1Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research, Karlsruhe, Germany
2University of Leeds, Leeds, UK
3Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
4University of Bremen, Institute of Environmental Physics, Bremen, Germany
5Department of Physics, University of Toronto, Toronto, Ontario, Canada
6University of Liège, Institute of Astrophysics and Geophysics, Liège, Belgium
7Environment Canada, Toronto, Ontario, Canada
8Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, Australia
9National Institute of Information and Communications Technology, Tokyo, Japan
10Fujitsu FIP Corporation, Tokyo, Japan
11Center for Global Environmental Research, National Institute for Environmental Studies (NIES), Japan
12Department of Environmental Studies, Graduate School of Environmental Studies, Tohoku University, Japan
13Atmospheric Environment Division, National Institute for Environmental Studies, Japan
14National Institute of Water and Atmospheric Research Ltd (NIWA), Lauder, New Zealand
15University of Denver, Department of Physics and Astronomy, Denver, CO, USA
16National Center for Atmospheric Research (NCAR), Boulder, CO, USA
17NASA Langley Research Center, Hampton, VA, USA
18Karlsruhe Institute of Technology (KIT), Steinbuch Centre for Computing, Karlsruhe, Germany
19ETH Zürich, Institute for Atmospheric and Climate Science (IACETH), Zürich, Switzerland
20National Ecological Observatory Network (NEON), Boulder, CO, USA
21Swedish Institute of Space Physics (IRF), Kiruna, Sweden
22Physical-Meteorological Observatory, World Radiation Center, Davos, Switzerland
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. These 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. The overall agreement between the measurements and models for the total column abundances and the seasonal cycles is good.
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. Their precision 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.
For the two chlorine species, a decrease is expected during this period because the emission of their prominent anthropogenic source gases (solvents, chlorofluorocarbons (CFCs)) was restricted by the Montreal Protocol 1987 and its amendments and adjustments. As most of the restricted source gases also contain fluorine, the HF total column abundance was also influenced by the above-mentioned regulations in the time period considered.
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.