Importance of reactive halogens in the tropical marine atmosphere: a regional modelling study using WRF-Chem

Badia, Alba; Reeves, Claire E.; Baker, Alex R.; Saiz-Lopez, Alfonso; Volkamer, Rainer; Koenig, Theodore K.; Apel, Eric C.; Hornbrook, Rebecca S.; Carpenter, Lucy J.; Andrews, Stephen J.; Sherwen, Tomás; von Glasow, Roland

doi:https://doi.org/10.5194/acp-19-3161-2019

Articles | Volume 19, issue 5

https://doi.org/10.5194/acp-19-3161-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-19-3161-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 5

Research article

|

12 Mar 2019

Research article |

| 12 Mar 2019

Importance of reactive halogens in the tropical marine atmosphere: a regional modelling study using WRF-Chem

Alba Badia, Claire E. Reeves, Alex R. Baker, Alfonso Saiz-Lopez, Rainer Volkamer, Theodore K. Koenig, Eric C. Apel, Rebecca S. Hornbrook, Lucy J. Carpenter, Stephen J. Andrews, Tomás Sherwen, and Roland von Glasow

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Preprint (discussion started on 14 Nov 2017)
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Interactive discussion

Status: closed

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

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RC1: 'Model, Halogens, Troposphere', Anonymous Referee #2, 12 Dec 2017
RC2: 'Review of Badia et al. “Importance of reactive halogens in the topical marine atmosphere: A regional modelling study using WRF-Chem”', Anonymous Referee #1, 19 Jan 2018
- AC2: 'RC1: 'Review of Badia et al. 2016', Anonymous Referee #1, 19 January 2018', Alba Badia, 01 Nov 2018
AC1: 'Review of ““Importance of reactive halogens in the tropical marine atmosphere: A regional modelling study using WRF-Chem” by Alba Badia et al.', Alba Badia, 01 Nov 2018

Peer-review completion

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

AR by Alba Badia Moragas on behalf of the Authors (07 Nov 2018) Author's response

ED: Referee Nomination & Report Request started (13 Nov 2018) by Aurélien Dommergue

RR by Anonymous Referee #1 (21 Dec 2018)

Suggestions for revision or reasons for rejection

The authors have addressed all of the reviewer comments, except for the comments by both reviewers regarding the mass transfer expression used. Given the importance of gas-aerosol interactions for halogen chemistry, the authors must address this point.

The authors should review the recent book from Brasseur and Jacob, Modeling of Atmospheric Chemistry, 2017, Section 5.5.2. The issue of mass transport limitation by gas diffusion is very well described in this section. Specifically, Brasseur and Jacob point out that "The free molecular regime generally applies to stratospheric aerosols. ... Tropospheric aerosols are often in the transition regime." This section then goes on to describe how to treat mass transfer using the expression of Schwartz (1986) for the transition regime, which is generally within 10% of the exact solution.

The authors must demonstrate that they are in the free molecular regime in order to generally use their equation 2 throughout their model runs. The expression for the mass transfer rate coefficient in the free molecular regime, which the authors use, is given in Equation 5.117 (k_T in uptake rate expression given by 5.114) in Brasseur and Jacob. If the runs the authors present are not in the free molecular regime, then it is necessary to re-complete at least one model run with the more correct mass transfer rate coefficient, given in equation 5.121 in Brasseur and Jacob. The authors should discuss how the model results change (if they do) upon using the more correct expression. The authors argue that this will add additional computational cost in large scale models and as a result the mass transfer expression requires simplification in their runs. This is not convincing as the more correct expression will not add significant computational costs. However, I recognize that it is likely unfeasible to re-complete all the model runs for this study with the updated expression due to the large number of runs. This paper can be accepted if they can demonstrate how using the correct expression impacts the results in their case and discuss this briefly in the paper.

Even if other large scale model studies have used the the same expression (free molecular regime), we should not continue to generally use an expression that we know is incorrect for aerosol-gas interactions in the troposphere.

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ED: Publish subject to minor revisions (review by editor) (03 Jan 2019) by Aurélien Dommergue

AR by Alba Badia Moragas on behalf of the Authors (25 Jan 2019) Author's response Manuscript

ED: Publish as is (29 Jan 2019) by Aurélien Dommergue

AR by Alba Badia Moragas on behalf of the Authors (08 Feb 2019) Manuscript

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Short summary

The oceans have an impact on the composition and reactivity of the troposphere through the emission of gases and particles. Thus, a quantitative understanding of the marine atmosphere is crucial to examine the oxidative capacity and climate forcing. This study investigates the impact of halogens in the tropical troposphere and explores the sensitivity of this to uncertainties in the fluxes and their chemical processing. Our modelled tropospheric O_x loss due to halogens ranges from 20 % to 60 %.