Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes.
The abundance of chlorine in the Earth's atmosphere increased considerably during the 1970s to 1990s, following large emissions of anthropogenic long-lived chlorine-containing source gases, notably the chlorofluorocarbons. The chemical inertness of chlorofluorocarbons allows their transport and mixing throughout the troposphere on a global scale, before they reach the stratosphere where they release chlorine atoms that cause ozone depletion. The large ozone loss over Antarctica was the key observation that stimulated the definition and signing in 1987 of the Montreal Protocol, an international treaty establishing a schedule to reduce the production of the major chlorine- and bromine-containing halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of half a per cent to one per cent per year, in line with expectations. Remote-sensing data have revealed a peak in stratospheric chlorine after 1996, then a decrease of close to one per cent per year, in agreement with the surface observations of the chlorine source gases and model calculations. Here we present ground-based and satellite data that show a recent and significant increase, at the 2σ level, in hydrogen chloride (HCl), the main stratospheric chlorine reservoir, starting around 2007 in the lower stratosphere of the Northern Hemisphere, in contrast with the ongoing monotonic decrease of near-surface source gases. Using model simulations, we attribute this trend anomaly to a slowdown in the Northern Hemisphere atmospheric circulation, occurring over several consecutive years, transporting more aged air to the lower stratosphere, and characterized by a larger relative conversion of source gases to HCl. This short-term dynamical variability will also affect other stratospheric tracers and needs to be accounted for when studying the evolution of the stratospheric ozone layer.
Institute of Astrophysics and Geophysics, University of Liège, Liège 4000, Belgium.
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E
Mahieu
National Centre for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
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MP
Chipperfield
Department of Physics, University of Bremen, Bremen 28334, Germany.
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J
Notholt
Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe 76021, Germany.
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T
Reddmann
Department of Atmospheric and Planetary Science, Hampton University, Hampton, Virginia 23668, USA.
J
J
Anderson
1] Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, USA [2] Department of Chemistry, University of York, York YO10 5DD, UK [3] Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
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PF
Bernath
Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe 76021, Germany.
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T
Blumenstock
National Center for Atmospheric Research, Boulder, Colorado 80307, USA.
M T
MT
Coffey
National Centre for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
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Dhomse
National Centre for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
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W
Feng
Institute of Astrophysics and Geophysics, University of Liège, Liège 4000, Belgium.
B
B
Franco
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA.
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L
Froidevaux
School of Chemistry, University of Wollongong, Wollongong, New South Wales 2522, Australia.
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DW
Griffith
National Center for Atmospheric Research, Boulder, Colorado 80307, USA.
J W
JW
Hannigan
Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe 76021, Germany.
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F
Hase
National Centre for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
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R
Hossaini
School of Chemistry, University of Wollongong, Wollongong, New South Wales 2522, Australia.
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NB
Jones
National Institute for Environmental Studies (NIES), Tsukuba, Ibaraki 305-8506, Japan.
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I
Morino
Graduate School of Environmental Studies, Tohoku University, Sendai 980-8578, Japan.
I
I
Murata
National Institute for Environmental Studies (NIES), Tsukuba, Ibaraki 305-8506, Japan.
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H
Nakajima
Department of Physics, University of Bremen, Bremen 28334, Germany.
M
M
Palm
School of Chemistry, University of Wollongong, Wollongong, New South Wales 2522, Australia.
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C
Paton-Walsh
Department of Atmospheric and Planetary Science, Hampton University, Hampton, Virginia 23668, USA.
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JM
Russell
Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe 76021, Germany.
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M
Schneider
Institute of Astrophysics and Geophysics, University of Liège, Liège 4000, Belgium.
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C
Servais
National Institute of Water and Atmospheric Research (NIWA), Lauder 9352, New Zealand.
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D
Smale
1] Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada [2] Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada.
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Walker