the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jan 2025
Origin, size distribution and hygroscopic properties of marine aerosols in the south-western Indian Ocean: report of 6 campaigns of shipborne observations
Abstract. Marine aerosol observations from 6 shipborne campaigns carried out in 2021 and 2023 in the southwest Indian Ocean are presented. A set of aerosol instruments is used to study the spatial and temporal variability of the aerosol size distribution, cloud condensation nuclei (CCN), activation diameters, and hygroscopicity (κ). Total number of aerosols (Na) shows concentration above 1500 cm-3 in polluted areas, and between 100–1500 cm-3 in the open ocean. CCN measurements (0.2 %, 0.4 % supersaturation) range from 40 to 500 cm-3. At 0.2 % (0.4 %) supersaturation, the average activation diameter is 104 (76) nm and κ is 0.36 (0.25). Using a back-trajectory model, the aerosol data were classified into three source regions. Aerosols are hydrophobic in the continental group (κ from 0.1 to 0.13), hydrophilic in the Subtropical Indian Ocean group (κ from 0.24 to 0.4), and intermediate values are found in the Southern Indian Ocean group (κ from 0.17 to 0.22). Subtropical Indian Ocean κ increases with stronger wind speeds. Southern Indian Ocean κ decreases significantly (between 0.2 % and 0.4 % supersaturation) with stronger wind speeds, probably due to a higher concentration of organic species on the smallest particle surface. High aerosol concentration events are presented. Pollution related to air masses passing through a well-developed continental boundary layer. Nucleation triggered by clear skies between two cloudy periods. Arrival of air masses at the ship’s location after a precipitation event.
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RC1: 'Comment on egusphere-2024-3747', Anonymous Referee #1, 25 Feb 2025
General comments:
This manuscript by Dournaux et al. provides the characteristics on aerosol number concentrations, particle size distributions, CCN number concentrations and their hygroscopicity in several ocean regions, and discusses with the different origins between sea surface and atmosphere.
The statistical analysis, consisting of several cruise data including oceanic, atmospheric observations and meteorological elements, is important for the scientific knowledge.
The various case studies, such as anthropogenic pollution and new particle formation, are also interesting topics.
However, the overall format and tone of the paper is not consistent and there are mixed of well-structured paragraph and single sentence statements without clear causal relationships, I would like all co-authors to review the format of the paper so that it is consistent and also the structure in the manuscript is clear.
The English language in general needs to be improved.
Publication can be considered after the following points and issues have been addressed.
Specific comments:
My main concerns are:
(1) Each topic and result is presented; there are different regions that are relatively clean and mainly from marine sources, while others are affected by pollution from continental sources. The data are interesting, but the description needs to be clear, especially in the abstract, how they are ultimately linked and what the authors want to argue as a conclusion or implication of the results, not separately in individual sections.
(2) Some sentences are suggested with kappa values and previous studies but not analysed in this study itself, e.g. processes and chemical compositions, it is desirable to be specified that these are observations in this study or indications from the previous studies or literature.
(3) Regarding point (1), although this manuscript is divided into sections, I recommend that the necessary data be reported in detail first and then discussed, or merged into some sections with related topics.
For example, the topic of kappa is related to both the origin of the aerosol mass and the biological activity, so it should be discussed after the description of Figure 8.
(Abstract)
L8-11:
You describe the kappa increase in the subtropical Indian Ocean and the kappa decrease in the southern Indian Ocean, and you suggested the same reasons (with high wind speed in both cases). What is the difference between the kappa values in both cases?
Also, if the local emission with wind at the sea surface is large, the highly hygroscopic components (sea salt and sulphate) may also contribute in addition to biological organic components.
L11-13:
Last 4 sentences, revise to make the cause and effect clear between sentences.
If you want to pick up some case studies, should be written so, with your indication or suggestion of results.
(Manuscript)
L203-208:
Why do you explain the details only for the CCNC here? In this section it is necessary to briefly describe the observation information like other instruments. If you want to describe more, move to the introduction or experimental sessions.
L284-288 :
I cannot follow this part. Do you want to describe the range and the percentage of the total number concentrations? The most important thing is the representative characters rather than those of the small fraction (<10%), and the good explanation is the same as described at L299-300. It is better to use the same format here, i.e. "mostly (97%) xxx... and the rest is xxx...".)
L291:
"in this mode": What mode or range do you mean? please clarify.
L292:
"in this mode": What mode or range do you mean? Please clarify.
L306-310:
"Along this transect, the percentage of aerosols in the free troposphere is higher than..."
Can you add here the percentages of air masses from the free troposphere exactly?
L311-315:
"Peaks of Na concentration are also visible..."
Can you add here the numerical concentrations, as described in the other three regions (north of Madagascar, south-east coast of Africa, eastern part of the Indian Ocean)?
L321:
weak variation? or low concentration? It's not common as "weak concentrations".
L373-389:
The CCN-derived kappa values might give some indication, but in fact the kappa values in the ambient are determined by the combination of fractions of several chemical components, so you should describe carefully if you don't have chemical composition data or evidence.
For example, if kappa were between 0.6 and 1.0 (L386), why do you think the combination of organic matter and sulphate? not the contribution of sea salt?
If kappa were around 0.2 (L380), why do you think the organics? Are they transported or local emissions? Because it could be explained by other combinations, such as hydrophobic and hygroscopic components (BC and sulphate). If you have supporting data and reasons (e.g. biological activity), you should describe them here.
L376-377:
"which is in line with the previous observations of Na and CCN..." Please add references to the previous studies.
L404-406:
You mentioned the median and interquartile values only in this part, but you discussed the averaged kappa except this sentence.
If you want to discuss the median values, it should be added to the values and discussions before the part. If not, I recommend deleting it.
L463-464:
"This result is consistent with the low nanophytoplankton abundance in this group, which makes the κ values closer to NaCl κ"
I find the aerosols in the subtropical Indian Ocean had the most hygroscopic values among the three difference groups in Figure 8a, but it is much different from the kappa of NaCl or sea salt (about 1.2-1.3 of the kappa), cannot say "closer". Please revise.
Figure 8 and L461-476:
I cannot follow the explanation of Figure 8.
The organic fraction tends to be high in the fine particles especially high biological activity period and they can act as surfactants and affect to the CCN activation ability, as you suggested.
However, I think that the organic fractions at 76 nm and 105 nm, around the two activation diameter at 0.2% and 0.4%, is not so much different, indeed mass fraction of 60-125 nm was presented in the same size range in O’Dowd et al., 2004.
Recently, primary bioaerosol concentrations by local emission at remote oceans are well correlated with wind speed, corresponding to the wind-driven upwelling from the sea surface to the atmosphere (Kawana et al., 2021), and also discuss the marine (primary) bioaerosol formation for the submicron particles (Santander et al., 2021) by fluorescence methods in seawater and ambient samples.
Do you have any insights to explain for your results?
Can you also look at the kappa variability at 0.2% SS?
L562:
"κ values of organic species (0.22-0.24) are observed at 0.4% SS during storm events".
You derived the kappa values, but can you say these values is for organic species? or is this based on the indication from the literature?
Technical comments:
L3 (and in manuscript):
Total number of aerosols: Na should be changed to Ntotal or Naero etc, Na leads to confusing as sodium.
L177:
Use the same format in the paragraph, although you have used both number (i.e. 19 instruments) and English (three OPC-N3).
e.g. Nineteen instruments for gas and particles and remote sensing...?
L178:
Among the nineteen instruments, seven instruments...?
L208-210 (and in manuscript):
Change to the subscript, e.g. NOx, O3, ...
L259:
FLEXible PARTicle Model (FLEXPART)... the word should be spelled out the first time it appears.
L339 (and in manuscript):
Why don't you spell out "CCN number concentrations" as NCCN, like Ntotal? It's a frequency and usually NCCN is used.
Table 2:
"D = 1.7 nm" in coarse mode over the subtropical Indian Ocean -> "D = 1.7 um"
L438:
"Larger N" -> Larger Total Number Concentrations (N)
L556-557:
CCN-100 measurements -> CCN measurements
Figure captions:
Revise to use the same format with figure description and caption, i.e. (a) wind speed, (b) wave height, ... in Figure 2 (and other figures).
References:
1) Kawana, K., Matsumoto, K., Taketani, F., Miyakawa, T., and Kanaya, Y.:
Fluorescent biological aerosol particles over the central Pacific Ocean: covariation with ocean surface biological activity indicators,
Atmos. Chem. Phys., 21, 15969–15983, https://doi.org/10.5194/acp-21-15969-2021, 2021.
2) O’Dowd, C. D., Facchini, M. C., Cavalli, F., Ceburnis, D., Mircea, M., Decesari, S., Fuzzi, S., Yoon, Y. J., and Putaud, J.-P.:
Biogenically driven organic contribution to marine aerosol, Nature, 431, 676–680, https://doi.org/10.1038/nature02959, 2004.
3) Santander, M. V., Mitts, B. A., Pendergraft, M. A., Dinasquet, J., Lee, C., Moore, A. N., Cancelada, L. B., Kimble, K. A., Malfatti, F., and Prather, K. A.:
Tandem Fluorescence Measurements of Organic Matter and Bacteria Released in Sea Spray Aerosols,
Envrion. Sci. Technol., 55, 5171–5179, 2021.
Citation: https://doi.org/10.5194/egusphere-2024-3747-RC1 -
RC2: 'Comment on egusphere-2024-3747', Anonymous Referee #2, 25 Mar 2025
General comments
Dournaux et al. present observations of aerosol number size distribution, cloud condensation nuclei, activation diameters, and hygroscopicity from six different shipborne marine campaigns in the southwest Indian Ocean.
The presented results are interesting and offer valuable insight into marine aerosols. The scientific basis of the study, including the campaigns and the analysis, are mainly solid and there are no major scientific issues. However, the manuscript is noticeably unevenly written, and has issues both in the grammar of the writing and the structure of the text itself. In addition, I find the discussion on the manuscript lacking. The authors fail to properly discuss the implications and argue the importance of their results. As it is, the manuscript reads more like a measurement report.
If the authors go through the manuscript carefully and address these issues presented, I believe this manuscript by Dournaux et al. has the potential to be published, and be published as a research article.
Specific comments
Abstract:
L11-L13: Poorly written, rewrite this as e.g., “High aerosol concentration events are presented, and include: pollution related air masses…”
Introduction
The introduction is a bit hard to read and has some structural and grammatical issues. Many of the paragraphs are too long, and therefore very heavy to read. The text in general lacks flow. Note that the readers typically pay most attention to the start and end of paragraphs, while skimming through the middle.
In addition, the main objectives of the study are not clearly stated.
L16: “Among them, marine aerosols...”. It would put these values more into perspective, if it were also stated how large of a fraction this is relative to the total global aerosol emissions.
L17: Please define “marine aerosol” as aerosol including all types of particles found over the oceans, regardless of point of origin.
L37: The paragraph starting here lasts for one and half pages and is difficult to read. Revise the structure so that it can be divided into multiple shorter paragraphs.
L111: “This paper is organized as follows” could start a new paragraph. I would also add the objectives/aims of the study to this paragraph, before the structure of the paper is described.
Campaigns overview and in situ conditions observed
2.2 Atmospheric and oceanic conditions observed: the whole section is one, long paragraph. Revising the structure would make it both easier to read and present the relevant information in a more clear manner.
L183: “The distance between the inlets and the instruments (8 m) was carefully chosen” Can you give a little bit more details about this as the distance between the inlets and the instruments is quite long and could result in a significant amount of diffusion and deposition losses.
L189: Can you discuss a bit on the impact of measuring the dry particles on your results?
L211: Can you specify, which variables have which time step?
Spatial and temporal variability of marine aerosols properties
As a general comment for this section (and the following section 6) and its subsections, there is a lot of results and observations. However, I would like more discussion and emphasis on their relevance. In addition, at some parts it is hard to remember what has been already discussed before in this section and how it is related to what is discussed later on.
L291: Please specify, which mode.
L293: “The difference in the location of high and low aerosol concentrations reflects…” This sentence would make more sense placed before the discussion on the processes starts, i.e., before “Sedimentation and below-cloud scavenging…”
L296: “… nucleation of sulfuric acid” To my knowledge, this is not fully accurate as a) the nucleation mechanism in varying marine environments is still poorly understood, b) both iodic acid and methane sulfonic acid have been in previous studies been identified as participating in the nucleation process in marine environments.
L308-L309: “Along this transect, the percentage of aerosols in the free troposphere...” I may have missed something, but what is the basis for this? Where does the information on the percentage of aerosols in free troposphere come from?
L322: Please specify why these values suggest that the air masses have aged.
L339: CN is defined, but not used aside from defining the ratio CCN/CN, which is not actually discussed in this manuscript.
Aerosols size distribution based on their air mass origin
6.1 Size distribution of marine aerosols: This whole subsection is one, too long, paragraph. Please revise. In addition, there should be more discussion on the relevance of the results.
L438: “Plus, the larger N in the Aitken mode of the continental air masses implies that there will be a more important coagulation process...”. I do not follow the line of reasoning here, please clarify.
L443-L450: Structurally, it would work better if throughout the section you first state what you have observed and then discuss it and possibly refer to a previous study, and then tell what else you have observed, discuss it and refer again to another study. Now the last 10 lines of this subsection are spent on referring to other studies without any discussion on how they relate to your study, while they should be spent on emphasizing your own results and their implications.
Focus on particular events
I would consider showing the same information, at least as supplementary material, for each of the cases.
Here, as in the previous sections, I wish there was more discussion of the results and not just reporting what was observed during these different cases. Do we learn anything new from these cases? How do these cases connect to previous studies? These cases are interesting, and it should be made more clear why they are interesting.
Conclusions
There’s quite a lot of detail, and the conclusions might be clearer if it was slightly more condense.
L555: Please be more exact and clarify what are the weather influences behind the highest concentrations.
Technical comments
L4, L42, rest of the manuscript: Please write Na using subscript and italics.
L29: “Low clouds, such as"
L30: “has” → “have”
L31: “changes, which”
L35-L36: "the lowest", "the greatest"
L85: > 1
L96: “to the authors knowledge” should come before “over”
L109: “composition, which”
L498: “clearly indicates” is repeated twice on sentences following each other, please consider rephrasing.
Citation: https://doi.org/10.5194/egusphere-2024-3747-RC2
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