ALBERT

Description: 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.