1Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe, Germany
2Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
3Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
4Institute of Environmental Physics, University of Bremen, Bremen, Germany
5Department of Physics, University of Toronto, Toronto, Ontario, Canada
6National Center for Atmospheric Research (NCAR), Boulder, CO, USA
7Department of Physics and Astronomy, University of Denver, Denver, CO, USA
8Institute of Astrophysics and Geophysics, University of Liège, Liège, Belgium
9Environment Canada, Toronto, Ontario, Canada
10Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, Australia
11National Institute of Information and Communications Technology, Tokyo, Japan
12Fujitsu FIP Corporation, Tokyo, Japan
13Karlsruhe Institute of Technology (KIT), Steinbuch Centre for Computing, Karlsruhe, Germany
14Center for Global Environmental Research, National Institute for Environmental Studies (NIES), Japan
15Department of Environmental Studies, Graduate School of Environmental Studies, Tohoku University, Japan
16Atmospheric Environment Division, National Institute for Environmental Studies (NIES), Japan
17Swedish Institute of Space Physics (IRF), Kiruna, Sweden
18Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-IFU), Garmisch-Partenkirchen, Germany
19NASA Langley Research Center, Hampton, VA, USA
20ETH Zürich, Institute for Atmospheric and Climate Science (IACETH), Zürich, Switzerland
21Physical-Meteorological Observatory, World Radiation Center, Davos, Switzerland
22National Institute of Water and Atmospheric Research Ltd (NIWA), Lauder, New Zealand
23National Ecological Observatory Network (NEON), Boulder, CO, USA
Abstract. Time series of total column abundances of hydrogen chloride (HCl), chlorine nitrate (ClONO2), and hydrogen fluoride (HF) were determined from ground-based Fourier transform infrared (FTIR) spectra recorded at 17 sites belonging to the Network for the Detection of Atmospheric Composition Change (NDACC) and located between 80.05° N and 77.82° S. By providing such a near-global overview on ground-based measurements of the two major stratospheric chlorine reservoir species, HCl and ClONO2, the present study is able to confirm the decrease of the atmospheric inorganic chlorine abundance during the last few years. This decrease is expected following the 1987 Montreal Protocol and its amendments and adjustments, where restrictions and a subsequent phase-out of the prominent anthropogenic chlorine source gases (solvents, chlorofluorocarbons) were agreed upon to enable a stabilisation and recovery of the stratospheric ozone layer. The atmospheric fluorine content is expected to be influenced by the Montreal Protocol, too, because most of the banned anthropogenic gases also represent important fluorine sources. But many of the substitutes to the banned gases also contain fluorine so that the HF total column abundance is expected to have continued to increase during the last few years.
The measurements are compared with calculations from five different models: the two-dimensional Bremen model, the two chemistry-transport models KASIMA and SLIMCAT, and the two chemistry-climate models EMAC and SOCOL. Thereby, the ability of the models to reproduce the absolute total column amounts, the seasonal cycles, and the temporal evolution found in the FTIR measurements is investigated and inter-compared. This is especially interesting because the models have different architectures. The overall agreement between the measurements and models for the total column abundances and the seasonal cycles is good.
Linear trends of HCl, ClONO2, and HF are calculated from both measurement and model time series data, with a focus on the time range 2000–2009. This period is chosen because from most of the measurement sites taking part in this study, data are available during these years. The precision of the trends is estimated with the bootstrap resampling method. The sensitivity of the trend results with respect to the fitting function, the time of year chosen and time series length is investigated, as well as a bias due to the irregular sampling of the measurements.
The measurements and model results investigated here agree qualitatively on a decrease of the chlorine species by around 1% yr−1. The models simulate an increase of HF of around 1% yr−1. This also agrees well with most of the measurements, but some of the FTIR series in the Northern Hemisphere show a stabilisation or even a decrease in the last few years. In general, for all three gases, the measured trends vary more strongly with latitude and hemisphere than the modelled trends. Relative to the FTIR measurements, the models tend to underestimate the decreasing chlorine trends and to overestimate the fluorine increase in the Northern Hemisphere.
At most sites, the models simulate a stronger decrease of ClONO2 than of HCl. In the FTIR measurements, this difference between the trends of HCl and ClONO2 depends strongly on latitude, especially in the Northern Hemisphere.