Contrasting source contributions of Arctic black carbon to atmospheric concentrations, deposition flux, and atmospheric and snow radiative effects

Matsui, Hitoshi; Mori, Tatsuhiro; Ohata, Sho; Moteki, Nobuhiro; Oshima, Naga; Goto-Azuma, Kumiko; Koike, Makoto; Kondo, Yutaka

doi:https://doi.org/10.5194/acp-22-8989-2022

Articles | Volume 22, issue 13

https://doi.org/10.5194/acp-22-8989-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-22-8989-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 22, issue 13

Research article

|

12 Jul 2022

Research article |

| 12 Jul 2022

Contrasting source contributions of Arctic black carbon to atmospheric concentrations, deposition flux, and atmospheric and snow radiative effects

Hitoshi Matsui, Tatsuhiro Mori, Sho Ohata, Nobuhiro Moteki, Naga Oshima, Kumiko Goto-Azuma, Makoto Koike, and Yutaka Kondo

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Final revised paper (published on 12 Jul 2022)
Supplement to the final revised paper
Preprint (discussion started on 05 Jan 2022)
Supplement to the preprint

Interactive discussion

Status: closed

RC1: 'Comment on acp-2021-1091', Anonymous Referee #1, 08 Feb 2022

The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-1091/acp-2021-1091-RC1-supplement.pdf

Citation: https://doi.org/10.5194/acp-2021-1091-RC1
RC2: 'Comment on acp-2021-1091', Anonymous Referee #2, 27 Feb 2022

The authors present a thorough investigation into the source contributions of different Northern Hemisphere regions to black carbon in the Arctic, using the CAM-ATRAS aerosol microphysical module. The key novelty of the paper lies in the separate evaluation of BC surface concentrations, deposition, column loading, and atmospheric and bc-on-snow radiative forcing. The analysis is for the most part well documented and presented, and should be of broad interest to the BC-Arctic and aerosol modelling communities.

I mostly have questions and comments relating to the clarity of the presentation, and recommend publication in ACP after fairly minor revisions.

Major points:

1) My one major concern with the entire paper and analysis is the reliance on three years only, to represent a climatology. There is significant interannual variability in BC emissions, transport, loading, precipitation etc., which is not touched on in the analysis but which is crucial for understanding the observed conditions in the Arctic - and for a realistic model representation. I would urge the authors to either document whether the three years they have used really can be said to represent a climatology (e.g. using extended simulations, or, if this is not practical, longer time series from other models that are already available through AeroCom, CMIP6 or similar), or - preferably - to add discussion of the interannual variability in their results throughout. This would be a major addition, of course, but it would also markedly strengthen the conclusions and community relevance of the paper.

2) In the description of the simulations, I could not find the model setup. I assume you are running with nudged simulations for the years 2009-2011? (If not, the RF calculations presensented later would not be correct, so I hope this is the case.) I recommend documenting this is some more detail.

3) The global mean lifetime of BC in the baseline model is given as 5.6 days. This is at the upper end of recent estimates (see e.g. Lund et al. 2018 (https://www.nature.com/articles/s41612-018-0040-x), and could be expected to affect the transport of Asian BC into the Arctic. (Or rather, the processes that lead to this lifetime indicate that ageing and wet removal are slow enough to allow for transport into the Arctic.) However, the modelled lifetime, and therefore the type of results shown in this study, are very sensitive to how these processes are parameterized. There are currently no sensitivity studies of this in the manuscript. Would it be worth the effort to check how sensitive the results are to a realistic change in wet removal/ageing? If this dramatically changes the source region composition, then that is of course of high interest to the community as it will indicate a major source of model diversity in Arctic BC RF.

Minor points:

Figure 5: This is not a major point of the paper, but it seems to me that the model has essentially no interannual variability in BC on /in snow. There is a geographical variation, but for each location the model points all lie on a virtually straight line while the observations range over 1-2 orders of magnitude. This is perhaps worth mentioning? See also my first point above.

Figure 6: The purple regions are not easy to interpret. Is this the lowest color in the scale? (It seems so, but I had to zoom in on the colorbar on a large screen to see it.)

Line 285: "largest contributions to Arctic BC" -> this should be just "BC" I think. The figure shows the dominating source regions for the entire NH, not just the Arctic.

Line 312: AeroCom models -> AeroCom Phase II models (the RF range will differ for the various AeroCom phases)

Citation: https://doi.org/10.5194/acp-2021-1091-RC2
AC1: 'Comment on acp-2021-1091', Hitoshi Matsui, 28 Mar 2022

The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-1091/acp-2021-1091-AC1-supplement.pdf

Citation: https://doi.org/10.5194/acp-2021-1091-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Hitoshi Matsui on behalf of the Authors (28 Mar 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (04 Apr 2022) by Philip Stier

RR by Anonymous Referee #1 (15 Apr 2022)

ED: Publish subject to minor revisions (review by editor) (28 Apr 2022) by Philip Stier

Dear Authors

Thank you very much for your revised manuscript and the responses to the issues raised by the reviewers. The manuscript has now undergone re-review and I concur with the reviewer that most of the issues have now been sufficiently addressed.

Having said that, I also agree with the assessment that the explanation of the enhanced mass absorption coefficient for Asian BC would still benefit from a bit more scrutiny and improved explanation.

An understanding of this relies on the description of the aerosol microphysics. However, in the present form, insufficient detail is provided to understand key processes, in particular regarding the mixing state and the radiative properties:

- It is not entirely clear from the description, if all microphysical processes act on the full 2D bins or if they are estimated from simulating microphysical processes on the 17 size bins and subsequent allocation to mixing state bins.

- The calculation of radiative properties as "calculated theoretically (Bohren and Huffman, 1998;" is not sufficient. What theories are used and what are the actual references (noting that Bohren and Huffman is a full textbook not a specific reference)? How is mixing represented (effective medium approximations, core/shell,...) and what are the related uncertainties?

- And finally, the representation of the mixing state in microphysical models tends to be heavily affected by assumptions made during emissions (which often dominate over the actual microphysical processes). While some BC sources emit fairly pure particles, other sources have already a high degree of internal mixing. How is the mixing state dealt with at point of emission (or of formation of secondary organics)?

I have now accepted the manuscript subject to minor revisions and am looking forward to seeing your response and the revised manuscript.

Kind regards,
Philip Stier

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AR by Hitoshi Matsui on behalf of the Authors (08 May 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (05 Jun 2022) by Philip Stier

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Short summary

Using a global aerosol model, we find that the source contributions to radiative effects of black carbon (BC) in the Arctic are quite different from those to mass concentrations and deposition flux of BC in the Arctic. This is because microphysical properties (e.g., mixing state), altitudes, and seasonal variations of BC in the atmosphere differ among emissions sources. These differences need to be considered for accurate simulations of Arctic BC and its source contributions and climate impacts.