the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jan 2023
A mechanism of post-depositional processes affecting chlorine and its isotope in the upper snowpack of High Antarctic Plateau
Abstract. The main purpose of this work is to propose a mechanism of post-depositional processes affecting chlorine and its chlorine-36 cosmogenic nuclide in the upper snowpack of the High Antarctic Plateau. We suggest that the observed decrease of total chlorine content in the upper meters of the snowpack is due to a progressive release of the HCl content from ice. We also propose a consistent framework, combining diffusion in bulk ice and snow microstructure. The observation of the low chlorine content in ice at depth leads to the robust hypothesis that the chemical equilibrium of chlorine between the ice and the snowpack interstitial air (SIA) is close to zero. HCl is thought to diffuse in ice, and to be progressively released in the SIA, and exported to the Antarctic atmosphere by the wind-ventilation. The time required to expel all the mobile species of chlorine (i.e., HCl) from snow depends on the diffusion coefficient of chlorine in ice combined with the snow grain size and its evolution with depth. This work is synthesised in a model combining the microstructure evolution of the upper meters of a snowpack (changes in mean snow grain size) and the diffusion of chlorine in ice applied to single spherical grains. The variability observed in chloride concentration profiles with depth, at a same site but different sampling time or different snow pits, or among different sites of the High Antarctic Plateau, is mostly due to the variations in initial concentrations in HCl and sodium chloride (NaCl) species and the snow grain size evolution. This model offers a common framework for understanding the fate of chlorine in Antarctica, from coastal to inland locations, including low accumulation sites on the plateau, far from the ocean. Applications of this post-depositional model to chlorine and to 36Cl allows to picture a recycling mechanism of chlorine at the scale of Antarctica. In particular, the 36Cl concentration in the surface snow of the Vostok site illustrates this recycling mechanism and the persistent contamination of inland Antarctica by anthropogenic 36Cl originating from the marine nuclear tests of the 1950s to the 1970s.
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Status: closed
- RC1: 'Comment on egusphere-2022-1455', Florent Dominé, 08 Feb 2023
-
RC2: 'Comment on egusphere-2022-1455', Thorsten Bartels-Rausch, 13 Feb 2023
The manuscript «A mechanism of post-depositional processes affecting chlorine and its isotope in the upper snowpack of High Antarctic Plateau» by Giraud et al describes modeling work to advance our understanding of the fate of chlorine in the Antarctic snowpack (ll 20). This is a complex endeavor, as evident when one just considers the many processes and factors that determine chlorine levels in the snow – origin, chemical speciation, location in snow, phase changes, migration in snow, air-snow exchange, diffusion, snow metamorphism, and chemical reactions; to name a few. The introduction reflects this complexity, even if not all parameters and processes are discussed or mentioned. The modeling work presented is based on the diffusion of HCL in ice. When considering the snow microstructure, the model is capable of reproducing the observed chlorine trends in snowpacks.
In my understanding, it is a valid and useful approach to use models to test the importance of a specific process. Therefore, I’m very much in favor of a publication. However, taken that the other processes have not been considered or tested, I’d ask to rewrite any sentences that might imply a broader implication of the results. In other words, I’m questioning the “common framework for understanding the fate of chlorine”. I would argue that the model results show that -under the given assumptions- diffusion is an important process for the fate of HCl in snowpacks.
My second major concern is the processes I miss being discussed and incorporated into the model:
- The authors state that releasing HCl from the ice to the atmosphere might take decades. I’m very much wondering why temperature is not considered in the solid diffusion and transport of HCl through the porous snow.
- The temperature might also govern the phase of Chloride. Excuse my ignorance – but are temperatures always low enough to exclude the presence of liquid, even in nano pockets? The partitioning between liquid reservoirs and the air would be governed by Henry and not the solid-solution air equilibrium.
- Transport of trace gases through the porous snow (to reach the overlaying atmosphere) is heavily impacted by adsorption. The partitioning of sticky trace gases to ice surfaces acts as significant resistance to transport. It would slow the removal from the snowpack (Dominé, F., Albert, M. R., Huthwelker, T., Jacobi, H.-W., Kokhanovsky, A. A., Lehning, M., et al. (2008), Snow physics as relevant to snow photochemistry. Atmospheric Chemistry and Physics, 8(2), 171-208. doi:10.5194/acp-8-171-2008; Bartels-Rausch, T., Jacobi, H.-W., Kahan, T. F., Thomas, J. L., Thomson, E. S., Abbatt, J. P. D., et al. (2014), A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow. Atmospheric Chemistry and Physics, 14(3), 1587-1633. doi:10.5194/acp-14-1587-2014; Bartels-Rausch, T., Wren, S. N., Schreiber, S., Riche, F., Schneebeli, M., & Ammann, M. (2013), Diffusion of volatile organics through porous snow: impact of surface adsorption and grain boundaries. Atmospheric Chemistry and Physics, 13(14), 6727-6739. doi:10.5194/acp-13-6727-2013)
I’m confident that when tackling these questions in a major revision, the manuscript might be well suited for publication in TC. In the following, I list a few more questions that came to my mind while reading the manuscript:
Line 9: total chlorine, Are HCl and NaCl the only species detected or the only chlorine species present in the snowpack? What about other organic and inorganic chlorine species?
Line 14, diffusion coefficient of chlorine, and on the concentration gradient as driving force of diffusion, or not?
Line 30, (chloride Cl-, fluoride F-, 30 nitrate NO3-), Whether or not the anions are volatile depends on their chemical form, or not?
Line 42, erroneous age of the ice. Isn’t the age of ice cores often derived by a combination of techniques?
Line 49, alpine regions have. How well do these alpine snow models perform in arctic environments?
Line 55, Ice composition may be considered either as a bulk (Hutterli et al., 1999) or restricted to the surface of snow grains in a liquid-like layer (LLL) (Thomas et al., 2011), There might further be aerosol deposits, micro-pockets, and grain boundaries as location/reservoir for impurities. The presence of a LLL at the air-ice interface further depends on temperature and might be irrelevant at Antarctic temperatures. (Eichler, J., Kleitz, I., Bayer-Giraldi, M., Jansen, D., Kipfstuhl, S., Shigeyama, W., et al. (2017), Location and distribution of micro-inclusions in the EDML and NEEM ice cores using optical microscopy and in situ Raman spectroscopy. Cryosphere, 11(3), 1075-1090. doi:10.5194/tc-11-1075-2017; Bartels-Rausch, T., Jacobi, H.-W., Kahan, T. F., Thomas, J. L., Thomson, E. S., Abbatt, J. P. D., et al. (2014), A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow. Atmospheric Chemistry and Physics, 14(3), 1587-1633. doi:10.5194/acp-14-1587-2014)
Line 65, The time scale of this concentration decrease spans over decades (see section 1.2 and Fig. 1), suggesting slow processes. HCl is thought to diffuse into solid ice, but diffusion coefficients proposed by previous studies cover several orders of magnitude (see section 2.3.1), What role do grain boundaries play as a shortcut for diffusion in Antarctic snow? I have the feeling that -as diffusion via grain boundaries is orders of magnitude faster compared to solid state diffusion- this might be a larger source of uncertainty compared to the uncertainty in the solid state diffusion.
Line 73, observed an apparent shift of 36Cl profile compared to that of Cesium-137 (137Cs), non-mobile in the snowpack and produced during atmospheric nuclear tests at the same time period, Is this because Cs is deposited as solid while Cl as HCl?
Line 77, observed fine structures of, I don’t understand the meaning of this sentence.
Line 82, among other things. Please mention these other things explicitly.
Line 107, leading to a Cl-:Na+ ratio above 4, and even 25 in certain snow layers, values that are much higher than, What explains these high levels? What is the source of Cl- if not sea-salt aerosol?
Line 154, it implies that the ice-air equilibrium value for the mobile and volatile form of chlorine (HCl) at the surface of the snow grain is close to zero, I don’t understand this argument. As long as there is HCl in the grain, the system will go toward equilibrium.
Line 456, the diffusion coefficient and the mean grain size, and the concentration gradient?
Figure 1B: There appear to be 3 blue lines that are hard to see. Please specify in the legend what they are.
Citation: https://doi.org/10.5194/egusphere-2022-1455-RC2
Status: closed
- RC1: 'Comment on egusphere-2022-1455', Florent Dominé, 08 Feb 2023
-
RC2: 'Comment on egusphere-2022-1455', Thorsten Bartels-Rausch, 13 Feb 2023
The manuscript «A mechanism of post-depositional processes affecting chlorine and its isotope in the upper snowpack of High Antarctic Plateau» by Giraud et al describes modeling work to advance our understanding of the fate of chlorine in the Antarctic snowpack (ll 20). This is a complex endeavor, as evident when one just considers the many processes and factors that determine chlorine levels in the snow – origin, chemical speciation, location in snow, phase changes, migration in snow, air-snow exchange, diffusion, snow metamorphism, and chemical reactions; to name a few. The introduction reflects this complexity, even if not all parameters and processes are discussed or mentioned. The modeling work presented is based on the diffusion of HCL in ice. When considering the snow microstructure, the model is capable of reproducing the observed chlorine trends in snowpacks.
In my understanding, it is a valid and useful approach to use models to test the importance of a specific process. Therefore, I’m very much in favor of a publication. However, taken that the other processes have not been considered or tested, I’d ask to rewrite any sentences that might imply a broader implication of the results. In other words, I’m questioning the “common framework for understanding the fate of chlorine”. I would argue that the model results show that -under the given assumptions- diffusion is an important process for the fate of HCl in snowpacks.
My second major concern is the processes I miss being discussed and incorporated into the model:
- The authors state that releasing HCl from the ice to the atmosphere might take decades. I’m very much wondering why temperature is not considered in the solid diffusion and transport of HCl through the porous snow.
- The temperature might also govern the phase of Chloride. Excuse my ignorance – but are temperatures always low enough to exclude the presence of liquid, even in nano pockets? The partitioning between liquid reservoirs and the air would be governed by Henry and not the solid-solution air equilibrium.
- Transport of trace gases through the porous snow (to reach the overlaying atmosphere) is heavily impacted by adsorption. The partitioning of sticky trace gases to ice surfaces acts as significant resistance to transport. It would slow the removal from the snowpack (Dominé, F., Albert, M. R., Huthwelker, T., Jacobi, H.-W., Kokhanovsky, A. A., Lehning, M., et al. (2008), Snow physics as relevant to snow photochemistry. Atmospheric Chemistry and Physics, 8(2), 171-208. doi:10.5194/acp-8-171-2008; Bartels-Rausch, T., Jacobi, H.-W., Kahan, T. F., Thomas, J. L., Thomson, E. S., Abbatt, J. P. D., et al. (2014), A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow. Atmospheric Chemistry and Physics, 14(3), 1587-1633. doi:10.5194/acp-14-1587-2014; Bartels-Rausch, T., Wren, S. N., Schreiber, S., Riche, F., Schneebeli, M., & Ammann, M. (2013), Diffusion of volatile organics through porous snow: impact of surface adsorption and grain boundaries. Atmospheric Chemistry and Physics, 13(14), 6727-6739. doi:10.5194/acp-13-6727-2013)
I’m confident that when tackling these questions in a major revision, the manuscript might be well suited for publication in TC. In the following, I list a few more questions that came to my mind while reading the manuscript:
Line 9: total chlorine, Are HCl and NaCl the only species detected or the only chlorine species present in the snowpack? What about other organic and inorganic chlorine species?
Line 14, diffusion coefficient of chlorine, and on the concentration gradient as driving force of diffusion, or not?
Line 30, (chloride Cl-, fluoride F-, 30 nitrate NO3-), Whether or not the anions are volatile depends on their chemical form, or not?
Line 42, erroneous age of the ice. Isn’t the age of ice cores often derived by a combination of techniques?
Line 49, alpine regions have. How well do these alpine snow models perform in arctic environments?
Line 55, Ice composition may be considered either as a bulk (Hutterli et al., 1999) or restricted to the surface of snow grains in a liquid-like layer (LLL) (Thomas et al., 2011), There might further be aerosol deposits, micro-pockets, and grain boundaries as location/reservoir for impurities. The presence of a LLL at the air-ice interface further depends on temperature and might be irrelevant at Antarctic temperatures. (Eichler, J., Kleitz, I., Bayer-Giraldi, M., Jansen, D., Kipfstuhl, S., Shigeyama, W., et al. (2017), Location and distribution of micro-inclusions in the EDML and NEEM ice cores using optical microscopy and in situ Raman spectroscopy. Cryosphere, 11(3), 1075-1090. doi:10.5194/tc-11-1075-2017; Bartels-Rausch, T., Jacobi, H.-W., Kahan, T. F., Thomas, J. L., Thomson, E. S., Abbatt, J. P. D., et al. (2014), A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow. Atmospheric Chemistry and Physics, 14(3), 1587-1633. doi:10.5194/acp-14-1587-2014)
Line 65, The time scale of this concentration decrease spans over decades (see section 1.2 and Fig. 1), suggesting slow processes. HCl is thought to diffuse into solid ice, but diffusion coefficients proposed by previous studies cover several orders of magnitude (see section 2.3.1), What role do grain boundaries play as a shortcut for diffusion in Antarctic snow? I have the feeling that -as diffusion via grain boundaries is orders of magnitude faster compared to solid state diffusion- this might be a larger source of uncertainty compared to the uncertainty in the solid state diffusion.
Line 73, observed an apparent shift of 36Cl profile compared to that of Cesium-137 (137Cs), non-mobile in the snowpack and produced during atmospheric nuclear tests at the same time period, Is this because Cs is deposited as solid while Cl as HCl?
Line 77, observed fine structures of, I don’t understand the meaning of this sentence.
Line 82, among other things. Please mention these other things explicitly.
Line 107, leading to a Cl-:Na+ ratio above 4, and even 25 in certain snow layers, values that are much higher than, What explains these high levels? What is the source of Cl- if not sea-salt aerosol?
Line 154, it implies that the ice-air equilibrium value for the mobile and volatile form of chlorine (HCl) at the surface of the snow grain is close to zero, I don’t understand this argument. As long as there is HCl in the grain, the system will go toward equilibrium.
Line 456, the diffusion coefficient and the mean grain size, and the concentration gradient?
Figure 1B: There appear to be 3 blue lines that are hard to see. Please specify in the legend what they are.
Citation: https://doi.org/10.5194/egusphere-2022-1455-RC2
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