01 Sep 2020

01 Sep 2020

Review status: this preprint is currently under review for the journal ACP.

Cloud-scale modelling of the impact of deep convection on the fate of oceanic bromoform in the troposphere: a case study over the west coast of Borneo

Paul D. Hamer1,2, Virginie Marécal2, Ryan Hossaini3, Michel Pirre4,, Gisèle Krysztofiak4, Franziska Ziska5, Andreas Engel6, Stephan Sala6, Timo Keber6, Harald Bönisch6, Elliot Atlas7, Kirstin Krüger8, Martyn Chipperfield9, Valery Catoire4, Azizan A. Samah10,11, Marcel Dorf12, Phang Siew Moi11, Hans Schlager13, and Klaus Pfeilsticker14 Paul D. Hamer et al.
  • 1Norwegian Institute for Air Research (NILU), Kjeller, Norway
  • 2Centre National de Recherches Météorologiques, Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • 3Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK
  • 4Laboratoire de Physique et Chimiede l’Environnement et de l’Espace, CNRS and University of Orléans, UMR7328, Orléans, France
  • 5GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 6Institute for Atmospheric and Environmental Sciences, University of Frankfurt, Altenhöferallee 1, 60438 Frankfurt, Germany
  • 7University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149
  • 8Department of Geosciences, University of Oslo, Postboks 1022, Blindern, 0315 OSLO
  • 9The Institute for Climate & Atmospheric, Science, School of Earth and Environment, University of Leeds, Leeds, UK
  • 10National Antarctic Research Centre, University of Malaya, Kuala Lumpur 50603, Malaysia
  • 11Institute of Ocean & Earth Sciences, University of Malaya, 50603 Kuala Lumpur, Malaysia
  • 12Max Planck Institute for Chemistry, Department of Atmospheric Chemistry, Mainz, Germany
  • 13Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Atmosphärische Spurenstoffe, Münchner Straße 20, 82234 Oberpfaffenhofen-Wessling, Germany
  • 14Institute of Environmental Physics, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany
  • retired

Abstract. Coastal oceans emit bromoform (CHBr3) that can be transported rapidly to the upper troposphere by deep convection. In the troposphere, the spatial and vertical distribution of CHBr3 and its product gases (PGs) depend on emissions, chemical processing, transport by large scale flow, convection, and associated washout. This paper presents a modelling study on the fate of CHBr3 and its PGs in the troposphere. A case study at cloud scale was conducted along the west coast of Borneo, when several deep convective systems triggered in the afternoon and early evening of November 19th 2011. These systems were sampled by the Falcon aircraft during the field campaign of the SHIVA project. We analyse these systems using a simulation with the cloud-resolving meteorological model C-CATT-BRAMS at 2 × 2 km resolution that describes transport, photochemistry, and washout of CHBr3. We find that simulated CHBr3 mixing ratios and the observed values in the boundary layer and the outflow of the convective systems agree. However, the model underestimates the background CHBr3 mixing ratios in the upper troposphere, which suggests a missing source. An analysis of the simulated chemical speciation of bromine within and around each simulated convective system during the mature convective stage reveals that > 85 % of the bromine derived from CHBr3 and its PGs is transported vertically to the point of convective detrainment in the form of CHBr3 and that the remaining small fraction is in the form of organic PGs, principally insoluble brominated carbonyls produced from the photo-oxidation of CHBr3. The model simulates that within the boundary layer and free troposphere, the inorganic PGs are only present in soluble forms, i.e., HBr, HOBr, and BrONO2, and consequently, within the convective clouds, the inorganic PGs are almost entirely removed by wet scavenging. For the conditions of the simulated case study Br2 plays no significant role in the vertical transport of bromine. This likely results from the small simulated quantities of inorganic bromine involved, the presence of HBr in large excess compared to HOBr and the less soluble BrO, and the relatively quick removal of soluble compounds within the convective column. This prevalence of HBr is a result of the wider simulated regional atmospheric composition whereby background tropospheric ozone levels are exceptionally low.

Paul D. Hamer et al.

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Paul D. Hamer et al.

Paul D. Hamer et al.


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Short summary
Bromoform is a stratospheric ozone depleting gas released by seaweed and plankton that is transported to the stratosphere via convection in the tropics. We study the chemical interactions of bromoform and its derivatives within convective clouds using a cloud scale model and observations. Our findings are that soluble bromine gases are efficiently washed out and removed within the convective clouds and that most bromine is transported vertically to the upper troposphere in the form of bromoform.