Long-Term Trends of Inorganic Chlorine from Ground-Based Infrared Solar Spectra: Past Increases and Evidence for Stabilization

Long-term time series of hydrogen chloride (HCl) and chlorine nitrate(ClONO2) total column abundances have been retrieved from high spectralresolution ground-based solar absorption spectra recorded withinfrared Fourier transform spectrometers at nine NDSC(Network for the Detection of Stratospheric Change) sites in bothNorthern and Southern Hemispheres. The data sets span up to 24years and most extend until the end of 2001. The time series ofCly (defined here as the sum of the HCl and ClONO2 columns) from thethree locations with the longest time span records show rapid increasesuntil the early 1990s superimposed on marked day-to-day, seasonal andinter-annual variability. Subsequently, the buildup in Cly slowsand reaches a broad plateau after 1996, also characterized byvariability. A similar time evolution is also found in the totalchlorine concentration at 55 km altitude derived fromHalogen Occultation Experiment (HALOE) global observations since 1991.The stabilization of inorganic chlorine observed in both the totalcolumns and at 55 km altitude indicates that the near-global 1993organic chlorine (CCly) peak at the Earth?s surface has now propagatedover a broad altitude range in the upper atmosphere, though the timelag is difficult to quantify precisely from the current data sets dueto variability. We compare the three longest measured time serieswith two-dimensional model calculations extending from 1977 to 2010,based on a halocarbon scenario that assumes past measured trendsand a realistic extrapolation into the future.The model predicts broad Cly maxima consistent with the long-termobservations, followed by a slow Cly decline of 12-14% relative to thepeak by 2010. The data reported here confirm the effectiveness of theMontreal Protocol and its Amendments and Adjustments in progressivelyphasing out the major man-related perturbations of the stratosphericozone layer, in particular, the anthropogenic chlorine-bearing source gases