Summary

Su et al. conducted an analysis of the chemical composition of ambient aerosols collected during a research cruise in the Ross Sea, with a specific focus on calcium enrichment in sea spray aerosol. However, their claims of providing insight into the calcium enrichment process in sea spray aerosols are not convincing for several reasons. Firstly, the authors cannot claim that they exclusively probed sea spray aerosol since their measurements of ambient aerosol contained other aerosols derived from various sources, including blowing snow and ice, long-range transported aerosols, and secondary aerosols formed from gaseous precursors. Therefore, it is recommended that this claim be removed from the title and text. Secondly, the authors' analysis is limited to correlating calcium enrichment with environmental variables such as wind speed and air temperature, without providing statistical or in-depth analysis of the meteorological or oceanographic conditions. Consequently, the manuscript reads more like a measurement report than a research article. Furthermore, to enhance the manuscript's readability, it is necessary to improve the writing structure at the sentence, paragraph, and section levels. Therefore, I believe that the work should be rejected in its current form and resubmitted as a measurement report. However, if the authors do wish to pursue publication of this work as an original research article, they should perform a more comprehensive analysis of their data and the conditions under which their measurements were made. Detailed suggestions for improvement are provided below.

Major points

• The manuscript would benefit from improved writing clarity and readability. Specifically, the authors could consider breaking down long sentences into shorter ones and using an active voice to formulate their ideas more clearly. Verb tenses are also used inconsistently throughout the text, and this should be addressed. In the minor points below, I provide specific examples to help the authors improve their writing.
• In my opinion, the description of the methods used to obtain the samples is not adequately detailed. Firstly, the text in section S2 should be integrated into the main manuscript. Secondly, I suggest including statements about the efficiency of particle and gas collection in the sampler in the methods section of the manuscript. While I assume this has been previously tested, it would be helpful to have a statement such as "As shown in previous studies, 99% of particles entering the sampler were retained on the impaction plate." If not all particle sizes are efficiently sampled, a statement about the effective particle size ranges sampled by the system should be included instead. The same applies to the gas phase sampling in the denuder. It would be helpful to know how effective it is and what fraction of standard acid/base gases are sampled by this system.
• I believe that the main manuscript would benefit from the inclusion of two key figures. Firstly, a map displaying the measurement stations would help readers orient themselves. Secondly, a schematic of the sampling system should also be included in the main manuscript. Additionally, it might be helpful to include a map showing the distribution of calcium enrichment as part of the results section to enhance the reader's understanding of the observations.
• In my opinion, the content of text S1 appears to be more suitable for the introduction and discussion sections of the manuscript rather than as supplementary information. If the authors consider this information important for readers to understand their study, they should incorporate it into the main manuscript. The supplementary information should not be used for discussions that are integral to the main content of the study.
• Could the authors provide more details on how they prevented their research vessel from affecting their measurements? It is unclear whether they used a pollution control system or sector analysis to reduce potential ship emissions during their measurements. Additionally, while they used a PM10 cyclone to capture larger particles, it is not clear whether they took measures to eliminate the influence of smaller particles being emitted from the ship. To improve the clarity of their study, the authors should provide more information on these points.
• The authors propose that blowing snow may be responsible for the observed Ca2+ enrichment in the aerosols they collected. However, this contradicts their earlier assertion that their study provides insights into Ca2+ enrichment in sea spray aerosol. Furthermore, their analysis to support this conclusion lacks depth. While the association between sea-ice coverage and Ca2+ provides a clue, the authors should conduct a more thorough investigation. For example, they could explore how long a specific air mass they sampled spent over ice and examine correlations between wind speed and sea-ice during sampled air-mass trajectories.

Minor points

Line 23 – I suggest this sentence is rephrased as follows: “Although calcium is known to be enriched in sea spray aerosols (SSA), the factors that control its enrichment remain ambiguous.” Calcium can not have a mixing state – it is SSA that has a mixing state.

Line 24 – I suggest the authors break this sentence in two to improve the clarity and readability and to make clear the research objectives and the source of data: “In this study, we examine how environmental factors affect the distribution of water-soluble calcium (Ca2+) in sea spray aerosols (SSA). We obtained our data from observations taken during a research cruise on R/V Xuelong in the Ross Sea, Antarctica, from December 2017 to February 2018.”

Line 27 – In order to improve the clarity of the sentence I suggest the authors use active voice and rephrase to make it a statement of the study’s findings: “Our observations show that the enrichment of Ca2+ in aerosol samples is enhanced under specific conditions, including lower temperatures (< -3.5 ℃), lower wind speeds (< 7 m s-1), and the presence of sea ice.”

Line 28 – I suggest the authors break this sentence in two to improve the readability. I also suggest the use of “inaccurate” rather than “neglected” to more accurately describe the potential problem with current estimates of Ca2+ enrichment: “Our analysis of individual particle mass spectra revealed that a significant portion of calcium in SSA is likely bound with organic matter (in the form of a single-particle type, OC-Ca). This finding suggests that current estimates of Ca2+ enrichment based solely on water-soluble Ca2+ may be inaccurate.”

Line 31 – I suggest the authors rephrase this sentence as a statement of the study's unique contribution: “Our study is the first to observe a single-particle type dominated by calcium in the Antarctic atmosphere.”

Line 32 – I suggest the authors rephrase this sentence to clarify the specific aspect of the modeling that needs to be addressed and to make it a recommendation based on the study's findings: “Our findings suggest that future Antarctic atmospheric modeling should take into account the environmental behavior of individual OC-Ca.”

Line 34 - I suggest the authors improve the clarity and readability of the sentence by rephrasing it as a statement of the study's importance: “With the ongoing global warming and retreat of sea ice, it is essential to understand the mechanisms of calcium enrichment and the mixing state of individual particles to better comprehend the interactions between aerosols, clouds, and climate during the Antarctic summer.”

Introduction in general - The text could benefit from using active voice to make it more engaging and easier to read. For example, instead of "The extent of enrichment and chemical signature of calcium may affect some physicochemical properties of SSA," use "Calcium enrichment and chemical signature can affect the physicochemical properties of SSA."

Introduction in general - Break up long sentences: Some sentences are quite long and complex, which can make them difficult to read and understand. Breaking them up into shorter, more concise sentences could help.

Introduction in general – Use consistent verb tense and try to present conclusions first and then the sources that support the conclusions to make it easier for readers to follow. For example Line 58, "A growing number of studies have shown that calcium (Ca2+) is significantly enriched in SSA relative to bulk seawater (Table S1) (Keene et al., 2007; Hara et al., 2012; Cochran et al., 2016; Salter et al., 2016; Cravigan et al., 2020; Mukherjee et al., 2020)" could be rephrased as “Several studies have demonstrated a significant enrichment of calcium (Ca2+) in SSA compared to bulk seawater, as presented in Table S1 and documented by Keene et al. (2007), Hara et al. (2012), Cochran et al. (2016), Salter et al. (2016), Cravigan et al. (2020), and Mukherjee et al. (2020).”

Introduction in general - Define acronyms when they are first used: Some acronyms are used without being defined, which can be confusing for readers who are not familiar with the field. For example, "SSA" is used multiple times without being defined until later in the text.

Introduction in general - Some points could be clarified or expanded upon to help readers understand the context better. For example, on line 53 what do the authors mean by “the most efficient gelling agent”?

Line 69 – This sentence does not read well and it is unclear exactly what the authors mean. I assume that what the authors mean is that our current understanding of the enrichment of Ca2+ in SSA is the result of measurements of only water-soluble Ca2+. If that is the case the authors need to make this point plainly. The authors then need to inform the reader of what alternatives there are. Presumably, measurement approaches that determine not only the amount of water-soluble Ca2+ but also insoluble Ca2+ in the form of calcareous shell debris or the like could be used. Here would be a good point to outline the difference clearly.

Line 70 – I would argue that the authors need to do a better job of describing the two hypotheses for why Ca2+ may be enriched in SSA. As they state one possible mechanism is the complexing of Ca2+ to organic matter. A second possible mechanism Ca2+ ions are enriched close to the water surface in the form of ionic clusters most probably with carbonate ions.

Methods section 2.1  – I suggest the authors separate the information in this section into three distinct paragraphs to improve organization. E.g. one starting “Our study focused on the Ross Sea region of Antarctica (50 to 78° S, 160 to 185° E) (see Fig. S1), where we conducted two separate observation campaigns aboard the R/V Xuelong…”. A second paragraph starting “The first observation campaign (Leg I) took place from December 2-20, 2017, during the sea ice period. The second campaign (Leg II) was conducted…” and a third starting “The sampling design for Leg I and Leg II aimed to investigate the differences in atmospheric aerosol characteristics…” etc.

Methods section 2.1  – I suggest the authors use more precise terminology (e.g., "observation campaigns" instead of "observations carried out"; "sampling design" instead of "sampling")

Methods section 2.1 – I suggest the authors remove redundant or unnecessary phrases, such as "hereafter" and "when the ocean was covered by sea ice" (since this is already clear from the "sea ice period" description).

Methods section 2.2 – Again I think it would help clarity if the authors rephrased using active voice e.g. “We measured various meteorological parameters, such as ambient temperature, relative humidity (RH), wind speed, and true wind direction using an automated meteorological station located on the top deck of the R/V Xuelong…” and “To determine the type of air masses, we used the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectories (HYSPLIT, version 4.9) model to perform 72-hour back trajectory analysis…” and “Additionally, we obtained the monthly sea ice fraction from the Sea Ice Concentration Climate Data Record with a spatial resolution of 25 km…” etc.

Line 137 - Always be specific. As such this would read better as: “Throughout the observations, the mean Na+ and Ca2+ mass concentrations were 364.64 ng m-3 (ranging from 6.66 to 4580.10 ng m-3) and 21.20 ng m-3 (ranging from 0.27 to 334.40 ng m-3), respectively, which were more than 10 times higher than the detection limits.”

Line 140 – The sentence makes no sense to me. How can wind speed alone be used to rule out the potential influence of the research vessel on their measurements?

Line 180 – You need to state that 0.038 is the molar ratio and not the mass ratio of Ca2+ to Na+ since the latter is approximately 0.066 (for every gram of sodium in seawater, there are about 0.066 grams of calcium).

This manuscripts present data on Ca enrichment in sea spray aerosol. This is an interesting topic in atmospheric research because of the CCN activation of sea spray aerosol particles. The mechanism of Ca enrichment is not yet fully understood. The data set itself (from a ship cruise at Antarctica) is rare and valuable.

However, there are some weaknesses in the presentation and interpretation of the results. Especially the single particle results are presented as measured, but connection to the main question (i.e. the mechanism behind the Ca enrichment) is not made clear. I don't think that it is unexpected that Ca and organics are internally mixed in sea spray particles. However, how the organic Ca helps to explain the Ca enrichment is not clear to me. 

Thus, I think that there are major revisions necessary before the manuscript can be accepted for ACP.

My comments and concerns are listed in the following:

General comments

Manuscript structure: 

The main text of the manuscript is rather short. Many of the important and interesting discussions regarding methods and uncertainties are only found in the supplement. This is unsual for an ACP manuscript and make me suspect that the manuscript had been originally submitted somewhere else.

I recommend moving the following parts of the supplement into the main text:

S1: merge with section 4

S2: merge with section 2.3

from S3: lines 123 to 141: move to section 4

S6: move to section 3.2

S8: move to main text, maybe as an additionally results subsection

Calcium enrichment:

It is known that calcium is enriched in sea spray. The data from the IGAC instrument confirm this nicely. The results show that the highest enrichmet factors are found at low temperate, low wind speed and sea ice conditions. This is a solid result, but I am no expert in this field and can not judge whether this is new or not.

Single-particle analysis:

The abstract suggests that the controlling factors of the Ca enhancement wich are still unknown are studied in this manuscript. 

It is not clear to me what the results of the mansucript mean for the controlling factors.

A calcium-dominated OC-Ca particle type is detected. This particle type dominates the Ca-containing particle types, but it is not clear if these particles really represent microgels. The process how the biological organic material and the calcium end up in the same particle can not be identified from SPMS data alone.

Particle size range:

The difference in the size rane of the two techniques is only discussed in the supplement. This is an important point when it comes to comparing the results of the two techniques. I suggest to move this part (supplement lines 123 - 141) into the main text (see comments above).

Furthermore, it is not only the size range, but also a comparison between number fractions or peak intensities (SPMS) and mass concentrations (IGAC). This should also be discussed.

The OC-CA particle size distribution as displayed (Fig 3 i) centers around 1 µm, but this may be an instrumental effect (transmission and detection efficieny of the SPAMS). A plot of particle number fractions per size bin versus particle size will better show the contribution of each particle type. 

 

Trajectories:

More trajectories are needed. Either finer time steps (one trajectory per hour) or the "ensemble mode" of HYSPLIT, where many trjectories are calculated per start time with slight differences in the starting conditions. Only through such ensembles one can see the variations between individual trajectories and can judge the reliability of the backward calculation.

Specific points

Section 3.2

What do you mean by "were further refined with an ion signal of m/z 40 [Ca]+."? Was there any threshold applied to the intensity of m/z 40 or were all mass spectra selected that had a signal > 0 at m/z 40? Please explain. 

Maybe this should be explained already in section 2.3.2 (lines 160-170)?

Figure 3:

i) the size distributions look like Gauss fits. Please confirm if they are. I suggest using log scales for both axes. For the diameter axis, this is very common in aerosol science, and for the y-axis it will make particle types with low concentrations better visible. On a log scale, the Gauss fits should be replaced by lognormal fits.

Minor points

Title: remove the "through", e.g.:

Enrichment of calcium in sea spray aerosol: Bulk measurements and individual particle analysis during the R/V Xuelong cruise over the Ross Sea, Antarctica

Heading of section 2.2:

Meteorological