the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Mar 2026
Impact of anticyclonic eddies on the spatial distribution and emission of non-methane hydrocarbons in the northern South China Sea
Abstract. Non-methane hydrocarbons (NMHCs) are important trace active gases that exert significant impacts on climate. Ubiquitous mesoscale eddies likely act as a key physical process regulating the marine emission of these gases, yet the underlying mechanisms remain poorly understood. Herein, we characterized the distributions and emissions of NMHCs in the South China Sea, with particular emphasis on the impacts of an anticyclonic eddy. Significantly lower NMHC concentrations were observed within the eddy-controlled region (201 ± 101 pmol L−1) relative to the reference sites (433 ± 62.5 pmol L−1). Downwelling in the anticyclonic eddy core reduced surface nutrient availability, suppressing the biological production and surface concentrations of alkanes and isoprene, whereas lower alkene levels were mainly driven by weakened photochemical production. NMHC sea-to-air fluxes dropped by 56 % within the eddy, which further diminished ozone and secondary organic aerosol generation by 59% and 60%, respectively. Overall, our findings elucidate the regulatory role of mesoscale eddies in NMHC dynamics, highlighting their critical function in shaping marine trace gas cycling and associated environmental consequences.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-1212', Anonymous Referee #1, 29 Apr 2026
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AC1: 'Reply on RC1', Hong-Hai Zhang, 02 Jun 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-1212/egusphere-2026-1212-AC1-supplement.pdf
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AC1: 'Reply on RC1', Hong-Hai Zhang, 02 Jun 2026
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RC2: 'Comment on egusphere-2026-1212', Anonymous Referee #2, 07 May 2026
This manuscript discussed the impact of an anticyclonic eddy on the distribution, production, and emission of NMHCs in the South China Sea. The study combines field observations and deck incubation experiments, providing valuable insights into the coupling between physical oceanographic processes and marine trace gas variability. However, several issues need to be addressed, particularly regarding the clarity of interpretation, the description of methodological details, and redundancy in certain parts of the text. Some conclusions require more precise wording to avoid potential overinterpretation. Overall, the manuscript is well structured, and the results are potentially of interest to the community. After these issues are carefully addressed, it could be suitable for publication.
Major comments:
Line 22: “which further diminished ozone and secondary organic aerosol generation by 59% and 60%” it may be misleading and could be interpreted as referring to total atmospheric O₃ and SOA production. It is more likely that these reductions apply only to the fraction attributable to NMHC emissions from the ocean. The authors should revise this statement to explicitly indicate that the reported decreases refer to NMHC-derived contributions, in order to avoid overinterpretation.
Line 49: “exerting substantial impacts on atmospheric chemical processes within the marine boundary layer.” are there any specific impacts about atmospheric chemical processes from the reference? it seems repetitive of the statement in Line 28 the “exert substantial influences on atmospheric reactivity and global climate patterns”. The authors are encouraged to either specify the distinct processes (e.g., OH reactivity, ozone formation pathways, etc.)
Line 67-79: The background description of mesoscale eddies in the South China Sea is somewhat limited. Authors should provide more information about the eddies in SCS, like numbers, types, frequent, already reported eddies impacts on marine biogeochemistry, etc.
Line 89: more information about AE is needed. Clarify how to determined its life? What kind of data is depending on? “sampled at the end of its intensification” how many stages are typically defined in eddy evolution, and what criteria were used to define these stages in this study? A clearer description of the formation, evolution, and decay of the AE would strengthen the interpretation of the results.
Line 109: Only 8 species of NMHCs were measurement. I think it is crucial to point this study focus on limited species in the abstract, e. g., “eight C2-C5 NMHCs”, so that readers can better understand the scope of the results.
Minor issues:
Line 25: change “consequences” to effects.
Line 53: “well established”? I think it is only well recognized that eddies have important influence on marine biogeochemistry.
Line 60: how to understand “reshape phytoplankton community dynamics”, delete “dynamics”
Line 61: change “phytoplankton productivity” to “primary production” or “phytoplankton biomass”
Line 84: how can “survey period” revealed those? Rewrite this sentence.
Line 119: provide the calibration curves in the SI
Line 223: “37.3 ± 25.1 129 ± 47.1,” missed a ,
Line 225: “85.8%–383% higher than those of alkanes” chang it to “xx-xx times higher than”
Line 227: change “AE-dominated” to “AE-affected”
Line 250: “far exceeding the changes observed for other compounds” list the specific changes of other compounds.
Line 267: why are there so many empty cells in the table 1? Authors should provide the rates as mean ± standard deviation based on triplicate experiments.
Line 273: “Surface nutrients were nearly undetectable within the AE,” if undetectable, say undetectable, else show the data
Line 281: provide the t-test results
Line 300:” because the zonal extent of the study region was only 4°” It would be more intuitive to express the zonal extent in kilometers or nautical miles rather than 4°.
Line 306: provide the t-test results
Line 323: “Assessing the impact of NMHCs on global climate feedback requires precise estimates of their sea-to-air exchange fluxes.” This sentence is meaningless in the beginning of the paragraph.
Line 348: change to “demonstrating that the anticyclonic eddy suppressed the release of NMHCs.”
Citation: https://doi.org/10.5194/egusphere-2026-1212-RC2 -
AC2: 'Reply on RC2', Hong-Hai Zhang, 03 Jun 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-1212/egusphere-2026-1212-AC2-supplement.pdf
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AC2: 'Reply on RC2', Hong-Hai Zhang, 03 Jun 2026
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RC3: 'Comment on egusphere-2026-1212', Anonymous Referee #3, 27 May 2026
This paper reported the observation results of NMHCs in the south China Sea. Observation was well organized. NMHCs were lower concentration within the eddy-controlled region. This would be precious observation results.
OH reactivity, Ozone formation potential, and particle formation from the atmospheric light NMHCs were discussed. But the calculated values (LOH, OFP, PSOAP) were just compared with other observation sites. I think if the estimation of these values of DMS would be informative, especially PSOAP. If DMS observation was not available, just assume suitable DMS concentration and compare these values. Then we can evaluate the importance of light NMHCs over the surface ocean.
Line 164-167: In the explanation of Eq.(1), units of Fux, k, Cw, Ca, (and H) are given (pptv etc.). These are correct for dimension. But these are not directly to use for calculation.
This is also for Eq(6). (NMHC concentration would be molecules/cm^3 for calculation. )
Also same for Eq(7), Eq(8). (the unit of NMHC concentration should be micro g/m^3 for OFP etc.)
Question about Deck incubation experiments.
In Table 1 , there are four “Photochemical production rates” : natural light, UVB, UVA, PAR
“natural light” seems to be the sum of UVB + UVA + PAR.
There are four experiments (Line 144-148).
(1) Natural light
(2) PAR + UVA
(3) PAR
(4) (dark)
Do you deduce the “Photochemical production rates” as follows?
Natural light: (1)-(4)
UVB : [(1)-(4)] – [(2)-(4)]
UVA: [(2)-(4)] – [(3)-(4)]
PAR: (3)-(4)
Citation: https://doi.org/10.5194/egusphere-2026-1212-RC3 -
AC3: 'Reply on RC3', Hong-Hai Zhang, 04 Jun 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-1212/egusphere-2026-1212-AC3-supplement.pdf
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AC3: 'Reply on RC3', Hong-Hai Zhang, 04 Jun 2026
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- 1
General comments: This study was designed to investigate the impact of anticyclonic eddy systems on the distribution and oceanic efflux of an array of non-methane hydrocarbon compounds (NMHCs) and the projected atmospheric impact in the South China Sea. The measurements in this manuscript show that 1) the concentrations and oceanic emissions of NMHCs were significantly lower in eddy-influenced stations compared to the reference stations; 2) the photoproduction of NMHCs was slower in eddy-influenced waters compared to the reference waters. This work is of significance because the biogeochemical cycling of NMHCs and other volatile organic compounds (VOCs) has not been thoroughly studied in mesoscale eddies. However, the most critical issue with this work, in my opinion, is the lack of clarification on the method of statistical analysis on multiple occasions that has compromised the quality of this manuscript in telling a compelling story. That’s being said, statistical analyses must be performed throughout the manuscript to truly tell if these differences are significant, particularly when error propagation is involved in this work. Please see the texts below for the major and minor issues that I believe need to be addressed for the manuscript to be of a better quality. Overall, I think this work is well designed and carries significant scientific merits by providing new knowledge to the study of oceanic NMHCs in the system of mesoscale eddies. Thereby I believe it’s suited for publication in Biogeosciences after revision.
Major issues and recommendation: 1) This study relies on the comparison made for the measurements between eddy-impacted water and reference waters. However, the uncertainty associated with these measurements and the method of statistical analysis are missing on many occasions in the manuscript. For instance, in the Abstract (Lines 16-19), the authors claimed that the difference between the two means of NMHC concentrations was significant. What statistical analysis was performed that led to this conclusion? What is the sample number? Can authors clarify if the data are normally distributed? This is just an example. Authors should check through manuscripts and ensure proper statistical analyses are coupled with comparisons of means.
2) The authors should acknowledge that only four photochemical experiments were done in the field that a robust assessment cannot be made here in any capacity for the photoproduction rates of NMHCs between eddy-influenced and reference stations. I understand that field work is labor intense and it’s of great value to collect preliminary results. For future, 1) more photochemical experiments should be carried out; 2) I recommend authors think about what the rationales are that a difference is expected for their photoproduction in eddy vs reference stations; 3) Authors should also measure the CDOM absorbance in these waters to see if there is a difference in absorbance; 4) The most effective approach to study spectral dependence of a photochemical process is to determine wavelength-dependent apparent quantum yields. The crude spectral study, using optical treatments, is less valuable as it’s well known that marine photochemistry is a UV-controlled process particularly during field work at sea. I’d rather invest the valuable sea time in performing more photochemical experiments that would afford one a larger dataset.
Minor issues: The writing of this manuscript in English language could benefit from further polishing for a more concise and accurate communication of your research.
Specific comments:
Line 16: Get rid of “particular”
Line 17: Were these concentrations determined in water samples from the near-surface?
Lines 21-22: It’s not that the flux dropped by 56% in the eddies. I believe you meant that the flux in the eddies is 56% lower than reference sites. Please have this sentence rewritten if that’s true.
Line 29: Add a dot (•) to the side of OH to make a radical.
Line 74: Get rid of “systematically”, and I’d avoid this type of “empty” words in scientific writing.
Line 76: Get rid of “formation”.
Line 95: Add “photochemical” before incubation experiments.
Line 93: Can you clarify what that 0.5 m/s on the figure to the right shows?
Line 100: Just simply say “samples were collected in duplicate at 4 stations”.
Line 144: Change “light” to optical.
Line 145: To my knowledge, the term “visible light” rather than PAR is used more often in photochemical studies.
Lines 159-160: Are there any studies supporting this statement about photolysis of NMHCs in seawater? If so, provide the references. If not, get rid of this statement since it’s meaningless. I highly doubt any of these NMHCs photolyze in ambient seawater. What would be the photolysis mechanism(s)?
Line 211: Authors should take time to verify the scales on the y-axes of these plots. As an example, the scale for the DOC plot seems to be too broad if you look at the color scheme in the plot. There is no dark red in the plot.
Lines 218-219: please provide statistics for the DOC comparison like you did for Chl a. Are these two means truly statistically different?
Line 22, section 3.2: As a general comment, the concentration or standing stock of an NMHC species measured is always a net result of its production and removal. Are there any previous studies that investigated the impact of eddies on bacterial activity or more specifically on the consumption rates of NMHCs or any other biogenic products?
Lines 247-248: Make your writing concise by simply saying “In 0.2µm-filtered seawater”. In the method, you already clarified about the purpose of filtration.
Lines 252-254: what is the sample number n?
Lines 257-260: what is the sample number for eddy, eddy edge, and refence stations? Very important to show these information.
Line 266, Table 1: Change “natural light” to Full spectrum because that’s the comparison you are showing Full spectrum vs Specific spectral bands (e.g. UVB). Please clarify why there are these blank spaces. Experiments were not performed at these stations or data were problematic? Also, please provide the concentrations in your dark controls as it’s so important to know the level of [ ]s in the dark. You said that triplicate quartz tubes were irradiated. Can you provide the uncertainty associated with these measurements? Can you actually say with confidence that the production in PAR is significant since the production is so low?
Line 281: these differences in Chl a are statistically different? They are definitely not substantially different.
Line 298: It’s not entirely accurate to say “wavelength dependence” because the way you did in this work can only give you an idea of the crude spectral dependence. The best way to do it is using a monochromatic system or even a polychromatic system with narrower spectral range.