Quantified effect of seawater biogeochemistry on the temperature dependence of sea spray aerosol fluxes
Abstract. Future change in sea surface temperature may influence climate via various air-sea feedbacks and pathways. In this study, we investigate the influence of surface seawater biogeochemical composition on the temperature dependence of sea spray number emission fluxes. Dependence of sea spray fluxes was investigated in different water masses (i.e. subantarctic, subtropical and frontal bloom) with contrasting biogeochemical properties across a temperature range from ambient (13–18 °C) to 2 °C, using seawater circulating in a plunging jet sea spray generator. We observed sea spray total concentration to increase significantly at temperatures below 8 °C, with an average 4-fold increase at 2 °C relative to initial concentration at ambient temperatures. This temperature dependence was more pronounced for smaller size sea spray particles (i.e. nucleation and Aitken modes). Moreover, temperature dependence varied with water mass type and so biogeochemical properties. While the sea spray flux at moderate temperatures (8–11 °C) was highest in frontal bloom waters, the effect of low temperature on the sea spray flux was highest with subtropical seawaters. The temperature dependence of sea spray flux was also inversely proportional to the seawater cell abundance of the cyanobacterium Synechococcus, which facilitated parameterization of temperature dependence of sea spray emission fluxes as a function of Synechococcus for future implementation in modelling exercises.
Karine Sellegri et al.
Status: final response (author comments only)
RC1: 'Comment on acp-2022-790', Anonymous Referee #1, 16 Mar 2023
- AC1: 'Reply on RC1', Karine Sellegri, 05 Jun 2023
RC2: 'Comment on acp-2022-790', Anonymous Referee #2, 21 Mar 2023
- AC2: 'Reply on RC2', Karine Sellegri, 05 Jun 2023
Karine Sellegri et al.
Karine Sellegri et al.
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This paper presents measurements of sea spray aerosol particle number size distributions and calculated fluxes for particles produced from four different water masses during one sampling period. Surface tension measurements were done on water samples at different temperatures, and relationships between sea spray flux and water temperature are presented for the different water masses. This work is put in the context of previous work and relates the nanophytoplankton numbers to cloud condensation nuclei as well. Overall, this paper is interesting and relevant. Understanding sea spray production flux as a function of water temperature will be a good contribution to global modeling. There are some areas of the paper that can be improved, mostly through clarification of the methods and presentation of the results. Some general and specific comments are noted below.
The abstract states that future changes in seawater temperature will be important with climate change. In that case, why were such low temperatures chosen for the experiments? 2°C is outside of the range of the ambient temperatures measured (13-15°C). It would be helpful to have a comment on the global distribution of current sea surface temperatures and how they may change. For example, what fraction of the world’s ocean has a sea surface temperature at 2°C? How will that change in the future? Is it physically relevant to cool these specific water masses to 2°C? The statement made in the Conclusions is not fully supported. This study cooled ambient water, and many other factors will change with a warming ocean. It would have been more relevant to warm up the ambient water or present a larger range of temperatures.
The Methods should contain more details on the experiments and calculations, as mentioned below in the Specific Comments. There are some pieces in the Results and Discussion that should be moved into the Methods (i.e., the calculation of FCCN). Additionally, while references are included, there are some places that could use more explanation in the calculations. It also seems unnecessary to use “CCN” when it is simply defined as all particles over 100 nm. That could just be stated as its own variable.
Could you add some comments on how the temperature of the air would affect the particle number flux? The air temperature may influence the lifetime of the bubble at the surface. Would a large gradient in the temperature from the surface of the seawater to the air change the lifetime of the bubbles at the sea surface? What air temperature were these experiments done at? Was the air temperature in the headspace held constant, or was the whole system cooled?
More text needs to be included discussing role of different factors on the surface tensions measured. The temperature at the time of measurement affects the surface tension, as stated. Additionally, salt concentrations in the water can change the surface tension as well. Because these are different water masses, it is likely that their salinities also change. Reporting a total surface tension might not be relevant, unless it is in the context of these changes in temperature and salinity.
The discussion of the different biological factors influencing the particle flux at lower temperatures needs to be clarified. These are not necessarily species that would live at these temperatures. And it seems like any surfactants that would be in the water would have already been emitted at the ambient temperature. Is it possible that these species could die at these temperatures and then emit more surfactants or organics? Some more discussion of this would be useful. Towards the end of the paper, it is stated that the bubble films are more stable at colder temperatures due to the stabilization from surfactants. Why would there be more stabilization of surfactants at colder temperatures? This implies there are more or different surfactants when the temperature is changed.
The Results and Discussion section is somewhat short and could use more discussion of what these results mean. Some sections as mentioned below, could use more support for the statements. The Conclusions section starts as more of a discussion that could be moved up to the Results and Discussion. It would be better to have the key findings summarized in the Conclusions, and it should be clear what was measured in this study. The comparisons to the previous studies could be discussed in the previous section.
Line 105: Nothing to change. Just noting that March 2020 must have been an extra stressful time to be on a research cruise, and it is great that you were still able to finish the cruise and complete the experiments.
Line 121: FCCN is used in this sentence but has not yet been defined. Because it is defined later, it might be better to remove it from this sentence, or move the definition up. It seems like “F” is for “flux”, but that should be stated explicitly here. And sub-scripting the “CCN” or not, should be consistent.
Line 122: Please expand this sentence. It is unclear what two fluxes are being compared. It is interesting and relevant that the wind speeds are related to the air entrainment in the plunging jet system. It would be helpful to have the wind speeds and corresponding flow rates that were used written out here.
Line 127: It would be helpful to have more details on the surface tension measurements. Were these all done on board the ship, with fresh seawater, directly after sampling? How did the ship movement contribute to any uncertainties in these measurements? What volume of sample was collected for the temperature gradient experiments? Were these mixed to ensure a constant temperature throughout the sample?
Line 132: How were the samples in the 10L carboys stored to prevent changes in the chemistry and biology? Please add more details on the aliquots and their storage prior to analyses. Were these analyzed on board the ship or later in the laboratory?
Line 166: The paragraph prior to this line could be moved into the Methods section. This is mainly describing the water masses that were sampled and their dates. Starting at Line 166, there are some results from the measurements.
Line 176: Add more explanation on how all of the particles greater than 100 nm can be considered CCN at 0.2% supersaturation.
Figure 1 should be improved. The y-axes labels all appear to be stretched. The panels should be merged together into one figure, with one common x-axis. The shading and labels are nice to have. It would be helpful if this figure also had sea surface temperature and salinity. Maybe b and c could be combined, and SST could be added to a on the right axis. In d, why are there more surface tension markers than flux markers? Because the flux was calculated from the size distributions which were measured continuously, it seems like those could be at higher resolution or the same resolution as the surface tension. Error bars should also be added to these markers, especially the flux and the surface tension, to see any overlap in variability. (Also, change “STT” to “SST” in caption.)
In Figure 2 it is not clear what are measurements from this study and those from previous studies. The text states that there are four other datasets, but I do not see markers for the Sea2Cloud data. “SO” needs to be defined as well. If that is the Sea2Cloud data, then I am not sure what the fifth dataset is that is referenced in the text. Overall, the caption could be a little more descriptive.
Line 190: I think this equation and all of the description of the FCCN calculations should be moved up into the methods. It would be helpful to have just an FCCN equation as well.
Line 202: This should be Figure 3.
Figures 3, 4, and 5 could be combined into a multi-panel figure, since they all have the same x-axis. Figures 3 and 5 should be combined, since they both contain sea spray flux as a function of temperature. It would be easier to compare the figures if they were together.
Line 266: Interesting result that there is a shift in the shape of the size distribution.
Figure 6 needs a legend to describe the different colors. Additionally, some of the marker colors do not match the line colors (i.e., black dashed line with orange x’s). It would be useful to have the same color scheme as Figure 7, to be consistent.
Line 283: Can you add a little more description on the calculations going into Figure 7. It seems like it would be interesting to compare the Dp values shown in Table 1 for both temperature ranges.
Line 294: This explanation is not consistent. Why would there be higher concentrations of surfactant in colder waters that would further stabilize the bubble film? Surfactant concentration is not the only thing contributing to the surface tension and thus bubble lifetime.
Line 348: More explanation is needed to make this claim. It is not entirely clear that the results of this study support the idea that with a warmer ocean, there will be less sea spray flux. The temperatures measured here were colder than ambient. In order to make this statement, it would have been better to warm the water instead. There are a lot of factors that will change in a warming climate, so this needs to be clarified.