1Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe, Germany
2Swedish Institute of Space Physics (IRF), Kiruna, Sweden
Abstract. Trends of hydrogen chloride (HCl), chlorine nitrate (ClONO2) and hydrogen fluoride (HF) column abundances above Kiruna (Northern Sweden, 67.84° N, 20.41° E) derived from nearly 14 years (1996–2009) of measurement and model data are presented. The measurements have been performed with a Bruker 120 HR (later Bruker 125 HR) Fourier transform infrared (FTIR) spectrometer and the model used was KASIMA (KArlsruhe SImulation model of the Middle Atmosphere). To calculate the long-term trends, a linear function combined with an annual cycle was fitted to the data using a least squares method. The precision of the resulting trends was estimated with the so-called bootstrap resampling method. The relative trends were calculated on the basis of the linear fit result on 1 January 2000, 12:00 UTC.
For hydrogen fluoride, both model and measurements show a positive trend that seems to decrease in the last few years. This suggests a stabilisation of the HF total column abundance. For the summer data (August to November), the FTIR trend of (+1.25 ± 0.28)%/yr agrees within errors with the KASIMA one of (+1.55 ± 0.11)%/yr.
The trends determined for HCl and ClONO2 are significantly negative over the time period considered here. This corresponds to the expectations because the emission of their precursors (chlorofluorocarbons and hydrochlorofluorocarbons) has been restricted in the Montreal Protocol in 1987 and its amendments and adjustments. The relative trend for ClONO2 from the FTIR measurements amounts to (−3.28 ± 0.56)%/yr and the one for HCl to (−0.81± 0.23)%/yr. KASIMA simulates a weaker decrease: For ClONO2, the result is (−0.90 ± 0.10)%/yr and for HCl (−0.17± 0.06)%/yr. Part of the difference between measurement and model data can be explained by sampling and the stronger annual cycle indicated by the measurements. There is a factor of about four between the trends of HCl and ClONO2 above Kiruna for both measurement and model data. The absolute values of ClONO2 and HF calculated by KASIMA agree quite well with the FTIR measurements while KASIMA tends to underestimate the HCl column abundances.