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The first concerted multi-model intercomparison of halogenated very\udshort-lived substances (VSLS) has been performed, within the framework of the\udongoing Atmospheric Tracer Transport Model Intercomparison Project\ud(TransCom). Eleven global models or model variants participated (nine\udchemical transport models and two chemistry–climate models) by simulating the\udmajor natural bromine VSLS, bromoform (CHBr3) and dibromomethane\ud(CH2Br2), over a 20-year period (1993–2012). Except for three model\udsimulations, all others were driven offline by (or nudged to) reanalysed\udmeteorology. The overarching goal of TransCom-VSLS was to provide a\udreconciled model estimate of the stratospheric source gas injection (SGI) of\udbromine from these gases, to constrain the current measurement-derived range,\udand to investigate inter-model differences due to emissions and transport\udprocesses. Models ran with standardised idealised chemistry, to isolate\uddifferences due to transport, and we investigated the sensitivity of results\udto a range of VSLS emission inventories. Models were tested in their ability\udto reproduce the observed seasonal and spatial distribution of VSLS at the\udsurface, using measurements from NOAA's long-term global monitoring network,\udand in the tropical troposphere, using recent aircraft measurements –\udincluding high-altitude observations from the NASA Global Hawk platform. \udThe models generally capture the observed seasonal cycle of surface CHBr3\udand CH2Br2 well, with a strong model–measurement correlation\ud(r  ≥  0.7) at most sites. In a given model, the absolute\udmodel–measurement agreement at the surface is highly sensitive to the choice\udof emissions. Large inter-model differences are apparent when using the same\udemission inventory, highlighting the challenges faced in evaluating such\udinventories at the global scale. Across the ensemble, most consistency is\udfound within the tropics where most of the models (8 out of 11) achieve best\udagreement to surface CHBr3 observations using the lowest of the three\udCHBr3 emission inventories tested (similarly, 8 out of 11 models for\udCH2Br2). In general, the models reproduce observations of CHBr3\udand CH2Br2 obtained in the tropical tropopause layer (TTL) at various\udlocations throughout the Pacific well. Zonal variability in VSLS loading in the\udTTL is generally consistent among models, with CHBr3 (and to a lesser\udextent CH2Br2) most elevated over the tropical western Pacific during boreal winter. The models also indicate the Asian monsoon during boreal\udsummer to be an important pathway for VSLS reaching the stratosphere, though\udthe strength of this signal varies considerably among models. \udWe derive an ensemble climatological mean estimate of the stratospheric\udbromine SGI from CHBr3 and CH2Br2 of 2.0 (1.2–2.5) ppt,\ud ∼  57 % larger than the best estimate from the most recent World\udMeteorological Organization (WMO) Ozone Assessment Report. We find no\udevidence for a long-term, transport-driven trend in the stratospheric SGI of\udbromine over the simulation period. The transport-driven interannual\udvariability in the annual mean bromine SGI is of the order of ±5 %,\udwith SGI exhibiting a strong positive correlation with the El Niño–Southern Oscillation (ENSO) in the\udeastern Pacific. Overall, our results do not show systematic differences between\udmodels specific to the choice of reanalysis meteorology, rather clear\uddifferences are seen related to differences in the implementation of\udtransport processes in the models

atmospheric chemistry and physics

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