the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Jun 2024
Representation of iron aerosol size distributions is critical in evaluating atmospheric soluble iron input to the ocean
Abstract. Atmospheric aerosol deposition acts as a major source of soluble (bioavailable) iron in open ocean regions where it limits phytoplankton growth and primary production. The aerosol size distribution of emitted iron particles, along with particle growth from mixing with other atmospheric components, is an important modulator of its long-range transport potential. There currently exists a large uncertainty in the particle size distribution of iron aerosol, and the role of aerosol size in shaping global soluble iron deposition is thus unclear. In this study, we couple a sophisticated microphysical, size-resolved aerosol model with an iron-speciated and -processing module to disentangle the impact of iron emission size distributions on soluble iron input to the ocean, with a focus on anthropogenic combustion and metal smelting sources. We first evaluate our model results against a global-scale flight measurement dataset for anthropogenic iron concentration and find that the different representations of iron size distribution at emission, as adopted in previous studies, introduces a variability in modeled iron concentrations over remote oceans of a factor of 10. Shifting the iron aerosol size distribution toward finer particle sizes (<1 μm) enables longer atmospheric lifetime (a doubling), promoting atmospheric processing that enhances the soluble iron deposition to ocean basins by up to 50 % on an annual basis. Importantly, the monthly enhancements reach 110 % and 80 % over the Southern Ocean and North Pacific Ocean, respectively. Compared with emission flux uncertainties, we find that iron emission size distribution plays an equally important role in regulating soluble iron deposition, especially to the remote oceans. Our findings provide implications for understanding the effects of atmospheric nutrients input on marine biogeochemistry, including but not limited to iron, phosphorus, and others.
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RC1: 'Comment on egusphere-2024-1454', Anonymous Referee #2, 08 Jul 2024
This manuscript reports on a numerical modelling study that examines the impact of the size distribution of iron-bearing anthropogenic emissions on the transport and deposition of soluble iron to the oceans. The paper is extremely well written and the work is presented clearly and concisely. I have no hesitation in recommending that it is suitable for publication in ACP with only minor revisions.
Specific Comments
Line 1. The work reported focusses exclusively on the size distribution at emission of anthropogenic iron. The title of the paper does not reflect this. I suggest that “Representation of iron aerosol size distributions of anthropogenic emissions is critical in evaluating atmospheric soluble iron input to the ocean” would give a better representation of the subject of the paper.
L 84. Given the dominance of dust as a source of Fe, it might be helpful for the reader to be given a little more detail on the representation (and validation) of the size distribution of dust Fe in the model. Similarly, the model described identifies various Fe-bearing minerals in anthropogenic emissions (L 100 – 101). Presumably at least some of these minerals also occur in dust. How does the model treat these minerals in dust, and how are the anthropogenic and dust fractions handled in the comparison to the aircraft observations (e.g. Fig. 3)?
L 96-97. Please comment on the implications of the assumption of internal mixing of Fe with all other aerosol components for your results.
L 189 – 190. Please explain this statement further.
L 223 – 224. Perhaps some clarification is needed here? The differences referred to do appear to be more pronounced over much of the global ocean, but not over the Southern Ocean.
L 225 – 227. Is the solubility enhancement referred to here relevant to dust Fe, anthropogenic Fe, or Fe in general?
Technical Corrections
L 102. Delete “taken into account”.
L 117. Also add “ of anthropogenic emissions” to this section heading?
L 139. “A similar…”
L 190. “… used as an …”
L 250. “iron source”, rather than “iron emission”?
Fig. 8. Please add a description of the two panels. (b) shows a percentage difference. What is this difference relative to?
Citation: https://doi.org/10.5194/egusphere-2024-1454-RC1 - AC2: 'Reply on RC1', Mingxu Liu, 09 Sep 2024
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RC2: 'Comment on egusphere-2024-1454', Mónica Zamora Zapata, 10 Jul 2024
This study shows the effects of varying the iron emission size distribution on global-scale simulations to diagnose iron concentration in the atmosphere and ocean deposition. Four different size distributions are studied, properly justified, and their results are compared to aircraft observations, allowing them to conclude on the closest simulated distributions. The methods lack some details about the iron emission and deposition processes. Finally, some comments on the possible biases of using a global model could be useful, reflecting on the possibility of higher resolution models being helpful in further elucidating this phenomenon. Therefore, I recommend a minor revision. Some minor comments follow:
In the abstract, it would be great to highlight which findings are new.
L55 are all of these papers working with global models? Is there a variety of local to global approaches and if so, how does grid resolution and microphysics models differ?
L118 Is it also possible that iron emissions have a different size distribution in other parts of the world? How could this simplification be considered for future work?
L95 I don't completely follow how the emissions are prescribed or calculated. Is it homogeneous over all surfaces in the world? Same with its absorption. Do they change over time?
Fig. 4,5,6: Are these plots derived from yearly averaged values?
Citation: https://doi.org/10.5194/egusphere-2024-1454-RC2 - AC1: 'Reply on RC2', Mingxu Liu, 09 Sep 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-1454', Anonymous Referee #2, 08 Jul 2024
This manuscript reports on a numerical modelling study that examines the impact of the size distribution of iron-bearing anthropogenic emissions on the transport and deposition of soluble iron to the oceans. The paper is extremely well written and the work is presented clearly and concisely. I have no hesitation in recommending that it is suitable for publication in ACP with only minor revisions.
Specific Comments
Line 1. The work reported focusses exclusively on the size distribution at emission of anthropogenic iron. The title of the paper does not reflect this. I suggest that “Representation of iron aerosol size distributions of anthropogenic emissions is critical in evaluating atmospheric soluble iron input to the ocean” would give a better representation of the subject of the paper.
L 84. Given the dominance of dust as a source of Fe, it might be helpful for the reader to be given a little more detail on the representation (and validation) of the size distribution of dust Fe in the model. Similarly, the model described identifies various Fe-bearing minerals in anthropogenic emissions (L 100 – 101). Presumably at least some of these minerals also occur in dust. How does the model treat these minerals in dust, and how are the anthropogenic and dust fractions handled in the comparison to the aircraft observations (e.g. Fig. 3)?
L 96-97. Please comment on the implications of the assumption of internal mixing of Fe with all other aerosol components for your results.
L 189 – 190. Please explain this statement further.
L 223 – 224. Perhaps some clarification is needed here? The differences referred to do appear to be more pronounced over much of the global ocean, but not over the Southern Ocean.
L 225 – 227. Is the solubility enhancement referred to here relevant to dust Fe, anthropogenic Fe, or Fe in general?
Technical Corrections
L 102. Delete “taken into account”.
L 117. Also add “ of anthropogenic emissions” to this section heading?
L 139. “A similar…”
L 190. “… used as an …”
L 250. “iron source”, rather than “iron emission”?
Fig. 8. Please add a description of the two panels. (b) shows a percentage difference. What is this difference relative to?
Citation: https://doi.org/10.5194/egusphere-2024-1454-RC1 - AC2: 'Reply on RC1', Mingxu Liu, 09 Sep 2024
-
RC2: 'Comment on egusphere-2024-1454', Mónica Zamora Zapata, 10 Jul 2024
This study shows the effects of varying the iron emission size distribution on global-scale simulations to diagnose iron concentration in the atmosphere and ocean deposition. Four different size distributions are studied, properly justified, and their results are compared to aircraft observations, allowing them to conclude on the closest simulated distributions. The methods lack some details about the iron emission and deposition processes. Finally, some comments on the possible biases of using a global model could be useful, reflecting on the possibility of higher resolution models being helpful in further elucidating this phenomenon. Therefore, I recommend a minor revision. Some minor comments follow:
In the abstract, it would be great to highlight which findings are new.
L55 are all of these papers working with global models? Is there a variety of local to global approaches and if so, how does grid resolution and microphysics models differ?
L118 Is it also possible that iron emissions have a different size distribution in other parts of the world? How could this simplification be considered for future work?
L95 I don't completely follow how the emissions are prescribed or calculated. Is it homogeneous over all surfaces in the world? Same with its absorption. Do they change over time?
Fig. 4,5,6: Are these plots derived from yearly averaged values?
Citation: https://doi.org/10.5194/egusphere-2024-1454-RC2 - AC1: 'Reply on RC2', Mingxu Liu, 09 Sep 2024
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