the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jul 2023
Mechanisms controlling giant sea salt aerosol size distributions along a tropical orographic coastline
Abstract. Sea salt aerosol (SSA) is a naturally occurring phenomenon that arises from the breaking of waves and consequent bubble bursting on the ocean's surface. The resulting particles exhibit a bimodal distribution, spanning orders of magnitude in size which introduce significant uncertainties when estimating the total annual mass of SSA on a global scale. Although estimates of mass and volume are significantly influenced by the presence of giant particles (dry radius > 1 µm), effectively observing and quantifying these particles proves to be challenging. Additionally, uncertainties persist regarding the contribution of SSA production along coastlines, but preliminary studies suggest that coastal interactions may increase SSA concentrations by orders of magnitude. Moreover, our knowledge regarding the vertical distribution of SSA in the marine boundary layer remains limited, resulting in significant gaps in understanding vertical mixing of giant aerosol particles and the specific environmental conditions facilitate their dispersion. By addressing these uncertainties, particularly in regions where SSA constitutes a substantial percentage of total aerosol loading, we can enhance our comprehension of the complex relationships between the air, sea, aerosols, and clouds.
A case study conducted on the Hawaiian Island of O'ahu offers insight into the influence of coastlines and orography on the production and vertical distribution of giant SSA size distributions. Along the coastline, the frequency of breaking waves is accelerated, serving as an additional source of SSA production. Furthermore, the steep island orography generates strong and consistent uplift during onshore trade wind conditions, facilitating vertical mixing of SSA particles along windward coastlines. To investigate this phenomenon, in-situ measurements of SSA size distributions for particles with dry radii (rd) ≥ 2.8 µm were conducted for various altitudes, ranging from approximately 80–650 m altitude along the windward coastline and 80–250 m altitude aboard a ship offshore. Comparing size distributions on and offshore confirmed significantly higher concentrations along the coastline, with 2.7–5.4 times greater concentrations than background open-ocean concentrations for supermicron particles. These size distributions were then analyzed in relation to critical environmental variables influencing SSA production and atmospheric dynamics. It was found that significant wave height exhibited the strongest correlation with changes in SSA size distributions. Additionally, simulated SSP trajectories provided valuable insight into how production distance from the coastline impacts the horizontal and vertical advection of SSA particles of different sizes under varying trade wind speeds. Notably, smaller particles demonstrated reduced dependence on local wind speeds and production distance from the coastline, experiencing minimal dry deposition and high average maximum altitudes relative to larger particles. This research not only highlights the role of coastlines in enhancing the presence and vertical mixing potential of giant SSA but also emphasizes how it is important to consider the influence of local factors on aerosol observations at different altitudes.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(15832 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1387', Anonymous Referee #1, 26 Jul 2023
The manuscript “Mechanisms controlling giant sea salt aerosol size distributions along a tropical orographic coastline” presents measurements of sea salt aerosols, including giant and ultragiant particles, from a novel device both on the coast and in the open ocean. Notable differences in the overall SSA concentrations are noted between the two sites, as are differences in the SSA-SD shape parameters and vertical profiles. Some of the mechanisms are illustrated by trajectory tracking in a WRF run of the area.
In my opinion, this is a great study and a very well-written manuscript. It is well-referenced and clear, and a lot of details were attended to properly. It also provides very reliable measurements of a data-scarce topic: SSA and giant particles in particular, but also comparing coastal to open ocean conditions. A lot remains to be understood about how universal the SSA-SD shape parameters are, and this study does the field a service by so clearly demonstrating some of the key differences that can occur. I only have a few very minor points for the authors to consider, but otherwise think that this would make a great contribution to ACP.
Specific comments:
- Line 9: Maybe should be “facilitating”?
- Sentence starting on line 45: Maybe consider “whitecap fraction” explicitly as one of the things that has been analyzed a lot? It seems to fit with the way the literature has been broken up here – production has been linked with those things listed (U10, SST, SSS) but also whitecap fraction.
- Figure 2: It might be nice to have a legend in the figure itself, explaining the symbols (so the reader doesn’t have to sift through the caption)
- Line 177: I’m not sure what the “Bellows’ 12 m” wind speed is here. I searched around the manuscript but didn’t see any description of what this meant. So I’m left a little uncertain of why it was regressed to the WeatherFlow, and which one was ultimately used for the reported U10 values
- Line 206: What is meant by “the Shin and Hong PBL scheme for the grid resolution”? Is the phrase “for the grid resolution” supposed to be there?
- Line 231: Are these using the averaged wind profiles? Or the time-varying profiles from some set frequency of WRF output? It appears to be the average, so it’s probably worth stating explicitly here
- Line 245: This isn’t necessarily a suggestion for this study, but in the future it might be more realistic to use the PBL scheme parameters to approximate this variability
- Line 267-269: If I’m reading figure 6 correctly, this sentence is really only referring to figure 6b (at specific heights), right? The COAST samples in 6a don’t always exceed W53 at the wind speeds mentioned in this sentence. However I might be misunderstanding because the next paragraph explicitly introduces fig 6b.
- Line 314: “chances” should be “changes”
- Line 315: “strong” should be “stronger”.
- Line 351: I’m not sure the units are needed after “concentrations”
12: Figure 4: It would be helpful if this figure had a panel “c” which showed the average RH profile that these are based on
13: Line 410 (or thereabouts): Somewhere at the end of this section I think it’s important to point out the relatively simple way of doing the trajectory simulations. I think it’s totally appropriate for what the authors are trying to do, but the treatment of turbulence in particular is only very crudely represented. I think the authors do a good job of only drawing conclusions which are supported by this method – I just think that a brief 1-sentence reminder at the end of the processes (especially turbulence) which were left out and could lead to significant differences in some of the details.
14: Lines 435-436: Elsewhere it’s spelled “Hawai’i”
Citation: https://doi.org/10.5194/egusphere-2023-1387-RC1 -
AC1: 'Reply on RC1', Katherine L. Ackerman, 03 Sep 2023
We want to thank the reviewer for the time and effort they put into reviewing our manuscript - your recommendations have improved our paper tremendously, and offered us interesting considerations for our future research! Please see more specific responses in the attached file.
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RC2: 'Comment on egusphere-2023-1387', Anonymous Referee #2, 01 Aug 2023
This manuscript presents the results of a sea salt aerosol (SSA) study in coastal zone (Hawaii) at moderate wind speeds (Trade winds up to 7 m/s). Super-micron SSA size distributions (dry radii > 2.8 mm) were measured with a miniature impaction instrument, mini-GNI, specifically designed for these observations. The size distributions in coastal zone were compared to SSA size distributions in open ocean and differences up to a factor of 5 are reported. Several mini-GNIs were deployed on a kite to obtain the vertical profiles of SSA number concentrations from 80 to 650 m height. Various environmental data were collected in situ (wind, wave height, air temperature, relative humidity) to identify the most suitable forcing variables for the observed SSA size distributions. Data generated with WRF model were used to simulate SSA trajectories and study the effects of the coastal topography on the SSA vertical dynamics.
Super-micron size distributions of SSA, as well as their vertical profiles, in coastal zone are indeed severely understudied. Though fewer and less frequently occurring, large SSA particles, with their large areas and volumes, contribute disproportionately to various aerosol effects, from scattering and absorption to formation of cloud condensation nuclei. The differences between open ocean and coastal zone conditions yield dramatic variations of SSA production, yet the governing processes are not well characterized. Studies like the one presented here are sorely missing and these new results represent a great new addition of data on SSA production and mixing as well as on processes affecting them.
This is a very well planned and executed study involving a myriad of observations and data. The manuscript is well organized and written. Considering the amount of used instrumentation, modeling, data collection, and analyses, the manuscript is concise and clear. The results are novel and represent valuable contribution to the representation and understanding of air-sea interaction processes. I recommend this manuscript for publication at ACP. I have minor questions and remarks. These are listed below by line number, not importance.
Line 23: Suggest change “SSP” to “sea salt particles” (thus avoid defining an acronym that is not further used in the abstract)
Lines 30 and 32: Acronyms SSA and SSP seem to be used interchangeably throughout the text. If so, better use “SSA” and “SSA particles” for consistency. If they are meant to represent some differences, it would be good to mention those here in lines 30-31 on first encounter.
Line 31: Symbol rd is used with both italic and non-italic (e.g., line ) subscript throughout the text and in the equations. Use one notation consistently. All other variables are non-italic. That is fine. Just check the ACP requirements for math/variables style.
Line 34: “orders of magnitude less often”—Is this to say that the large particles are fewer in number and occur less frequently? Or is this phrase strictly for the frequency of occurrence?
Line 68: Remove “the” to read “winds below it act to dilute the SSA concentration”
Line 96: Remove comma before “sea state”
Line 112: Remove “sea salt aerosol size distribution” because acronym SSA-SD is already defined in line 91.
Line 133: Introduce the concentration units in parentheses here on first definition of N and do not repeat them anymore; i.e., no need to include units in lines 134, 257, 258, 351, etc) unless they follow a specific number.
Line 133: Unless ACP writing style allows, you should not start a sentence with a symbol, like here starting with r2.8 and in line 135 with rd. There are several other cases (e.g., line 255). Likewise, should not start a sentence with a number as in line 235.
Line 136: After introducing r2.8. and B in lines 131-135, it would be good to specifically say here that you call these shape parameters and also give their physical meaning, besides their definition.
Table 1 caption: Change to Hs (m) and Per (s) for consistency with the listing of all other variables. Case 5 is missing in column “Sample #”; perhaps it has been discarded due to CE < 40% or some other reason. It would be good to mention it to avoid impression of typo on the order of Sample #.
Line 181: Altitude z is given in italic and non-italic. Here and everywhere, please use the same notation.
Line 202: Need to define what is “h-values”
Lines 205-209: Acronyms RRTMG, PBL, EPSSM, and NCAR are not defined. They are not used anywhere else. Better not to use acronyms in this case.
Line 253: “Cumulative” seems to mean averaged over a range of altitudes and a range of winds speeds. If so, better say this specifically to avoid any confusion with the statistical meaning of “cumulative.”
Line 262: Short-hand W53 is not introduced. Good place is line 160 regarding historical samples; introduce it as “Woodcock (1953, W53)”
Figure 7 caption: Put the units for U10 and Hs in parentheses.
Figure 9 caption: How were these altitude bins chosen? If the criterion for the altitude bin formation changed, would that affect the shape of the vertical profiles?
Line 364: I am not sure I see an order of magnitude change of the velocity. From surface to 1000 m altitude, the velocity changes at most from 0.1 to 0.35 m/s. Please paraphrase for clarity.
Line 392: Please fix typo to read “meaning they are unlikely to”
Line 439: Please remove “so when” to read “southern shores when SSTs are”
Line 440: Suggest change to read “across a period of one year”
Line 449: The citations should be all in one set of parentheses to read “(Porter and Clarke,1997; Blanchard et al., 1984)”
Line 463: Please fix the parentheses to read “Table 2 from Grythe et al. (2014) shows”
Citation: https://doi.org/10.5194/egusphere-2023-1387-RC2 - AC2: 'Reply on RC2', Katherine L. Ackerman, 03 Sep 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1387', Anonymous Referee #1, 26 Jul 2023
The manuscript “Mechanisms controlling giant sea salt aerosol size distributions along a tropical orographic coastline” presents measurements of sea salt aerosols, including giant and ultragiant particles, from a novel device both on the coast and in the open ocean. Notable differences in the overall SSA concentrations are noted between the two sites, as are differences in the SSA-SD shape parameters and vertical profiles. Some of the mechanisms are illustrated by trajectory tracking in a WRF run of the area.
In my opinion, this is a great study and a very well-written manuscript. It is well-referenced and clear, and a lot of details were attended to properly. It also provides very reliable measurements of a data-scarce topic: SSA and giant particles in particular, but also comparing coastal to open ocean conditions. A lot remains to be understood about how universal the SSA-SD shape parameters are, and this study does the field a service by so clearly demonstrating some of the key differences that can occur. I only have a few very minor points for the authors to consider, but otherwise think that this would make a great contribution to ACP.
Specific comments:
- Line 9: Maybe should be “facilitating”?
- Sentence starting on line 45: Maybe consider “whitecap fraction” explicitly as one of the things that has been analyzed a lot? It seems to fit with the way the literature has been broken up here – production has been linked with those things listed (U10, SST, SSS) but also whitecap fraction.
- Figure 2: It might be nice to have a legend in the figure itself, explaining the symbols (so the reader doesn’t have to sift through the caption)
- Line 177: I’m not sure what the “Bellows’ 12 m” wind speed is here. I searched around the manuscript but didn’t see any description of what this meant. So I’m left a little uncertain of why it was regressed to the WeatherFlow, and which one was ultimately used for the reported U10 values
- Line 206: What is meant by “the Shin and Hong PBL scheme for the grid resolution”? Is the phrase “for the grid resolution” supposed to be there?
- Line 231: Are these using the averaged wind profiles? Or the time-varying profiles from some set frequency of WRF output? It appears to be the average, so it’s probably worth stating explicitly here
- Line 245: This isn’t necessarily a suggestion for this study, but in the future it might be more realistic to use the PBL scheme parameters to approximate this variability
- Line 267-269: If I’m reading figure 6 correctly, this sentence is really only referring to figure 6b (at specific heights), right? The COAST samples in 6a don’t always exceed W53 at the wind speeds mentioned in this sentence. However I might be misunderstanding because the next paragraph explicitly introduces fig 6b.
- Line 314: “chances” should be “changes”
- Line 315: “strong” should be “stronger”.
- Line 351: I’m not sure the units are needed after “concentrations”
12: Figure 4: It would be helpful if this figure had a panel “c” which showed the average RH profile that these are based on
13: Line 410 (or thereabouts): Somewhere at the end of this section I think it’s important to point out the relatively simple way of doing the trajectory simulations. I think it’s totally appropriate for what the authors are trying to do, but the treatment of turbulence in particular is only very crudely represented. I think the authors do a good job of only drawing conclusions which are supported by this method – I just think that a brief 1-sentence reminder at the end of the processes (especially turbulence) which were left out and could lead to significant differences in some of the details.
14: Lines 435-436: Elsewhere it’s spelled “Hawai’i”
Citation: https://doi.org/10.5194/egusphere-2023-1387-RC1 -
AC1: 'Reply on RC1', Katherine L. Ackerman, 03 Sep 2023
We want to thank the reviewer for the time and effort they put into reviewing our manuscript - your recommendations have improved our paper tremendously, and offered us interesting considerations for our future research! Please see more specific responses in the attached file.
-
RC2: 'Comment on egusphere-2023-1387', Anonymous Referee #2, 01 Aug 2023
This manuscript presents the results of a sea salt aerosol (SSA) study in coastal zone (Hawaii) at moderate wind speeds (Trade winds up to 7 m/s). Super-micron SSA size distributions (dry radii > 2.8 mm) were measured with a miniature impaction instrument, mini-GNI, specifically designed for these observations. The size distributions in coastal zone were compared to SSA size distributions in open ocean and differences up to a factor of 5 are reported. Several mini-GNIs were deployed on a kite to obtain the vertical profiles of SSA number concentrations from 80 to 650 m height. Various environmental data were collected in situ (wind, wave height, air temperature, relative humidity) to identify the most suitable forcing variables for the observed SSA size distributions. Data generated with WRF model were used to simulate SSA trajectories and study the effects of the coastal topography on the SSA vertical dynamics.
Super-micron size distributions of SSA, as well as their vertical profiles, in coastal zone are indeed severely understudied. Though fewer and less frequently occurring, large SSA particles, with their large areas and volumes, contribute disproportionately to various aerosol effects, from scattering and absorption to formation of cloud condensation nuclei. The differences between open ocean and coastal zone conditions yield dramatic variations of SSA production, yet the governing processes are not well characterized. Studies like the one presented here are sorely missing and these new results represent a great new addition of data on SSA production and mixing as well as on processes affecting them.
This is a very well planned and executed study involving a myriad of observations and data. The manuscript is well organized and written. Considering the amount of used instrumentation, modeling, data collection, and analyses, the manuscript is concise and clear. The results are novel and represent valuable contribution to the representation and understanding of air-sea interaction processes. I recommend this manuscript for publication at ACP. I have minor questions and remarks. These are listed below by line number, not importance.
Line 23: Suggest change “SSP” to “sea salt particles” (thus avoid defining an acronym that is not further used in the abstract)
Lines 30 and 32: Acronyms SSA and SSP seem to be used interchangeably throughout the text. If so, better use “SSA” and “SSA particles” for consistency. If they are meant to represent some differences, it would be good to mention those here in lines 30-31 on first encounter.
Line 31: Symbol rd is used with both italic and non-italic (e.g., line ) subscript throughout the text and in the equations. Use one notation consistently. All other variables are non-italic. That is fine. Just check the ACP requirements for math/variables style.
Line 34: “orders of magnitude less often”—Is this to say that the large particles are fewer in number and occur less frequently? Or is this phrase strictly for the frequency of occurrence?
Line 68: Remove “the” to read “winds below it act to dilute the SSA concentration”
Line 96: Remove comma before “sea state”
Line 112: Remove “sea salt aerosol size distribution” because acronym SSA-SD is already defined in line 91.
Line 133: Introduce the concentration units in parentheses here on first definition of N and do not repeat them anymore; i.e., no need to include units in lines 134, 257, 258, 351, etc) unless they follow a specific number.
Line 133: Unless ACP writing style allows, you should not start a sentence with a symbol, like here starting with r2.8 and in line 135 with rd. There are several other cases (e.g., line 255). Likewise, should not start a sentence with a number as in line 235.
Line 136: After introducing r2.8. and B in lines 131-135, it would be good to specifically say here that you call these shape parameters and also give their physical meaning, besides their definition.
Table 1 caption: Change to Hs (m) and Per (s) for consistency with the listing of all other variables. Case 5 is missing in column “Sample #”; perhaps it has been discarded due to CE < 40% or some other reason. It would be good to mention it to avoid impression of typo on the order of Sample #.
Line 181: Altitude z is given in italic and non-italic. Here and everywhere, please use the same notation.
Line 202: Need to define what is “h-values”
Lines 205-209: Acronyms RRTMG, PBL, EPSSM, and NCAR are not defined. They are not used anywhere else. Better not to use acronyms in this case.
Line 253: “Cumulative” seems to mean averaged over a range of altitudes and a range of winds speeds. If so, better say this specifically to avoid any confusion with the statistical meaning of “cumulative.”
Line 262: Short-hand W53 is not introduced. Good place is line 160 regarding historical samples; introduce it as “Woodcock (1953, W53)”
Figure 7 caption: Put the units for U10 and Hs in parentheses.
Figure 9 caption: How were these altitude bins chosen? If the criterion for the altitude bin formation changed, would that affect the shape of the vertical profiles?
Line 364: I am not sure I see an order of magnitude change of the velocity. From surface to 1000 m altitude, the velocity changes at most from 0.1 to 0.35 m/s. Please paraphrase for clarity.
Line 392: Please fix typo to read “meaning they are unlikely to”
Line 439: Please remove “so when” to read “southern shores when SSTs are”
Line 440: Suggest change to read “across a period of one year”
Line 449: The citations should be all in one set of parentheses to read “(Porter and Clarke,1997; Blanchard et al., 1984)”
Line 463: Please fix the parentheses to read “Table 2 from Grythe et al. (2014) shows”
Citation: https://doi.org/10.5194/egusphere-2023-1387-RC2 - AC2: 'Reply on RC2', Katherine L. Ackerman, 03 Sep 2023
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Katherine L. Ackerman
Alison D. Nugent
Chung Taing
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(15832 KB) - Metadata XML