the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jul 2024
Wind-driven Emission of Marine Ice Nucleating Particles in the Scripps Ocean-Atmosphere Research Simulator (SOARS)
Abstract. Sea spray aerosol (SSA) represent one of the most abundant natural aerosol types, contributing significantly to global aerosol mass and aerosol optical depth, as well as to both the magnitude and uncertainty of aerosol radiative forcing. In addition to their direct effects, SSA can also serve as ice nucleating particles (INPs), which are required for the initiation of cloud glaciation at temperatures warmer than ∼-36 °C. This study presents initial results from the CHaracterizing Atmosphere-Ocean parameters in SOARS (CHAOS) mesocosm campaign, which was conducted in the new Scripps Ocean-Atmosphere Research Simulator (SOARS) wind-wave channel at the Scripps Institution of Oceanography. SOARS allows for isolation of individual factors, such as wave height, wind speed, water temperature, or biological state, and can carefully vary them in a controlled manner. Here, we focus on the influence of wind speed on the emission of SSA and INPs. Unlike recent measurements from the Southern Ocean, real-time and offline INP observations during CHAOS exhibited opposite relationships with wind speed, which may be related to sampling inlet differences. Changes in the INP activated fraction, dominant INP particle morphology, and INP composition were seen to vary with wind. Seawater ice nucleating entity concentrations during CHAOS were stable over time, indicating changes in atmospheric INPs were driven by wind speed and wave-breaking mechanics rather than variations in seawater chemistry or biology. While specific emission mechanisms remain elusive, these observations may help explain some of the variability in INP concentration and composition that have been seen in ambient measurements.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-2159', Anonymous Referee #1, 07 Aug 2024
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RC2: 'Comment on egusphere-2024-2159', Anonymous Referee #2, 20 Aug 2024
This work was well designed, the experiments and analysis were thoroughly carried out. I especially appreciate the detailed description of the experiments, which could be really helpful for other scientists in the community to conduct similar work. Overall, this study is excellent.
I have only one comment. The “less clear trend with wind speed” of INP concentration from IS measurements was attributed to “different averaging times, differences in inlet orientations or locations, or differences in the aerosol sampled”. I have no objections about the possibility of these factors. However, there might be two other reasons, which could be somehow more important. First, for IS method the particles were formed from evaporation of seawater droplets, later collected on filters, and then washed off by water. The resulted solution should be very similar to the original seawater, except they might have different concentrations. It should have similar ice-nucleating properties as the seawater, which could be confirmed by the similar slopes of the INP spectra for filters in Figure 2a and those for seawater in Figure A7. Therefore, IS actually measured seawater-like solutions. As seawater used in experiments kept the same, it is understandable IS method did not see the trend as CFDC method. Second, the two methods measured INP for particles with different sizes. CFDC measured polydispersed particles. The different size distribution of particles generated by different wind speed would of course result in different INP concentrations. However, the IS equivalently measured several groups (meaning each dilution) of monodispersed seawater-droplets generated particles. For each group (one dilution), the droplets had similar size and might have similar INP properties. If you maintain the concentration of solution by using different amount of water to wash the filters according to the mass collected, you might get even less clear trend with wind speed for IS measurements.
Citation: https://doi.org/10.5194/egusphere-2024-2159-RC2 -
RC3: 'Comment on egusphere-2024-2159', Anonymous Referee #3, 13 Sep 2024
Review of “Wind-driven Emission of Marine Ice Nucleating Particles in the Scripps Ocean-Atmosphere Research Simulator (SOARS)” by Moore et al.
This work addresses the transfer of INPs from water to air via wind induced wave breaking processes.
The authors present an impressive new facility, the Scripps Ocean-Atmosphere Research Simulator (SOARS) wind-wave channel and describe results from a mesocosm campaign named CHAOS (CHaracterizing Atmosphere-Ocean parameters in SOARS). The authors have studied the effect of windspeed on emission of sea spray aerosol and properties of ice nucleating particles from seawater. A wide range of advanced instrumentation was applied to analyze sea spray aerosol and INPs.
Overall, the manuscript presents an amazing new facility and a large body of data, information and thoughts which can serve as inspiration for other studies.
I have some suggestions for improving the manuscript. After consideration of the comments/suggestions below, I recommend publication and look forward to following future findings from the new SOARS facility.
Main comments
As I understand, the results presented in the manuscript are based on two fillings of the channel with seawater each lasting several days. I miss one or more overview figures showing how the experiments evolved with time and showing how basic parameters such as temperature of water and air, as well as windspeed varied with time from start to end in each of the two experiments. It could also be marked when sampling of different types were performed. Such figures could be provided as supporting material. This will help the reader get a better basis for the discussion.
Abstract:
Regarding the sentence: “unlike recent measurements from the SOurther Ocean, real-time and offline INP observations during CHAOS exhibited opposite relationships with wind speed which may be related to sampling inlet differences”
I think the abstract would be more informative if the authors explained the relationship from this study explicitly and then say it is opposite to previous field work.
Methods:
The level of detail given varies, but I assume it is to focus on what is important for the current data-set – for example, it is stated that wind turbine setpoints are calibrated using a TSI Inc. model 9545-A air velocity meter while no details are given on the camera used to infer whitecap coverage. If this manuscript will serve as a reference for future studies in the facility it might be useful if such details are provided. I also realize that wind speed is in focus in this work and that other details may be given in future publications which the authors already mention are in progress or planning, and so perhaps that is why they are omitted?
Were the solar simulators turned on during the current study? What were the brand and wavelength spectrum of the solar simulators and the PAR LEDs?
What were the temperatures of the seawater and the ambient air during the two periods, did they vary?
This is a huge facility and water volume, how is the channel cleaned?
Iine 160: It is a bit unclear what/when the sampling periods took place and how long they were. Sampling periods for different types of sampling (e.g. aerosol filters, INP) could be indicated in overview figures as suggested above.
Line 275: why is a density of 2 assumed?. I think there is a typo in the unit: should be g cm-3 not g cm-1?
Figure A3: how should a transmission efficiency larger than one be understood? I suggest to also show the corresponding figure using the corrected values of density etc.
Figure A4: The description of normalization and figure A4 would benefit from further explanation. Under which conditions are the data taken – is the slope the correction factor? What are the uncertainties? Is n300 nm sometimes used for normalization – this is difficult to understand from the text.
In lines 150-153 white cap fraction during CHAOS is discussed, but no data or values are given, is it possible to give such numbers?
As I understand the wind speeds shown in all figures are the values extrapolated to 10 m height (U10) from the measured windspeed at 0.6 m in SOARS – is this correct? The extrapolation is based on Hsu et al. 1994 – I suggest providing the value of p used, also, I was wondering, is it necessary/justified to give the extrapolated U10 windspeed with two decimals (m/s) on basis of Hsu et al?
Results and discussion
Figure 2A –To avoid misunderstandings, I suggest that the equations for activated fraction, surface site density and volume site density are provided. Is surface and volume densities also normalized only to particles larger than 500 nm? (perhaps it does not matter so much as the larger particles dominate these distributions?)
I suggest showing the surface and volume distributions for the size distributions in Figure A5 in and additional figure.
Regarding the INE measurements: it is concluded: “The INE stability across multiple fills of the SOARS channel and over time with the same water indicates the observed INP-wind speed relationships were driven by wind-wave interactions rather than biological activity” and in the abstract it says that sea water ice nucleating entity concentrations during CHAOS were stable over time and therefore changes in INPs were driven by windspeed and wavebreaking mechanisms and not seawater chemistry or biology. Were any data on seawater chemical composition or biology available to further support this interesting conclusion? This could be interesting to look further into in future studies.
Minor comments
What is the material of the paddle?
The sentence about SMA (line 25) is in between two sentences about SSA – since SMA is not SSA this is a bit confusing. I suggest moving the SMA sentence to the end of the paragraph.
Figure A1: it would be nice with indication of length scales on the figure.
Line 116: The function of the MERV 8 and potassium permanganate filters should be explained.
Line 198: I find something is strange in the formulation of this sentence - how can filters be collected without airflow?
Something is wrong with the URL to the Hsu paper in the reference list.
Citation: https://doi.org/10.5194/egusphere-2024-2159-RC3
Data sets
CHAOS CFDC INP measurements Kathryn A. Moore and Paul J. DeMott https://datadryad.org/stash
CHAOS IS filter + seawater INP measurements Kathryn A. Moore, Thomas C. J. Hill, and Samantha Greeney https://datadryad.org/stash
CHAOS INP AFM measurements Chamika K. Madawala, Alexei V. Tivanski, and Kathryn A. Moore https://datadryad.org/stash
CHAOS Aerosol size distribution measurements (SMPS + APS) Raymond J. Leibensperger III https://datadryad.org/stash
CHAOS Aerosol size distribution measurements (SEMS + APS) Christopher D. Cappa https://datadryad.org/stash
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