Enrichment of calcium in sea spray aerosol through bulk measurements and individual particle analysis during the R/V Xuelong cruise over the Ross Sea, Antarctica
Abstract. Calcium is known to be enriched in sea spray aerosols (SSA), but its controlling factors and individual mixing states remain ambiguous. Here, we investigate the impact of various environmental factors on the water-soluble calcium (Ca2+) distribution in SSA through R/V Xuelong cruise observations over the Ross Sea, Antarctica, from December 2017 to February 2018. We observed enhanced Ca2+ enrichment in aerosol samples at lower temperatures (< -3.5 °C), lower wind speeds (< 7 m s-1) and in the presence of sea ice. Further individual particle mass spectral analysis indicated that considerable fractions of calcium in SSA likely bind with organic matter (a single-particle type, OC-Ca), which may be neglected in current water-soluble estimation of Ca2+ enrichment. Also, this is the first time that a calcium-dominated single-particle type has been observed in the Antarctic atmosphere. We suggest that a broader focus on individual OC-Ca and its subsequent environmental behavior should be included in future Antarctic atmospheric modeling. Given the context of global warming and sea ice retreat, an understanding of the mechanisms of calcium enrichment and mixing state of individual particles involved is valuable for further recognizing the aerosol-cloud-climate interactions in the Antarctica summer.
Bojiang Su et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2023-322', Anonymous Referee #1, 21 Apr 2023
- AC1: 'Reply on RC1', Bojiang Su, 06 Jun 2023
RC2: 'Comment on egusphere-2023-322', Anonymous Referee #2, 11 May 2023
- AC2: 'Reply on RC2', Bojiang Su, 06 Jun 2023
Bojiang Su et al.
Bojiang Su et al.
Viewed (geographical distribution)
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.
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).