Articles | Volume 16, issue 14
Atmos. Chem. Phys., 16, 9163–9187, 2016
Atmos. Chem. Phys., 16, 9163–9187, 2016

Research article 26 Jul 2016

Research article | 26 Jul 2016

A multi-model intercomparison of halogenated very short-lived substances (TransCom-VSLS): linking oceanic emissions and tropospheric transport for a reconciled estimate of the stratospheric source gas injection of bromine

R. Hossaini1,a, P. K. Patra2, A. A. Leeson1,b, G. Krysztofiak3,c, N. L. Abraham4,5, S. J. Andrews6, A. T. Archibald4, J. Aschmann7, E. L. Atlas8, D. A. Belikov9,10,11, H. Bönisch12, L. J. Carpenter6, S. Dhomse1, M. Dorf13, A. Engel12, W. Feng1,4, S. Fuhlbrügge14, P. T. Griffiths5, N. R. P. Harris5, R. Hommel7, T. Keber12, K. Krüger14,15, S. T. Lennartz14, S. Maksyutov9, H. Mantle1, G. P. Mills16, B. Miller17, S. A. Montzka17, F. Moore17, M. A. Navarro8, D. E. Oram16, K. Pfeilsticker18, J. A. Pyle4,5, B. Quack14, A. D. Robinson5, E. Saikawa19,20, A. Saiz-Lopez21, S. Sala12, B.-M. Sinnhuber3, S. Taguchi22, S. Tegtmeier14, R. T. Lidster6, C. Wilson1,23, and F. Ziska14 R. Hossaini et al.
  • 1School of Earth and Environment, University of Leeds, Leeds, UK
  • 2Department of Environmental Geochemical Cycle Research, JAMSTEC, Yokohama, Japan
  • 3Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 4National Centre for Atmospheric Science, Cambridge, UK
  • 5Department of Chemistry, University of Cambridge, Cambridge, UK
  • 6Department of Chemistry, University of York, Heslington, York, UK
  • 7Institute of Environmental Physics, University of Bremen, Bremen, Germany
  • 8Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, USA
  • 9Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan
  • 10National Institute of Polar Research, Tokyo, Japan
  • 11Tomsk State University, Tomsk, Russia
  • 12Institute for Atmospheric and Environmental Sciences, Universität Frankfurt/Main, Frankfurt, Germany
  • 13Max-Planck-Institute for Chemistry, Mainz, Germany
  • 14GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 15University of Oslo, Department of Geosciences, Oslo, Norway
  • 16School of Environmental Sciences, University of East Anglia, Norwich, UK
  • 17National Oceanic and Atmospheric Administration, Boulder, USA
  • 18Institute for Environmental Physics, University of Heidelberg, Heidelberg, Germany
  • 19Department of Environmental Sciences, Emory University, Atlanta, USA
  • 20Department of Environmental Health, Rollins School of Public Health, Emory University, Atlanta, USA
  • 21Atmospheric Chemistry and Climate Group, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain
  • 22National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
  • 23National Centre for Earth Observation, Leeds, UK
  • anow at: Lancaster Environment Centre, Lancaster University, Lancaster, UK
  • bnow at: Lancaster Environment Centre/Data Science Institute, Lancaster University, Lancaster, UK
  • cnow at: Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, CNRS-Université d'Orléans, Orléans, France

Abstract. The first concerted multi-model intercomparison of halogenated very short-lived substances (VSLS) has been performed, within the framework of the ongoing Atmospheric Tracer Transport Model Intercomparison Project (TransCom). Eleven global models or model variants participated (nine chemical transport models and two chemistry–climate models) by simulating the major natural bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2), over a 20-year period (1993–2012). Except for three model simulations, all others were driven offline by (or nudged to) reanalysed meteorology. The overarching goal of TransCom-VSLS was to provide a reconciled model estimate of the stratospheric source gas injection (SGI) of bromine from these gases, to constrain the current measurement-derived range, and to investigate inter-model differences due to emissions and transport processes. Models ran with standardised idealised chemistry, to isolate differences due to transport, and we investigated the sensitivity of results to a range of VSLS emission inventories. Models were tested in their ability to reproduce the observed seasonal and spatial distribution of VSLS at the surface, using measurements from NOAA's long-term global monitoring network, and in the tropical troposphere, using recent aircraft measurements – including high-altitude observations from the NASA Global Hawk platform.

The models generally capture the observed seasonal cycle of surface CHBr3 and CH2Br2 well, with a strong model–measurement correlation (r  ≥  0.7) at most sites. In a given model, the absolute model–measurement agreement at the surface is highly sensitive to the choice of emissions. Large inter-model differences are apparent when using the same emission inventory, highlighting the challenges faced in evaluating such inventories at the global scale. Across the ensemble, most consistency is found within the tropics where most of the models (8 out of 11) achieve best agreement to surface CHBr3 observations using the lowest of the three CHBr3 emission inventories tested (similarly, 8 out of 11 models for CH2Br2). In general, the models reproduce observations of CHBr3 and CH2Br2 obtained in the tropical tropopause layer (TTL) at various locations throughout the Pacific well. Zonal variability in VSLS loading in the TTL is generally consistent among models, with CHBr3 (and to a lesser extent CH2Br2) most elevated over the tropical western Pacific during boreal winter. The models also indicate the Asian monsoon during boreal summer to be an important pathway for VSLS reaching the stratosphere, though the strength of this signal varies considerably among models.

We derive an ensemble climatological mean estimate of the stratospheric bromine SGI from CHBr3 and CH2Br2 of 2.0 (1.2–2.5) ppt,  ∼  57 % larger than the best estimate from the most recent World Meteorological Organization (WMO) Ozone Assessment Report. We find no evidence for a long-term, transport-driven trend in the stratospheric SGI of bromine over the simulation period. The transport-driven interannual variability in the annual mean bromine SGI is of the order of ±5 %, with SGI exhibiting a strong positive correlation with the El Niño–Southern Oscillation (ENSO) in the eastern Pacific. Overall, our results do not show systematic differences between models specific to the choice of reanalysis meteorology, rather clear differences are seen related to differences in the implementation of transport processes in the models.

Final-revised paper