This manuscript by Xu et al. presents one-year ambient measurements on organosulfates and nitrogen-containing organic compounds in PM_2.5 collected from an isolated Sansha island in the South China Sea. To my knowledge, this work represents the first concurrent field observation resolving molecular signatures of organic sulfur and organic nitrogen aerosols in Sansha Island. The authors found that the proportion of aliphatic and aromatic organosulfates in the total organosulfates was significantly higher in Sansha than in other Chinese cities investigated. This is a very interesting finding. Furthermore, the authors demonstrated that marine biogenic sources may play an important role for the production of aliphatic/aromatic organosulfur compounds and nitrogen-containing organic compounds, with relatively little from long-range transport and shipping-derived emissions. I believe this may be a special observation case to demonstrate that marine organisms can provide important aliphatic or aromatic precursors for the formation of aliphatic and aromatic organosulfates and nitrogen-containing organic compounds in the tropical ocean atmosphere.

In general, the manuscript is well-written and logically structured, and it presents a wealth of valuable observational data. I have only a few minor suggestions, and I believe it would be suitable for publication in ACP once these suggestions are addressed.

Specific comments

1. I can understand directly comparing the quantitative organosulfate concentrations between Sansha and other Chinese cities. However, I also found that the authors made a comparison between the signal intensity of various nitrogen-containing organic compounds in Sansha and the results from previous field studies.Due to different instruments or methods used in the determination of nitrogen containing organic compounds in different studies, there may be differences in the types and ionization efficiencies of nitrogen containing organic compound measured. Therefore, how did the author correct or explain these issues.
2. The authors present a large amount of satellite-based data. I understand that conducting year-long observations on such data in field sites with underdeveloped infrastructure is quite challenging. Therefore, I suggest that the authors incorporate more literature on observational studies carried out in the South China Sea to validate the reliability of the trends in chlorophyll-a or sea surface temperature calculated using satellite data. Since the various analytical results of this study were closely linked to the trends of the parameters, it would suffice to demonstrate the reliability of the trend changes in the key parameters.
3. This study quantified up to 92 types of organosulfates. However, the authors utilized surrogate standards. I recognize that reference standards for many organic sulfur compounds are difficult to obtain or prepare, which is why numerous studies rely on surrogate standards for quantifying organosulfates. Therefore, I suggest the authors clarify the limitations of this methodology.
4. Lines 285-286: ‘…aromatic-derived CHON+ type Re NOCs…’I suggest changing it to ‘For Re NOCs, the mean signal intensity proportion of aromatic-derived CHON+ in the total signal intensity of CHON+ compounds …’.
5. Lines 295-297:Please reorganize this sentence to enhance the logical coherence of the context.
6. Line 302: This should be ‘The subsequent discussions…’
7. Line 305: …Temporal variations of OSs and NOCs…
8. Line 329: Please delete ‘very’.
9. Line 350: Please delete ‘from different sources’.
10. Has the structure of the two important organosulfate markers in Figure 6 been verified?

General comments:

This manuscript presented the quantification of OSs and measurements of nitrogen-containing organic compounds (NOCs) in PM_2.5 collected in the Sansha region over a one-year period. The proportion of aliphatic and aromatic OSs in the total OSs was significantly higher in Sansha than in other Chinese cities investigated. They concluded that the emissions of marine organisms can contribute to the formation of both typical BSOA and aliphatic- and aromatic-derived SOA in this sea area with few anthropogenic sources of pollution. This study can provide a scientific bases for studying on the potential origins and formation of tropical marine aerosol OSs and NOCs.

The manuscript is mostly well written and easy to read. The results and discussion are detailed and convincing through the comparative analysis of results. The manuscript deserves publication after the authors take care of the following minor revision described below.

Specific comments:

1. Page 5: Why was Sansha chosen as the research area? How does the intensity of marine biological activities there compare with those in other tropical seas?
2. Page 7: the authors mention that all OSs were undetectable in the blank samples. However, the specific handling procedures for the blank samples were not specified. It is recommended to provide more details on the quality control of the blank samples.
3. Page 9: Why did you choose the forward mode with the thermodynamically metastable state when using ISORROPIA-II to predict the pH value?
4. Page 10: Were the OSs and NOCs detected in the study reported in other marine aerosols? Are their abundances geographically specific?
5. Page 11: Figure 1 compares the OSs and NOCs data of different cities, but does not specify whether the sampling times and analysis methods of these data are consistent. It is suggested that a description of the comparability of the methods be added.
6. Page 14: Is it possible that the formation of NOCs in low NH₄⁺ environments occurs through other pathways (such as reactions involving organic amines)?
7. Page 20: the authors mention that the above results suggest that ship emissions were not the primary factor in controlling the abundance of aerosol OSs and NOCs in the Sansha area. It is suggested to provide specific percentages to illustrate this point, and to elaborate on the results shown in Figure 4.
8. Page 21: the authors mention that a Mantel test correlation analysis was conducted to further investigate the impact of different factors on the formation of OSs and NOCs in the Sansha area. However, the criteria for variable selection (such as why Na⁺ was chosen instead of other ions) were not explained. It is suggested to provide the basis for variable selection.

This study reports the molecular composition and characteristics of marine organic aerosols in Sansha, South China Sea, and especially focuses on the OSs and NOCs. The data is very informative and valuable. The following comments need to be addressed before being published on ACP.

General comments:

This work highlights the contribution of marine emissions to aromatic/aliphatic organic sulfur and nitrogen aerosols. My major concern is the potential influence of biomass burning on the aromatic/aliphatic sulfur or nitrogen, or other organic compounds in marine aerosols over the South China Sea.

Many shipboard cruise observations have suggested the obvious impacts of biomass burning emissions on the marine organic aerosol formation over the South China Sea, including studies from Chinese researchers. The authors also report that higher concentrations of OSs and NOCs and more fire pots were observed during December to March. Biomass burning has been proved to be one Thus, more solid and detailed evidence should be provided to exclude the possible impacts of biomass burning emissions on the formation of sulfur/nitrogen organic aerosols.

The K+ concentrations in June-August are much higher than during other months. I may suggest analyzing the data during Aug. 20—May (without the data in Jun-Aug) and during Jun-Aug separately for the correlation analysis in Fig. 5 or the PCA analysis in Fig. 6. It should also be noted that previous studies have reported that the K+ in biomass burning aerosols would decrease rapidly during long-range transport in the atmosphere.

There have been some studies on the seasonal variations of seawater isoprene or other VOCs. I may suggest the authors combining the observation results on the seawater VOCs to validate the seasonal variations of satellite-derived Chl-a and calculated isoprene concentration reported here. Does the data here follow the same seasonal variation trend as reported in previous studies?

Specific comments:

Lines 287-291: The authors argue that the much lower NH4+ aerosols lead to the limited formation of Re-NOCs. Is the gaseous NH3 lower in Sansha than in other cities? Reactions between carbonyl compounds and gaseous NH3 could also form CHON+. Considering the higher air temperature in Sansha, the fraction of NH3 in the gas phase could be much higher in Sansha than in other cities.

The authors compare the NOCs compositions in Sansha and other sites. However, different analysis MS instruments or measurement parameters were used in different studies. This might be the primary reason for the different NOC compositions.

This manuscript presents an observational study on organosulfates (OSs) and nitrogen-containing organic compounds (NOCs) in fine particulate matter in an island in the South China Sea. The source of marine emissions has been emphasized though other source contribution of long-range transport could still be found. This study highlights the contribution of marine biogenic emissions for Oss and NOCs in the oceanic area. I would recommend it to be published in ACP after moderate revision. My comments/suggestions are listed below.

1. Line 254-255, Page 12: As the proportions of OSa in the total OSs are higher at both inland and ocean sites, how can we differentiate between the anthropogenic and oceanic sources of OSa based on its percentage in aerosol samples?
2. Line 295-297, Page 14: The assertion that atmospheric oxidation capacity does not constrain OS formation is contradicted by the significant correlation between OSs and O₃ mentioned in 3.2.
3. Ling 330, Page 16: Given that both marine biological activities and anthropogenic sources contribute to atmospheric sulfate, what are the non-sea salt and sea salt proportions of the total sulfate in aerosol samples, and are there any differences in correlations of OSs versus these sulfate fractions?
4. Line 388-389, Page 19: How to explain the coincidence between the maximum of biomass burning intensity and elevated levels of OSs and aromatic NOCs during winter and spring months, assuming the SCS atmosphere is unaffected by air mass long-range transport?
5. Line 392-394, Page 19: The inconsistent variation trends between nss-K⁺ and the surrounding continental fire point density alone cannot conclusively demonstrate that the SCS atmosphere is not impacted by air mass long-range transport. Are there additional anthropogenic or terrestrial tracer concentration patterns strengthening this assumption?
6. Line 415-417, Page 20: Does the inverse relationship between isoprene/monoterpenes and OSs/aromatic NOCs contradict the previous allusion that the predominance of isoprene and monoterpenes-derived NOCs over aromatic NOCs results from marine emissions of isoprene and monoterpenes?
7. Line 439-441, Page 22: Could the author provide the exact Mantel’s r and p value from the Mantel test, since the threshold of >4 is too broad for meaningful interpretation?
8. Line 487, Page 25: How can Ca²⁺ indicate marine sources given its negative correlation with Na⁺ and Cl^- in Figure 5?
9. Line 502, Page 25: Did these two aromatic OSs dominate in the total aromatic OSs? How do other aromatic species correlate with these marine emission indicators?