Sensitivity of black carbon concentrations and climate impact to aging and scavenging in OsloCTM2–M7

Lund, Marianne T.; Berntsen, Terje K.; Samset, Bjørn H.

doi:https://doi.org/10.5194/acp-17-6003-2017

Articles | Volume 17, issue 9

https://doi.org/10.5194/acp-17-6003-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-17-6003-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 17, issue 9

Research article

|

16 May 2017

Research article |

| 16 May 2017

Sensitivity of black carbon concentrations and climate impact to aging and scavenging in OsloCTM2–M7

Marianne T. Lund, Terje K. Berntsen, and Bjørn H. Samset

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Final revised paper (published on 16 May 2017)
Supplement to the final revised paper
Preprint (discussion started on 09 Sep 2016)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of M. T. Lund et al.', Anonymous Referee #1, 19 Oct 2016
- AC1: 'Response to anonymous referee #1', M. T. Lund, 05 Jan 2017
SC1: 'Short comments on Lund et al.', Cenlin He, 31 Oct 2016
- AC2: 'Response to short comment by Cenlin He', M. T. Lund, 05 Jan 2017
RC2: 'Reviewing the BC sensitivity study by Lund et al.', Anonymous Referee #2, 02 Nov 2016
- AC3: 'Response to anonymous referee # 2', M. T. Lund, 05 Jan 2017

Peer-review completion

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

AR by Anna Wenzel on behalf of the Authors (06 Jan 2017) Author's response

ED: Referee Nomination & Report Request started (17 Jan 2017) by Toshihiko Takemura

RR by Anonymous Referee #1 (02 Feb 2017)

RR by Anonymous Referee #3 (15 Feb 2017)

Suggestions for revision or reasons for rejection

The paper represents a useful addition to the growing body of literature concerning the sensitivity of BC concentrations to parameterization uncertainties. The paper would benefit from a more detailed description of the modeling approach and discussion of key results. In particular, the meaning of results from sensitivity tests with modified scavenging of hydrophobic BC (ConvBC) and condensation of nitrate (NitCond) is not sufficiently clear.

Major comments:

How much does convective aerosol scavenging contribute to total deposition? How is convective scavenging and transport represented in the model? Is a mass flux scheme used and how are aerosols treated in that scheme? Are interactions between entrainment, detrainment, and scavenging processes in convective aerosol mass budgets accounted for?

Why do BC concentrations respond strongly to changes in the scavenging of hydrophobic BC in the convection? Various studies have shown that externally mixed BC particles are very rare in the remote atmosphere which implies that concentrations of hydrophobic BC should be quite low. This would likely imply a low sensitivity of model results to hydrophobic aerosol concentrations in model sensitivity studies. Please quantify the amount of hydrophobic BC in the model and further information about aging time scales etc.

What sources and sinks of nitrate are accounted for in the model? Do simulated aerosol size distribution respond to changes in nitrate mass and how is this response parameterized? Is ammonia accounted for in the calculation of the thermodynamic nitrate/HNO3 equilibrium and how is ammonia represented in the model?

Results from the study seem to differ from results of at least 2 other models in the Arctic (Mahmood et al., 2016). In these models, convective scavenging of BC mainly affects BC in the upper troposphere and lower stratosphere, which seems opposite to responses shown in Fig. 3 and 4. Why are concentrations near the surface sensitive to convective scavenging, which presumably occurs outside the Arctic? This does not seem plausible considering that transport of BC to the Arctic is mainly isentropic. Similar, aging processes are more likely to affect mid and upper tropospheric concentrations of BC in the Arctic than near-surface concentrations. It would be beneficial to change units to ppb in the graphs in order to illustrate impacts of scavenging processes relative to transport processes in the simulations.

A mean normalized bias (MNB) is introduced but is only applied to a few select simulation results, which limits the usefulness of this metric. In addition, the root mean square error, or similar metric, should be considered, too.

Further comments:

P 2 L 62: Allen and Landuyt (2014) seems relevant in this context, too.

P 4 L 112: Is below-cloud scavenging accounted for?

P 5 L 159: How is soluble material formed in the model? Through condensation?

P 7 L 224: 3D kernels are apparently available but global mean concentration profiles are used to calculate radiative forcings. Why? A lack of regional information limits the analysis for the Arctic in the second half of the paper.

P 10 L 307: AMAP models are shown to produce dramatically different seasonal variations in BC burdens in the Arctic (Mahmood et al., 2016). Currently available observational data sets are insufficient to validate seasonal variations in atmospheric BC burdens in the Arctic. Does this statement refer to near-surface concentrations?

P 11 L 332: Please quantify. What does "somewhat high" mean?

Fig 3: Include a profile from the control simulation for comparison with concentration differences.

P 15 L 451: "...in order for models to reproduce the HIPPO data...". Clearly, these profiles cannot be "reproduced" in models but perhaps the general agreement with observations has been improved? Also, this depends on the metric that is used. Typically, BC concentrations near the surface are systematically too low in models and this bias tends to become more severe at reduced lifetimes.

P 18 L 544: Please provide RFari due to BC from all sources from the model. How does it compare to the IPCC estimate? How do the individual values of RFari in Fig. 6 compare to this value?

P 20 L 606: For comparison, please provide the global mean surface temperature response that corresponds to RFari to BC from all sources. How do the temperature responses in Fig. 6 compare to this value?

References:

Allen, R. J., and W. Landuyt (2014), The vertical distribution of black car-
bon in CMIP5 models: Comparison to observations and the importance of convective transport, J. Geophys. Res. Atmos., 119, 4808-4835, doi:10.1002/2014JD021595.

Mahmood, R., K. von Salzen, M. Flanner, M. Sand, J. Langner, H. Wang, and L. Huang (2016), Seasonality of global and Arctic black carbon processes in the Arctic Monitoring and Assessment Programme models, J. Geophys. Res. Atmos., 121, 7100-7116, doi:10.1002/2016JD024849.

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ED: Reconsider after major revisions (17 Feb 2017) by Toshihiko Takemura

AR by Marianne T. Lund on behalf of the Authors (14 Mar 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (15 Mar 2017) by Toshihiko Takemura

ED: Publish as is (04 Apr 2017) by Toshihiko Takemura

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Short summary

This study investigates possibilities for improving the representation of black carbon (BC) distribution in a global atmospheric chemistry-transport model by exploring uncertainties in key processes controlling the removal of aerosols from the atmosphere. Our results provide an increased understanding of the processes contributing to uncertainties in the BC abundance and climate impact and underline the importance of more observations and experimental data further constrain models.