Impact of mixing state and hygroscopicity on CCN activity of biomass burning aerosol in Amazonia

Sánchez Gácita, Madeleine; Longo, Karla M.; Freire, Julliana L. M.; Freitas, Saulo R.; Martin, Scot T.

doi:https://doi.org/10.5194/acp-17-2373-2017

Articles | Volume 17, issue 3

https://doi.org/10.5194/acp-17-2373-2017

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

Special issue:

South AMerican Biomass Burning Analysis (SAMBBA)

https://doi.org/10.5194/acp-17-2373-2017

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

Articles | Volume 17, issue 3

Research article

|

15 Feb 2017

Research article |

| 15 Feb 2017

Impact of mixing state and hygroscopicity on CCN activity of biomass burning aerosol in Amazonia

Madeleine Sánchez Gácita, Karla M. Longo, Julliana L. M. Freire, Saulo R. Freitas, and Scot T. Martin

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Final revised paper (published on 15 Feb 2017)
Supplement to the final revised paper
Preprint (discussion started on 18 Apr 2016)

Interactive discussion

Status: closed

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

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RC1: 'Review report: Gacita et al', Anonymous Referee #1, 23 Jun 2016
RC2: 'Review of Sánchez Gácita et al., Impact of mixing state and hygroscopicity on CCN activity of biomass burning aerosol in Amazonia', Anonymous Referee #2, 29 Jun 2016
AC1: 'Response to the referees', Madeleine Sánchez Gácita, 11 Aug 2016

Peer-review completion

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

AR by Madeleine Sánchez Gácita Casagrande on behalf of the Authors (09 Sep 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 Oct 2016) by Hugh Coe

RR by Anonymous Referee #3 (23 Nov 2016)

RR by Anonymous Referee #2 (03 Jan 2017)

Suggestions for revision or reasons for rejection

Re-review of Sánchez Gácita et al.,

The manuscript has been fairly heavily revised and improved in many regards. In particular, the methodology section has been reduced where necessary and clarified significantly. The clarifications substantially improve the readability. Similarly, the removal of the field experiment descriptions from the main text helps to avoid distracting the reader.

Relating to my concern about the sensitivity to initial RH in the simulations, the authors state that they "found that the influence of the initial relative humidity was very low both to supersaturation and to activated fractions". This finding appears surprising though the additional figure in the supplementary material appears to confirm this statement. Whether this is a peculiarity for the particular simulation conditions or a general behaviour of the model configuration is unclear, but the statement that the findings are insensitive to the initialisation is convincing.

The last paragraph of the conclusion is very welcome and though it is still a little unclear about the actual recommendations being made for GCMs with more complex activation treatments. It is unlikely that a global modeller would degrade a model's parameterisation to allow for a regionally specific simplification if there was already a sophisticated treatment of mixing-state and kinetic limitation in activation. I guess the utility of the recommendations in the paper are in guiding the choices for less sophisticated representations, though I would still question whether the relative importance of capturing BB mixing-state or compositional effects in such models would convey as much benefit as addressing sub-grid updraught variability.

I also previously mentioned the omission of any consideration of recent work on surface properties other than the Christensen and Petters temperature dependence. The authors state that this is "not within the proposed scope of the submitted manuscript to approach this question". The authors then state that (p10, line 14 of annotated author response) "surface tension dependence on temperature is relevant to CCN activation". I am unsure how the roles of surface tension and bulk-to-surface partitioning can be outside the scope of the manuscript, but temperature dependence is considered to be in-scope. I would expect that, in order to set the priority for the study, it should be demonstrated that temperature effects were not just second order to the other surface tension dependences. An acknowledgement in the conclusions that the arguably first order effect is ignored seems rather dismissive.

The original abstract was too discursive and did not provide a quantitative precis of the key findings. The revised abstract makes some attempt to rectify this, but has become rather garbled. It seems to state that there can be a large sensitivity to mixing state and kinetic limitation if the composition is varied substantially outside the regimes found in the Amazon, but that neither are important if constrained to measured hygroscopicity values. If this is a correct interpretation of the abstract, then a clear statement to this effect would be welcomed - using the last paragraph of the conclusion as a guide, perhaps.

In general, the authors have provided a tighter and more focused version of the manuscript addressing my previous points and the paper can provide a useful contribution to the literature.

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ED: Reconsider after minor revisions (Editor review) (04 Jan 2017) by Hugh Coe

AR by Madeleine Sánchez Gácita Casagrande on behalf of the Authors (15 Jan 2017) Author's response Manuscript

ED: Publish as is (16 Jan 2017) by Hugh Coe

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Short summary

This study uses an adiabatic cloud model to simulate the activation of smoke aerosol particles in the Amazon region as cloud condensation nuclei (CCN). The relative importance of variability in hygroscopicity, mixing state, and activation kinetics for the activated fraction and maximum supersaturation is assessed. Our findings on uncertainties and sensitivities provide guidance on appropriate simplifications that can be used for modeling of smoke aerosols within general circulation models.