| 08 Aug 2025
Hydrothermal activity indirectly influences ice nuclei particles in seawater and nascent sea spray of the Subtropical Pacific Ocean
Abstract. Particles of marine origin may act as ice nuclei when clouds form and therefore influence cloud properties and lifetime. Here we investigate the abundance of Ice Nuclei Particles in bulk seawater (INPSW) collected in natural seawater of the Western Tropical South Pacific and in sea spray aerosol (INPSSA) artificially generated from the surface seawater. The study area was separated into two oligotrophic zones (the Melanesian Basin and the Western South Pacific Gyre), and a mesotrophic one (the Lau basin), characterized by high plankton biomass due iron fertilization by underwater hydrothermal activity of the Tonga volcanic arc. Our results show that INPSW were on average 80 % heat labile, strongly suggesting a biological origin. INPSW concentrations were two-fold higher in the Lau basin as compared to both oligotrophic areas at all freezing temperatures. This trend is consistent with a higher abundance of planktonic microorganisms, pigments and particulate organic carbon (POC) concentrations in the Lau basin. Over the whole cruise transect, medium to strong correlations were found between INPSW concentrations and pigments (notably with bacteriochlorophyll-a and carotene), bacterial abundance and POC. The heat stable fraction of INPSW exhibited correlations with Dissolved Organic Carbon (DOC) concentrations and were not as variable as the heat labile INPSW. In the nascent sea spray, INPSSA were also mostly heat labile in coherence with the INPSW. INPSSA were predominantly (60 %), submicron in size (presumed originating from film drops), but the supermicron INPSSA constituted 40 % of the INPSSA and were all heat labile (presumably originating from jet drops). Supermicron INPSSA were between 60 to 80 % heat stable with a high variability between samples, indicating different nature of the two fractions of INPs. Supermicron INPSSA were generally more abundant in the Lau basin, while submicron INPSSA did not exhibit any significant difference between the three regions. We report a transfer function of seawater INPs to SSA INPs of 1.70 m-2.LSW and 3.3 m-2.LSW for heat stable INPs, hinting that heat stable INPs were more efficiently transferred to the SSA. Our results suggest that hydrothermal activity indirectly enhances the INP concentration of surface waters, through boosting the biological activity, which results in increases of the ice forming ability of supermicron sea spray particle. Given the extent of hydrothermal activity throughout the global Ocean, its impact on cloud properties should be considered in future ocean-atmosphere interaction studies.
General comments
This is a very interesting and relevant manuscript investigating ice nucleating particles (INPs) in the Subtropical Pacific Ocean and the influence of hydrothermal activity on marine INPs. This study includes seawater samples collected during the TONGA campaign and sea spray aerosol samples generated from sea spray tank experiments during the campaign. They find that the hydrothermal activity indirectly influences INPs in seawater (as described by the title) through its stimulation of biological activity in the area. The studied regions are areas with little measurement data and the investigation of hydrothermal activity is a topic unexplored with respect to INPs. Therefore, the dataset is of high value to the scientific ice nucleation community.
The manuscript is well written and presented. However, the manuscript would benefit from added discussion/interpretation of the presented results and figures. In addition, the methods description is missing important details. See comments below.
Specific comments:
P1L30: This sentence needs to be reformatted. Is it correct understood that the first number is the total INP transfer and the second is the heat stable INP transfer? If yes, please state more clearly.
In the “hint” statement, please write what you are comparing to. “,hinting that heat stable INPs were more efficiently transferred to the SSA” compared to ...
P4L98: Description of the SSA experiments is lacking in detail. The authors reference papers of similar studies, however, the Sellegri et al. 2005 paper is possibly the same chamber (not clear) but run in that study without water or jets. I would remove this reference as it causes confusion. The tank described in Schwier et al. 2015 seems to be the same as described in the current study.
I suggest adding a few more details to the current description of the sea spray tank. The authors write “jets” in plural – are there several or is it a single plunging jet? If there are several please describe this as not many tanks have several. What flow rate of water through the jet was used? What flow rate of particle filtered air was used? Was the tank temperature regulated?
P6L151: Please check that ‘Sellegri et al 2005’ is the correct reference based on previous possible mix-up.
P6L156: Specify how blank samples were made. Were they also placed in the stage for ~24hours?
P6L159: Samples were collected in November 2019. How long were they stored frozen before INP measurement?
P6L165: The volume of liquid added to the Eppendorf tubes is different for SW and SSA measurements (200µL and 400µL, respectively). The Vali equation takes into account the volume difference when calculating the concentration of INPs. This should be specified in the text, so that the reader understands that the to dataset can be compared regardless of the difference in volume.
P6L168: What does it mean the samples were “being agitated”?
P6L168: The process of filling half the tubes and heating is described twice in the same section. I suggest keeping the following text (start line 172) and rewriting the text starting in line 168.
“Half of the Eppendorf© tubes were then filled with the untreated sample. The seawater samples were then subjected to the same heat treatment as the filter samples, and the second half of the Eppendorf© tubes was filled using the heated samples (Fig. 2).”
P6L170: Comment on the heat treatment study by Daily et al. 2022, where they find that wet INP heat tests at (> 90°C) have the potential to produce false positives. Why did the authors select 100°C?
Daily, Martin I., et al. "An evaluation of the heat test for the ice-nucleating ability of minerals and biological material." Atmospheric Measurement Techniques 15.8 (2022): 2635-2665.
P7L174: What is the limit of detection?
It would also be nice with an idea of the experimental uncertainty e.g. as a number of degrees Celsius. This would give the reader a better understanding of how significant differences between samples freezing temperatures are.
P7L178: Show a figure of the blank sample INP results in the appendix.
State what the “lower temperature that can be reached” is.
P7L180: It is interesting that the heat treatment increased the IN activity. Add a comment on whether this has been seen in other studies (see comment above about paper by Daily et al. 2022).
Section 3.1: It is some really nice data on the seawater characteristics, and the text describes a comparison of the three waters as is viewed in Figure 2. However, I am missing some discussion/interpretation of the comparisons. E.g. What does it mean that the different planktonic microorganisms are in higher abundance in the LAU surface water? Is this expected?
P9L205: The difference in salinity and surface tension between the three waters seems very small (very close to the standard deviation stated). Why is this important to mention?
P9L223: Elaborate on this sentence “The MEL were oligotrophic waters, and WGY were ultraoligotrophic waters.”. How does this fit with the seawater characteristics in Figure 2.
P10L228: It is confusing for the reader to have just read a section where the water types are separated into categories: MEL, LAU, WGY, and now in this section the INP samples are separated into new categories: INP_sw, INP_tot, INP_HS, INP_HL.
It needs to be clearly stated in the start of this section (3.2.1) what the new categories mean and from which waters these INP samples come from. This should also be added to the methods section – a specification of where the INP water samples and air samples where taken (If at all stations, then state that). Also state earlier in the text of section 3.2.1 that the comparison of water types was not possible.
What is the authors definition of heat stable INP (e.g. a specific decrease in IN activity or)?
P15L313: Elaborate on the strong correlation with dissolved iron.
P16L344: I suggest simply writing INP/aerosol instead of shortening to INP/aer as this is likely to confuse readers. I do not believe that “Aer.” is a commonly used abbreviation for units. Same for Figure A3.
Figure 7. In text above figure n_s = surface site density, but in figure caption n_s = nucleation site density. Select one name for the symbol.
Several different labels are written in the caption referring to the same thing. Ns_super = Unheated SUPM?? Use the same label for both legend, caption and text describing the figures.
What do the error bars represent?
P17L357: Split sentence after parenthesis. Elaborate on Córdoba2025 comment, if possible, give n_s values.
P17L375: Elaborate on how this observation contrasts that of McCluskey2018b.
P19L404: It is not clear to me what this transfer function means. Please explain in further detail why the equation ns/INP_sw is used and explain the resulting number in words. In addition, it would be nice to add a reference to other work that has determined a transfer function in a similar way and compare numbers.
The authors could also determine a flux of INPs from water to air using the sea spray tank experimental conditions.
Technical comments
P1L17: Change “study” to “studied”.
P1L18: add “to” after “due”.
P1L22: Consistently use capitol letters. Change to “Particulate Organic Carbon (POC)”.
P2L43: Remove s from “drops”.
P2L44: Consistently use capitol letters. Change to “Cloud Condensation Nuclei (CCNs)”.
P2L50: warmer temperatures compared to what?
P2L59: Split sentence between “coastal sites,” and “and heat labile”. It is confusing that the sentence starts by introducing a specific study and then later mentions other studies.
P3L66: missing parenthesis after references.
P3L73: Remove “the” in front of “bulk seawater” and “SSA”.
P3L91: I suggest replacing “West-East-West with” with “West-East-West reaching”.
Figure 1: I suggest increasing the font size in this figure, especially the legend is difficult to read.
Figure 1: Add to either figure or the caption which stations correspond to MEL, LAU and WGY. This is very helpful for the reader.
P4L100: Change “than” to “to”.
P5L119: Exchange link with details about instrument. The link is just to the campaign website.
P5L120: Add details on the dynotester instrument, e.g. company name.
P6L159: Remove “on land.”
P6L162: Specify the cooling rate for the experiments.
P7L176: Remove “either” or add another type of normalization. This sentence does not make sense.
Figure 2. What is the black square? I assume average/mean? Add this to caption along with a note that the y-axis’ are different and some are in log-scale. This is important when reader is comparing values from the three sites.
P9L220: Add to text how many sampling points for LAU?
P10L235: Add to caption what type of seawater SSW corresponds to.
P13L264: Should read Figure 5?
P13L275: The legend in figure 5. The mean datapoints are squares in the figure but not in the legend for red and blue.
P13L275: Use MEL and LAU as written in text not new abbreviations (Mel. B.).
P13L279: Pearsons’s test – missing an r.
P18L389: Should read Fig. B3 ?
P20L431: Specify what temperature? E.g. sea surface temperature, nucleation/freezing temperature.