the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Mar 2022
Sublimation of frozen CsCl solutions in ESEM: determining the number and size of salt particles relevant to sea-salt aerosols
Abstract. We identified the factors that, during the sublimation of a frozen CsCl solution, are important in generating fine salt particles as a possible source of salt aerosol. The number, size, and structure of the particles that remain after ice sublimation were investigated with respect to the concentration of the salt in the sample, the freezing method, and the sublimation temperature. The last-named aspect is evidently of primary importance for the preference of fine salt crystals over a large compact piece of salt. Independently of the concentration and freezing method tested, the sublimation of the frozen samples above the eutectic temperature (Teu) yielded a large compact piece of salt, namely, an improbable source of aerosol particles. Small salt particles that might be a source of atmospheric aerosols were formed predominantly at the temperatures below Teu, and their structures strongly depended on the concentration of the salt. For example, the sublimation of those samples that exhibited less than 8 psu (0.05 M) often produced small aerosolizable isolated particles readily able to be windblown. Conversely, the sublimation of 78 psu (0.5 M) samples led to the formation of relatively stable and largely interconnected salt structures. Presumably, our findings have important implications because the formed salt particles may assume the role of cloud condensation nuclei and ice-nucleating particles, affect polar atmospheric radiative forcing, and facilitate heterogeneous atmospheric chemistry.
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RC1: 'Comment on tc-2021-376', Gernot Nehrke, 17 May 2022
The manuscript represents a study on the growth of CsCl crystals within the chamber of an ESEM and, based on these observations, draws conclusions for the formation sea-salt aerosols. The relatively large effective cross section of Cs results in a good contrast in the BSD signal, allowing a good separation between the CsCl crystals and ice. The authors argue that CaCl can be used to study the formation of sea-salt crystals (mainly NaCl) due to the similarity in eutectic temperatures.
At this point I do not agree with the authors. The difference in solubility product of both minerals is huge (more than a factor of four). The formation of minerals and their size in from sea ice is a complex process that involves the presence of different mineral phases. For example, the mineral precipitation during the formation of frost-flowers follows a complex temperature profile. Often the increase in temperature is moderate in the beginning (the water is already close to the freezing temperature of sea-water) followed by a sharp drop in temperature. The cooling rates used in the ESEM study in which a droplet of distilled water containing various amounts of CsCl does not represent a situation that mimics the natural one. I don’t see an explanation how the different scenarios used (cooling rates, concentrations, and seeded vs. un-seeded) relate to natural processes. In an isolated, relatively clean system like the water droplet in an ESEM processes like supercooling are very likely, which will have a strong impact on the precipitation kinetics. This is not comparable to the natural system.
To summarize, I do not criticize the experiments itself, but I have my doubts that they allow any conclusions for the formation of “salt particles relevant to sea-salt aerosols). In my opinion the study would be more suitable for a journal like the “Journal of Crystal Growth” or “Crystal Growth and Design”.
Best regards
Gernot Nehrke
Citation: https://doi.org/10.5194/tc-2021-376-RC1 -
RC2: 'Comment on tc-2021-376', Anonymous Referee #2, 02 Jun 2022
This is a very interesting laboratory study investigating the impact of freezing method, salt concentration and sublimation temperature on the formation of salt aggregates after water ice removal via sublimation. The main finding is that the eutectic point represents a threshold below which salt aggregates are formed small enough to potentially form sea salt aerosol (SSA).
GENERAL COMMENTS
My main concern is regarding how the experiments presented do apply to the natural environment. A more critical assessment is required so the reader is aware that salty icy interfaces in the polar regions and associated processes may actually behave quite differently. I suggest to expand the discussion (section 4.6) and critical assessment of the experiment relevance, in particular on two points:
A) salt origin and transport - the experiment uses as a surrogate for the natural system a well mixed salt solution droplet, which is cooled down, then sublimated. However, the transport of salt to snow (which then gets airborne to relase SSA after sublimation) occurs primarily via upward migration driven by capillary action from the sea ice surface and atmospheric deposition of SSA (e.g. from open ocean and frost flowers), all well described in Dominé et al. (2004). Thus, the salt solution or particles are delivered to the surface of already frozen water, i.e. ice crystals. Therefore, the formed salt aggregates may be quite different in the natural environment than what is seen here, in agreement with that "structures of these residua therefore depend on the sizes of the ice crystals and the location of the salt in the frozen sample prior to sublimation" (L335).
B) freezing method - the first method, labeled as "spontaneous freezing" insinuates homogeneous freezing, which it is not. Ice nucleating particles introduced with the CsCl must be present in solution to initiate freezing at -11degC, way above the -37degC homogeneous freeze point. INPs could be anywhere in the droplet and initiate localised formation of an ice crystal, therefore I question the concept of a slowly upward moving freezing front (Fig1a), has this been observed? Have UHP water blanks been run to check for contamination and background? This needs clarification.
Furthermore, the title promises also some information on the size of salt particles/aggregates formed. However, only number densities are reported whereas particle size information is only semi-quantitative along the images. Has any attempt been made to extract from the image analysis also the fraction of observed residual particles falling into the coarse aerosol size range (0.5-30 μm)? This would importantly add to a more quantitative evaluation of the experiment.
Overall the paper is very long, in part due to the very descriptive discussion, and occasional repetitions of text from Section 3 in Section 4. I'd therefore recommend to combine Section 3 & 4 into a section "Results and discussion" & shorten the text; e.g. by remove all text/ references to chemical reactivity, pH, QY ... as these are rather speculative and were not part of this study; refer more often to Figure 15, which is a good summary.
In summary, I recommend some major revisions. More detailed comments are listed below.
SPECIFIC COMMENTS
L28 in general from the aerosol or ice (snow) phase containing bromide. The relative contribution of aerosol and snowpack source in the high latitudes is under debate
L32 source strength and their relative contribution
L41 better: ... not supported by ...
L52 spell out ESEM
L68 dry air?
L73 I suggest to move description and reason of choosing CsCl as a sea salt proxy to the method section
L222-240 I suggest moving this paragraph of describing choice/ advantages and limitations of CsCl as a sea salt proxy to the method section. You need this in order to understand the results.
L230 similar eutectic point between NaCl and CsCl supports this; however would differences in salt crystal structure itself impact the size, shape and number of salt structures formed after sublimation? CsCl due to the similarity in respective ion sizes forms a cubic lattice with eightfold coordination, whereas NaCl with the cation smaller than the anion forms octahedral structures with sixfold coordination. Please comment and expand the discussion accordingly.
L231 tasks?
L236-37 why mention if you don't share the details (composition, figure in appendix)? Either remove or add the required detail.
L274-77 Please clarify the terms 'stick-slip' mechanism and 'pined and mobile contact lines evaporation' and what they are supposed to mean in this context.
L278 this is a far too strong statement; it is true for the experimental conditions, however it remains far from clear if this is the case in the natural environment. Distinguish more critically between the this experiment and nature. Also what aerosol size range is being referred to?
L332 spontaneous freezing - typically homogeneous freezing without the addition of nucleators occurs at -37degC, however here freezing occurs at -12C so "warm" INPs are present (introduced with the CsCl); see general comments above.
L399-404 I don't think this is correct, please clarify. After lead opening in winter/spring new sea ice forms quickly and frost flowers grow on top of it into very cold air; the coldest air temperatures during MOSAiC were observed in March at around -42C (Shupe at el., 2022). I expect surface snow and filigran ice structures on the surface such as frost flowers to be in equilibrium with air temperature above.
L469-78 While potential implications of freezing on pH, reactivity etc are interesting I suggest to remove these (and references) as this is not relevant to the experiment of this study, pH was not measured ...
L500 clarify or define "aerosol-forming potential"; can you give an average particle size? what was the fraction of observed residual particles falling into the coarse aerosol category (0.5-30 μm)?Supplement
L8 it should really be here and throughout the text particle number density (number of particles per area or volume), or surface/ volume number density; the term particle density, in my view, refers to a composition dependent density, as in mass per volume, of the particle itself.
L14 surface density is ambiguous (see above), better surface number density or particle number density per area ...
REFERENCESDominé et al., The origin of sea salt in snow on Arctic sea ice and in coastal regions, Atmos. Chem. Phys., https://www.atmos-chem-phys.net/4/2259/2004/, 2004.
Shupe et al., Overview of the MOSAiC expedition - Atmosphere , Elementa, https://doi.org/10.1525/elementa.2021.00060, 2022.Citation: https://doi.org/10.5194/tc-2021-376-RC2
Status: closed
-
RC1: 'Comment on tc-2021-376', Gernot Nehrke, 17 May 2022
The manuscript represents a study on the growth of CsCl crystals within the chamber of an ESEM and, based on these observations, draws conclusions for the formation sea-salt aerosols. The relatively large effective cross section of Cs results in a good contrast in the BSD signal, allowing a good separation between the CsCl crystals and ice. The authors argue that CaCl can be used to study the formation of sea-salt crystals (mainly NaCl) due to the similarity in eutectic temperatures.
At this point I do not agree with the authors. The difference in solubility product of both minerals is huge (more than a factor of four). The formation of minerals and their size in from sea ice is a complex process that involves the presence of different mineral phases. For example, the mineral precipitation during the formation of frost-flowers follows a complex temperature profile. Often the increase in temperature is moderate in the beginning (the water is already close to the freezing temperature of sea-water) followed by a sharp drop in temperature. The cooling rates used in the ESEM study in which a droplet of distilled water containing various amounts of CsCl does not represent a situation that mimics the natural one. I don’t see an explanation how the different scenarios used (cooling rates, concentrations, and seeded vs. un-seeded) relate to natural processes. In an isolated, relatively clean system like the water droplet in an ESEM processes like supercooling are very likely, which will have a strong impact on the precipitation kinetics. This is not comparable to the natural system.
To summarize, I do not criticize the experiments itself, but I have my doubts that they allow any conclusions for the formation of “salt particles relevant to sea-salt aerosols). In my opinion the study would be more suitable for a journal like the “Journal of Crystal Growth” or “Crystal Growth and Design”.
Best regards
Gernot Nehrke
Citation: https://doi.org/10.5194/tc-2021-376-RC1 -
RC2: 'Comment on tc-2021-376', Anonymous Referee #2, 02 Jun 2022
This is a very interesting laboratory study investigating the impact of freezing method, salt concentration and sublimation temperature on the formation of salt aggregates after water ice removal via sublimation. The main finding is that the eutectic point represents a threshold below which salt aggregates are formed small enough to potentially form sea salt aerosol (SSA).
GENERAL COMMENTS
My main concern is regarding how the experiments presented do apply to the natural environment. A more critical assessment is required so the reader is aware that salty icy interfaces in the polar regions and associated processes may actually behave quite differently. I suggest to expand the discussion (section 4.6) and critical assessment of the experiment relevance, in particular on two points:
A) salt origin and transport - the experiment uses as a surrogate for the natural system a well mixed salt solution droplet, which is cooled down, then sublimated. However, the transport of salt to snow (which then gets airborne to relase SSA after sublimation) occurs primarily via upward migration driven by capillary action from the sea ice surface and atmospheric deposition of SSA (e.g. from open ocean and frost flowers), all well described in Dominé et al. (2004). Thus, the salt solution or particles are delivered to the surface of already frozen water, i.e. ice crystals. Therefore, the formed salt aggregates may be quite different in the natural environment than what is seen here, in agreement with that "structures of these residua therefore depend on the sizes of the ice crystals and the location of the salt in the frozen sample prior to sublimation" (L335).
B) freezing method - the first method, labeled as "spontaneous freezing" insinuates homogeneous freezing, which it is not. Ice nucleating particles introduced with the CsCl must be present in solution to initiate freezing at -11degC, way above the -37degC homogeneous freeze point. INPs could be anywhere in the droplet and initiate localised formation of an ice crystal, therefore I question the concept of a slowly upward moving freezing front (Fig1a), has this been observed? Have UHP water blanks been run to check for contamination and background? This needs clarification.
Furthermore, the title promises also some information on the size of salt particles/aggregates formed. However, only number densities are reported whereas particle size information is only semi-quantitative along the images. Has any attempt been made to extract from the image analysis also the fraction of observed residual particles falling into the coarse aerosol size range (0.5-30 μm)? This would importantly add to a more quantitative evaluation of the experiment.
Overall the paper is very long, in part due to the very descriptive discussion, and occasional repetitions of text from Section 3 in Section 4. I'd therefore recommend to combine Section 3 & 4 into a section "Results and discussion" & shorten the text; e.g. by remove all text/ references to chemical reactivity, pH, QY ... as these are rather speculative and were not part of this study; refer more often to Figure 15, which is a good summary.
In summary, I recommend some major revisions. More detailed comments are listed below.
SPECIFIC COMMENTS
L28 in general from the aerosol or ice (snow) phase containing bromide. The relative contribution of aerosol and snowpack source in the high latitudes is under debate
L32 source strength and their relative contribution
L41 better: ... not supported by ...
L52 spell out ESEM
L68 dry air?
L73 I suggest to move description and reason of choosing CsCl as a sea salt proxy to the method section
L222-240 I suggest moving this paragraph of describing choice/ advantages and limitations of CsCl as a sea salt proxy to the method section. You need this in order to understand the results.
L230 similar eutectic point between NaCl and CsCl supports this; however would differences in salt crystal structure itself impact the size, shape and number of salt structures formed after sublimation? CsCl due to the similarity in respective ion sizes forms a cubic lattice with eightfold coordination, whereas NaCl with the cation smaller than the anion forms octahedral structures with sixfold coordination. Please comment and expand the discussion accordingly.
L231 tasks?
L236-37 why mention if you don't share the details (composition, figure in appendix)? Either remove or add the required detail.
L274-77 Please clarify the terms 'stick-slip' mechanism and 'pined and mobile contact lines evaporation' and what they are supposed to mean in this context.
L278 this is a far too strong statement; it is true for the experimental conditions, however it remains far from clear if this is the case in the natural environment. Distinguish more critically between the this experiment and nature. Also what aerosol size range is being referred to?
L332 spontaneous freezing - typically homogeneous freezing without the addition of nucleators occurs at -37degC, however here freezing occurs at -12C so "warm" INPs are present (introduced with the CsCl); see general comments above.
L399-404 I don't think this is correct, please clarify. After lead opening in winter/spring new sea ice forms quickly and frost flowers grow on top of it into very cold air; the coldest air temperatures during MOSAiC were observed in March at around -42C (Shupe at el., 2022). I expect surface snow and filigran ice structures on the surface such as frost flowers to be in equilibrium with air temperature above.
L469-78 While potential implications of freezing on pH, reactivity etc are interesting I suggest to remove these (and references) as this is not relevant to the experiment of this study, pH was not measured ...
L500 clarify or define "aerosol-forming potential"; can you give an average particle size? what was the fraction of observed residual particles falling into the coarse aerosol category (0.5-30 μm)?Supplement
L8 it should really be here and throughout the text particle number density (number of particles per area or volume), or surface/ volume number density; the term particle density, in my view, refers to a composition dependent density, as in mass per volume, of the particle itself.
L14 surface density is ambiguous (see above), better surface number density or particle number density per area ...
REFERENCESDominé et al., The origin of sea salt in snow on Arctic sea ice and in coastal regions, Atmos. Chem. Phys., https://www.atmos-chem-phys.net/4/2259/2004/, 2004.
Shupe et al., Overview of the MOSAiC expedition - Atmosphere , Elementa, https://doi.org/10.1525/elementa.2021.00060, 2022.Citation: https://doi.org/10.5194/tc-2021-376-RC2
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