aerosol samples from a coastal site in China.  The main findings include that nearly all OOC was derived from fossil sources, fine-mode SOA was influenced by aqueous phase Fenton reactions and coarse mode SOA was derived from gas-phase oxidation.  I have a few major comments that need to be addressed before the manuscript is ready for publication.

General Comments

1. I have major concerns regarding the PMF analysis.   This has implications for the overall conclusions as the authors have defined SOA as the sum of two of the PMF species.  Specifically: 

• Why were only 5 variables included, rather than including more species from the IC, ICP or ACSM? The would strengthen the separation of the factors.
• How was nss-K⁺ derived? Based on the high Cl and NO3 in the coarse mode, it seems there is probably influence from sea salt.

The PMF results are also concerning and more validation is necessary. I would expect at least some CO₂⁺ and C₄H₉⁺ to be attributed to biomass burning.  It would be useful to also show the rest of the results from the PMF rather than just the EV plot.  

2. Should the aerosol size distribution units be µg m^-3?

3. According to table 1, coarse mode OOC is negatively correlated with the aromatic VOCs.  Similarly at line 255, the correlation between coarse mode biogenic OOC and monoterpenes and isoprene is discussed, but the correlations are negative.  I’m not sure how this supports these VOCs as precursors for the coarse mode OOC. 

Minor Comments

Line 61: “Dust particles mainly consist of aluminosilicate, sea salt, SiO2, CaCO3, and coated with secondary organic and inorganic aerosol components under an ambient environment…”  I would argue that sea salt particles are, by definition, not dust. 

Figure 2: What is included in the “Other” category?

Line 138: "These coarse modes contain more water insoluble components, it contains a variety of metal oxides (i.e., TiO2 and Fe2O3)".  Is this just a general comment, or were these measured? If so, how were these water-insoluble compounds measured? 

Line 163: Please include more details for these equations.  For example, what is f_modern?

Line 198:  Are these ion fragments from the ACSM or the PTR-MS?

Typographical

In general, I would advise the authors to carefully proof-read the manuscript for spelling and grammar.  Especially run-on sentences and incomplete sentences. 

Line 13: Radiocarbon is misspelled

Line 46, 47: SOA and VOC were already defined previously

Line 58: “While that formed on coarse particles was mostly neglected…” this clause is not clear

Line 79: PRD is not defined

This manuscript is an interesting effort to understand secondary organic aerosol formation in a coastal site in China. It could serve as a valuable guide for further complementary studies on the differences between fine-mode and coarse-mode oxygenated organic carbon origins. I recommend minor revisions for publication. Additionally, the text should undergo a careful review for grammar and fluency, with particular attention to punctuation and spaces.

General comments

• Why not utilize diagnostic ratios to attribute and support specific sources such as Mg²⁺/Na⁺, Cl⁻/Na⁺ (related to marine influence), and SO₄²⁻/NO₃⁻ (which some authors use to differentiate between stationary and vehicular sources) among the various particle sizes? It can be complementary to PMF.
• Please provide the robustness assessment of the Positive Matrix Factorization (PMF) results, including bootstrap mapping and displacement tests, and clarify how the three-factor solution was determined (in the manuscript).