Quantifying sources of black carbon in western North America using observationally based analysis and an emission tagging technique in the Community Atmosphere Model

Zhang, R.; Wang, H.; Hegg, D. A.; Qian, Y.; Doherty, S. J.; Dang, C.; Ma, P.-L.; Rasch, P. J.; Fu, Q.

doi:https://doi.org/10.5194/acp-15-12805-2015

Articles | Volume 15, issue 22

https://doi.org/10.5194/acp-15-12805-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-15-12805-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 22

Research article

|

18 Nov 2015

Research article |

| 18 Nov 2015

Quantifying sources of black carbon in western North America using observationally based analysis and an emission tagging technique in the Community Atmosphere Model

R. Zhang, H. Wang, D. A. Hegg, Y. Qian, S. J. Doherty, C. Dang, P.-L. Ma, P. J. Rasch, and Q. Fu

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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC C2984: 'Useful approach but improvements necessary', Anonymous Referee #1, 26 May 2015
RC C3756: 'A relevant study, but it needs to be compressed and focused on the key aspects', Anonymous Referee #2, 17 Jun 2015
AC C5847: 'Response to the Referees', Hailong Wang, 12 Aug 2015

Peer-review completion

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

AR by Hailong Wang on behalf of the Authors (12 Aug 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Aug 2015) by Dominick Spracklen

RR by Anonymous Referee #1 (31 Aug 2015)

Suggestions for revision or reasons for rejection

The manuscript has been substantially improved. However, the rationale of comparing snow column BC mixing ratios (BCC) and near-surface atmospheric concentrations of BC (BCS) is still unclear (i.e. page 12973, lines 6-11 in original manuscript; page 16, paragraphs following line 327 in revised manuscript). There is still a general lack of focus, as was pointed about by the other referee. The former is a critical shortcoming of the study because the authors hinge the second part of the paper and key conclusions on the proposition that biases in BCC and BCS somehow provide evidence for a missing source of BC in the model without sufficiently justifying this. For instance, the authors argue that deficiencies in emissions inventories are not likely the primary explanation for the model under-prediction of BCC (page 17, line 345). Instead the authors consider a lack of an additional source, and more specifically, a soil source of BC to be a more plausible explanation of model biases (page 18, line 368). It is still not clear how the authors come to this conclusion.

The use of comparisons of BCC and BCS biases to argue for missing processes (or emissions) in the model is misguided since both quantities are affected by fundamentally different processes, none of which are accounted for in the analysis in the manuscript. The importance of different processes for explaining differences in BCC and BCS biases is clearly evident from results presented in the manuscript. In particular, Fig. 7 shows that deposition and atmospheric concentrations of BC have very different spatial distributions. Furthermore, section 3.3.1 (page 19) provides a useful discussion of differences in atmospheric concentrations and deposition fluxes. However, the analysis of BCS and BCC does not account for any of this.

Generally, concentrations of BC in the snow (BCC) strongly depend on aerosol wet removal by precipitation and snow microphysical processes during the formation of the snowpack. In contrast, atmospheric near-surface concentrations (BCS) are probably more sensitive to local emissions and near-surface atmospheric processes, which occur on very different time scales than processes that affect BCC. Comparisons of biases in BCC and BCS in the manuscript are based on averages of these quantities over the same short (3 month) time period, without accounting for impacts of precipitation nor any other processes on analysis results. These are much serious problems than the sparsity of data which is mentioned as possible source of analysis uncertainty (page 16, line 334).

A fundamental re-think of the analysis approach is necessary. What information about model processes (or their lack thereof) can be derived specifically from an analysis of BCS and BCC? How do model biases in deposition rates and other processes affect the results? How plausible is a soil source of BC for explaining differences in simulated and observed BCC concentrations? Is it possible to constrain potential soil BC emissions by considering soil fraction and dust mobilization processes in model sensitivity simulations for a range of plausible conditions? Is there any evidence that regions with more persistent and widespread snow cover are less affected by soil BC emissions than regions with more exposed soil?

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RR by Anonymous Referee #2 (19 Sep 2015)

ED: Reconsider after minor revisions (Editor review) (09 Oct 2015) by Dominick Spracklen

Thanks for your response to reviews and your revised manuscript. One of the reviewers is happy with your revisions. However, the second reviewer has remaining concerns with your manuscript that need to be addressed before publication.

The following key concerns must be addressed before we can accept the manuscript for publication:

1) The rationale of comparing snow column BC mixing ratios (BCC) and near-surface atmospheric concentrations of BC (BCS) is still unclear (i.e. page 12973, lines 6-11 in original manuscript; page 16, paragraphs following line 327 in revised manuscript). Please clarify.

2) It is not clear why a missing soil source of BC is the most plausible explanation of model biases. Other explanations seem equally plausible. For example, could emissions and deposition both be underestimated by the model? This discussion around the soil source of BC needs to be further clarified. Other potential sources of bias need to be discussed. If you are confident that a missing soil source is responsible your reasoning needs to be clarified. The referee raises the following questions that you should consider: What information about model processes can be derived specifically from an analysis of BCS and BCC? How do model biases in deposition rates and other processes affect the results? Is it possible to constrain potential soil BC emissions by considering soil fraction and dust mobilization processes in model sensitivity simulations for a range of plausible conditions? Is there any evidence that regions with more persistent and widespread snow cover are less affected by soil BC emissions than regions with more exposed soil?

3) Generally, concentrations of BC in the snow (BCC) strongly depend on aerosol wet removal by precipitation and snow microphysical processes during the formation of the snowpack. In contrast, atmospheric near-surface concentrations (BCS) are probably more sensitive to local emissions and near-surface atmospheric processes, which occur on very different time scales than processes that affect BCC. Comparisons of biases in BCC and BCS in the manuscript are based on averages of these quantities over the same short (3 month) time period, without accounting for impacts of precipitation nor any other processes on analysis results. These are much serious problems than the sparsity of data which is mentioned as possible source of analysis uncertainty (page 16, line 334). Please add a discussion of these issues.

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AR by Hailong Wang on behalf of the Authors (19 Oct 2015) Author's response Manuscript

ED: Publish as is (05 Nov 2015) by Dominick Spracklen

AR by Hailong Wang on behalf of the Authors (10 Nov 2015) Manuscript

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Short summary

We use a global climate model with an explicit source tagging technique to quantify contributions of emissions from various geographical regions and sectors to BC in North America. Model results are evaluated against measurements of near-surface and in-snow BC. We found strong spatial variations of BC and its radiative forcing that can be quantitatively attributed to the various source origins, and also identified a significant source of BC in snow that is likely missing in most climate models.