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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 17 | Copyright
Atmos. Chem. Phys., 18, 13135-13153, 2018
https://doi.org/10.5194/acp-18-13135-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 12 Sep 2018

Research article | 12 Sep 2018

Quantifying the vertical transport of CHBr3 and CH2Br2 over the western Pacific

Robyn Butler1, Paul I. Palmer1, Liang Feng1, Stephen J. Andrews2, Elliot L. Atlas3, Lucy J. Carpenter2, Valeria Donets3, Neil R. P. Harris4,a, Stephen A. Montzka5, Laura L. Pan6, Ross J. Salawitch7, and Sue M. Schauffler6 Robyn Butler et al.
  • 1School of GeoSciences, University of Edinburgh, Edinburgh, UK
  • 2Department of Chemistry, Wolfson Atmospheric Chemistry Laboratories, University of York, York, UK
  • 3University of Miami, Department of Atmospheric Science, Miami, Florida, USA
  • 4Department of Chemistry, University of Cambridge, Cambridge, UK
  • 5National Oceanic and Atmospheric Administration, Boulder, Colorado, USA
  • 6National Center for Atmospheric Research, Boulder, Colorado, USA
  • 7University of Maryland, Department of Atmospheric and Oceanic Science, College Park, Maryland, USA
  • anow at: Centre for Atmospheric Informatics and Emissions Technology, Cranfield University, Cranfield, UK

Abstract. We use the GEOS-Chem global 3-D atmospheric chemistry transport model to interpret atmospheric observations of bromoform (CHBr3) and dibromomethane (CH2Br2) collected during the CAST and CONTRAST aircraft measurement campaigns over the western Pacific, January–February 2014. We use a new linearized, tagged version of CHBr3 and CH2Br2, allowing us to study the influence of emissions from specific geographical regions on observed atmospheric variations. The model describes 32%–37% of CHBr3 and 15%–45% of CH2Br2 observed variability during CAST and CONTRAST, reflecting model errors in vertical transport. The model has a mean positive bias of 30% that is larger near the surface, reflecting errors in the poorly constrained prior emission estimates. We find using the model that observed variability of CHBr3 and CH2Br2 is driven by open ocean emissions where there is deep convection. Atmospheric variability above 6km includes a significant contribution from coastal oceans, but it is still dominated by emissions from the open ocean and by older air masses that originate upwind. In the absence of reliable ocean emission estimates, we use a new physical age-of-air simulation to determine the relative abundance of halogens delivered by CHBr3 and CH2Br2 to the tropical transition layer (TTL). We find that 76% (92%) of air masses that originate from the ocean reach the TTL within two (three) atmospheric e-folding lifetimes of CHBr3 and almost all of them reach the TTL within one e-folding lifetime of CH2Br2. Over the duration of CAST and CONTRAST, and over our study region, oceans delivered a mean (range) CHBr3 and CH2Br2 mole fraction of 0.46 (0.13–0.72) and 0.88 (0.71–1.01)pptv, respectively, to the TTL, and a mean (range) Bry mole fraction of 3.14 (1.81–4.18)pptv from source gases to the upper troposphere.

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Natural sources of short-lived bromoform and dibromomethane are important for determining the inorganic bromine budget in the stratosphere that drives ozone loss. Two new modelling techniques describe how different geographical source regions influence their atmospheric variability over the western Pacific. We find that it is driven primarily by open ocean sources, and we use atmospheric observations to help estimate their contributions to the upper tropospheric inorganic bromine budget.
Natural sources of short-lived bromoform and dibromomethane are important for determining the...
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