Review of “Secondary formation dominated low molecular weight amines origins in aerosols over the marginal seas of China” by Yang et al.,

We would like to thank the authors for providing a comprehensive observational dataset of particulate amines over the Yellow Sea–Bohai Sea region. Filter-based measurements of low molecular weight amines in marine and coastal aerosols remain scarce, and the presented dataset adds valuable information on amine concentrations and speciation in this region, particularly in relation to co-measured inorganic and organic aerosol components.

At the same time, we suggest that the manuscript may be better suited for publication as a measurement report, with the primary emphasis placed on the observational dataset and its descriptive analysis. While the interpretations and proposed formation pathways are generally plausible and well-motivated by previous literature, many of the conclusions rely on correlation-based approaches and remain qualitative. Framing the study as a measurement report would highlight the clear strength of the work, especially the quality and breadth of the observations, while avoiding over-interpretation of secondary formation mechanisms and source contributions that cannot be constrained by the present dataset. Therefore, we suggest revising the manuscript title to reflect the measurement report, highlighting the detection of low molecular weight amines

This review reflects a joint assessment by two independent reviewers, and we have discussed the manuscript together. The comments below, therefore, represent a consolidated perspective of the submitted preprint.

General comments:

The authors present total suspended particle (TSP) filter-based measurements collected during a Chinese oceanographic cruise over the Yellow Sea and Bohai Sea (BS). Chemical analysis using ion chromatography was performed to quantify six major protonated amine species and several other organic and inorganic ions. Particle-phase amines have been reported in the Chinese coastal marine atmosphere due to their role in secondary aerosol formation. Considering the breadth of existing research on particulate amines, particularly in the Chinese coastal environment, the author should clearly highlight the novelty and added value of this work. Given that the measurements were conducted in 2018, the authors should clearly articulate how this dataset advances existing knowledge beyond what is already known.

There are numerous studies focused on the coastal/marine regions of China. While the authors have cited many of those, they have missed a few relevant studies (e.g., Chen et al., 2022; Huang et al., 2018; Du et al., 2021). In addition, a recent perspective review article by Kanawade and Jokinen (2025), which discusses challenges in detecting minute levels of amines in both gas- and particle-phases and emphasises the importance of such measurements, would be highly relevant.

Lines 19-22: Amines and other chemical components were not directly detected or measured in the air; rather, they were extracted from aerosol samples collected on filters. This distinction should be clearly stated throughout.

Line 72: Myriokefalitakis et al. assume that amines account for approximately one-tenth of the ocean-derived ammonia; however, this contribution may be up to three orders of magnitude lower in reality, which would substantially reduce the estimated role of amines in marine SOA formation. This limitation should be stated more clearly.

Line 120:125: These statements are very. They should be replaced with specific scientific questions or hypotheses that clarify the study’s added value to our understanding of amines in Chinese coastal regions, particularly in the YS-BS region.

Line 187 -194: TMDEA concentrations in TSP are compared with values from earlier studies that reported TMDEA either in PM10, PM2.5 or PM1. Further, most of these studies represent different seasons and sampling periods. While the reported concentrations appear broadly comparable, one-to-one quantitative comparisons are not appropriate. Grouping previous studies by region (e.g., SYS, NYS, BS) may be more informative, given the clear spatial variability of amine and other species. The authors should also comment on concentration differences arising from particle size cut-offs (TSP versus PMn). In my opinion, Section3.1 should focus primarily on studies conducted in the YS-BS region during spring (April), with only brief reference to other regions or seasons.

Amines exhibit a north-to-south decrease, and TMDEA is the predominant amine species in TSP over the YS-BS. However, TMDEA, despite constituting the largest fraction of total amines, does not appear to show a clear north-to-south gradient, whereas MA and EA do. This inconsistency should be addressed

Although the correlations are statistically significant at the 95% (?) confidence level, the number of samples is relatively small. The authors should discuss how the limited sample size may affect the interpretation of the results, particularly given that most analyses rely on a correlation-based approach (Figs. 2,3,4, and S6).

Additional comments on the interpretation of secondary formation processes:

The manuscript frequently refers to “secondary formation of amines” in the atmosphere. From an atmospheric chemistry perspective, it would be important to clarify more explicitly that low molecular weight amines are predominantly primary emissions in the gas phase, while their occurrence in the particle phase is largely driven by secondary particle-phase formation and partitioning processes (e.g., acid–base reactions, displacement reactions, and dissolution into aerosol liquid water). As currently written, some statements may be interpreted as implying the atmospheric production of new amine molecules, rather than secondary processing of primary gaseous amines.

Related to this point, the reported percentage contributions of nitrate- or sulfate-associated secondary formation should be interpreted with caution. These estimates are based on correlation analyses and ion-weighted regression approaches and therefore represent semi-quantitative indicators of association, rather than quantitative source apportionment or mechanistic yields. We recommend that the authors clearly state these limitations and frame the reported percentages accordingly.

Several of the proposed formation pathways (e.g. preferential sulfate-driven formation of TMDEA, displacement reactions involving ammonium salts) are chemically feasible and consistent with previous laboratory and theoretical studies. However, given that the dataset is based on filter-based TSP measurements and lacks gas-phase amine data or size-resolved aerosol information, these mechanisms cannot be uniquely constrained by the observations alone. A clearer distinction between observational evidence, inferred associations, and hypothesised mechanisms would strengthen the discussion.

Finally, while the manuscript discusses implications for new particle formation and aerosol growth, it should be emphasised that the present measurements do not directly probe nucleation or early growth processes, which are typically governed by submicron particles and gas-phase precursors. Any conclusions related to NPF should therefore be framed qualitatively, highlighting implications rather than direct observational evidence.

Minor comments:

Lines 17: modify as aerosol “physiochemical” properties

Line 103: For consistency, replace “the Bohai sea (BS) and Yellow Sea (YS)”  with “the Yellow Sea (YS) and Bohai Sea (BS)” throughout.

Line 130: Please clarify why a cyclone or any size-selective inlet was not used. Also indicate the sampling duration (e.g., 24 hours?)

Line 137: Explain acronyms SYS, NYS

Line 147: IC systems are used to separate, identify, and quantify compounds in aerosol samples collected on filters, not to measure or detect them directly in ambient air. The terminology should be revised accordingly.

Table S2: Some water-soluble inorganic ions are “0.0”. Please clarify whether these values were below DL or not detected. All chemical components should be reported in consistent units (e.g., in this case, ng m^-3); some values are currently given in µg m^-3.  Indicate whether values represent means or medians and include ranges (min-max). Align the “Component” column to the left. Please also indicate the number of data points available for SYS, NYS, and BS.

Line 171: Backward air-mass trajectories should specify the starting hour, given that sampling was conducted over ~24 hours. The authors should also clarify whether the air mass was homogeneous throughout the sampling period (24 hours) and whether it consistently originated from a particular direction. Additional trajectory analyses for individual sampling time points would help address potential air mass variability during the sampling period.

Line 241 & Fig. 2: The ratio seems stable (linear correlation). Is the difference between source areas significant?

Line 249: Amines are strong bases; ammonia is a weak base. I find it difficult to follow why the correlation between amines and ammonium relates to nucleation pathways without having particle number size distribution measurements. Line 269-270: OP-LW, OP-WP, WP-SP, SNA? Explain please.

Fig 1: Red-orange and blue-lighter blue colors too close to one another. Difficult to distinguish which is which. Could you use another color scheme?

Fig 2 & S5: Define line at 1.0? What is it for? What does the marker size represent?

Fig S4: Use a colorbar instead of symbol size to represent spatial distributions of amine, and different symbols for location (SYS, NYS and BC).

Line 330: Replace “6 isoprene SOA” with “Six isoprene SOA”

Line 402: Replace “Formula” with “Equation” throughout

References

Chen, D. et al. Competitive uptake of Dimethylamine and Trimethylamine against ammonia on acidic particles in marine atmospheres. Environ. Sci. Technol. 56, 5430–5439 (2022).

Du, W. et al. Particulate amines in the background atmosphere of the Yangtze River Delta, China: Concentration, size distribution, and sources. Adv. Atmos. Sci. 38, 1128–1140 (2021).

Huang, X., Kao, S.-J., Lin, J., Qin, X. & Deng, C. Development and validation of a HPLC/FLD method combined with online derivatization for the simple and simultaneous determination of trace amino acids and alkyl amines in continental and marine aerosols. PLOS ONE 13, e0206488 (2018).

Kanawade, V.P., Jokinen, T. Atmospheric amines are a crucial yet missing link in Earth’s climate via airborne aerosol production. Commun Earth Environ 6, 98 (2025). https://doi.org/10.1038/s43247-025-02063-0

This manuscript presented ship-based measurements of low molecular weight amines in aerosol samples from the Bohai Sea and Yellow Sea, China. They discussed the sources and secondary formation pathways of the studied amines based on an extensive chemical dataset, which has not been reported before. They also found that not only primary emissions but secondary formation was important for marine amines, and this was a very important finding. The topic was relevant to atmospheric chemistry and marine–continental interactions, and the manuscript was generally well organized and clearly written. I therefore recommend possible publication after minor revision.

From a presentation perspective, the in-depth discussion occasionally shifts from summarizing observational results to reviewing general chemical mechanisms reported in previous studies, without clearly indicating the added value of the present study. For example, several paragraphs focus on describing nitrate- and sulfate-related reaction pathways, while the specific constraints provided by the this cruise observations are less explicitly highlighted (but this part was very important). Clarifying which statements are directly supported by the measurements and which serve as contextual background would improve readability and focus.

I also suggest to remove all parts related to new particle formation as this part is not relevant for the current study. 

Details: 

1.    Lines 24–25

“corresponding to the declined influence of terrestrial air masses”

Replace “declined” with “decreasing”.

2.    Lines 31–32

“interacting with BSOA formation”

Consider rephrasing to clarify whether this refers to concurrent variability or shared environmental/chemical conditions.

3.    Lines 44–48

The sentence describing the ubiquity of amines in both the gas and particle phases is relatively long and could be split to improve readability.

4.    Lines 75–76

“associated with biological activities”

Please specify whether this refers to marine biological activity, microbial processes, or biological activity in general.

5    Lines 233–234

“suggest that MA, EA, and DMA shared similar sources”

Revise verb tense to present tense (“share similar sources”).

6    Lines 239–240

Consider revising “generally constitute a small fraction” to “constitute only a minor fraction”

7    Lines 270–272

not clear. 

8    Lines 284- 285

“distinct emission sources and formation mechanisms”

Please clarify what “distinct” refers to (e.g., primary vs secondary, marine vs terrestrial, nitrate vs sulfate).

9    Lines 550–552

“Different from other amines …”

Suggested revision: “In contrast to other amines, TMDEA was predominantly (>60%) generated …”

10. The manuscript would benefit from improvements in language clarity and figure presentation in general.