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Discussion papers | Copyright
https://doi.org/10.5194/acp-2017-903
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Nov 2017

Research article | 14 Nov 2017

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Alba Badia1, Claire E. Reeves1, Alex R. Baker1, Alfonso Saiz-Lopez2, Rainer Volkamer3,4, Eric C. Apel4, Rebecca S. Hornbrook4, Lucy J. Carpenter5, Stephen J. Andrews5, and Roland von Glasow1,† Alba Badia et al.
  • 1Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
  • 2Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain
  • 3Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
  • 4Earth System Laboratory, Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA
  • 5Wolfson Atmospheric Chemistry Laboratories (WACL), Department of Chemistry, University of York, York, United Kingdom
  • deceased, 6th September 2015

Abstract. This study investigates the impact of halogens on atmospheric chemistry in the tropical troposphere and explores the sensitivity of this to uncertainties in the fluxes of halogens to the atmosphere and the chemical processing. To do this the regional chemistry transport model WRF-Chem has been extended, for the first time, to include halogen chemistry (bromine, chlorine and iodine chemistry), including heterogeneous recycling reactions involving sea-salt aerosol and other particles, reactions of Br with volatile organic compounds (VOCs), along with oceanic emissions of halocarbons, VOCs and inorganic iodine. The study focuses on the tropical East Pacific using field observations from the TORERO campaign (January–February 2012) to evaluate the model performance.

Including all the new processes, the model does a reasonable job reproducing the observed mixing ratios of BrO and IO, albeit with some discrepancies, some of which can be attributed to difficulties in the model's ability to reproduce the observed halocarbons. This is somewhat expected given the large uncertainties in the air-sea fluxes of the halocarbons in a region where there are few observations of seawater concentrations. We see a considerable impact on the Bry partitioning when heterogeneous chemistry is included, with a greater proportion of the Bry in active forms such as BrO, HOBr and dihalogens. Including debromination of sea-salt increases BrO slightly throughout the free troposphere, but in the tropical marine boundary layer, where the sea-salt particles are plentiful and relatively acidic, debromination leads to overestimation of the observed BrO. However, it should be noted that the modelled BrO was extremely sensitive to the inclusion of reactions between Br and the VOCs, which convert Br to HBr, a far less reactive form of Bry. Excluding these reactions leads to modelled BrO mixing ratios greater than observed. The reactions between Br and aldehydes were found to be particularly important, despite the model underestimating the amount of aldehydes observed in the atmosphere. There are only small changes to Iy partitioning and IO when the heterogeneous reactions, primarly on sea-salt, are included.

Our model results show that the tropospheric Ox loss due to halogens is 31%. This loss is mostly due to I (16%) and Br (14%) and it is in good agreement with other estimates from state-of-the-art atmospheric chemistry models.

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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. Then, 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 the chemistry of halogens. Our model results show that the tropospheric Ox loss due to halogens is 31 %.
The oceans have an impact on the composition and reactivity of the troposphere through the...
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