Trends in atmospheric halogen containing gases since 2004

Abstract

The changes in the atmospheric concentration of 16 halogenated gases in the atmosphere have been determined using measurements made by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS). ACE-FTS has been used to measure the change in concentration between 2004 and 2010 of CCl 4, CF 4, CCl 3F (CFC-11), CCl 2F 2 (CFC-12), C 2Cl 3F 3 (CFC-113), CH 3Cl, ClONO 2, COF 2, COCl 2, COClF, CHF 2Cl (HCFC-22), CH 3CCl 2F (HCFC-141b), CH 3CClF 2 (HCFC-142b), HCl, HF and SF 6 between 30°N and 30°S. ACE-FTS measurements were compared to surface measurements made by the AGAGE network and output from the SLIMCAT three-dimensional (3-D) chemical transport model, which is constrained by similar surface data. ACE-FTS measurements of CFCs show declining trends which agree with both AGAGE and SLIMCAT values. There are problems with the ACE-FTS retrievals of CFC-113 and HCFCs, with work currently ongoing to correct these problems. At lower altitudes the volume mixing ratio (VMR) of these species increase with altitude. This is due to problems with the retrievals at high beta angle (the angle between the orbital plane and the Earth–Sun vector). Although some of the retrievals have problems, we are confident that the trends are generally reliable. The concentrations of HCFCs appear to be increasing with ACE-FTS, SLIMCAT and AGAGE all showing positive trends. ACE-FTS measurements of the decomposition products (COFCl and COCl 2) do not show any significant trends. SLIMCAT data show a negative trend for COFCl which corresponds to the decrease in CFC-11, its assumed major source, during this time. COF 2 measurements from ACE-FTS show an increasing trend, while SLIMCAT shows a decreasing trend again linked to its assumed production from CFCs. ClONO 2 is highly photosensitive, thus the ACE-FTS occultations have been divided into local morning and evening occultations. Evening measurements of ClONO 2 show a decreasing trend in VMR, while morning measurements show an increasing trend. The reason for this difference is not understood at this time. The SLIMCAT output used in this study was not saved as local sunrise and sunset: therefore, only 24 h mean fields are available for ClONO 2. These SLIMCAT data show a decreasing trend. SLIMCAT and ACE-FTS both show an increasing trend in the VMR of HF and a decreasing trend in the VMR of HCl. These results illustrate the success of the Montreal Protocol in reducing ozone depleting substances. The reduction in anthropogenic chlorine emissions has led to a decrease in the VMR of stratospheric HCl. The replacement of CFCs with HCFCs has led to an increase in the VMR of HF in the stratosphere. As chlorine-containing compounds continue to be phased out and replaced by fluorine-containing molecules, it is likely that total atmospheric fluorine will continue increasing in the near future.