Halogen occultation experiment confirmation of stratospheric chlorine decreases in accordance with the Montreal Protocol

Near-global time series of Halogen Occultation Experiment (HALOE) derived total Cl, F, and the Cl/F ratio were evaluated and compared with data from two ground-based CFC measurement programs and United Nations Environment Programme (UNEP) best-case scenarios for CFC emissions. The HALOE CI profiles confirm that the chlorine burden near the stratopause is decreasing and this decrease is driven by tropospheric decreases of methyl chloroform. The HALOE F profiles show that the fluorine burden near the stratopause is primarily influenced by CFC-12. The Cl/F ratio, which is insensitive to transport effects at 55 km, shows that the atmosphere near the stratopause is becoming more fluorinated since the inception of the HALOE mission and less chlorinated since the start of 1997. The temporal changes in the derived Cl, F, and Cl/F ratio at 55 km are in accord with UNEP projections and in situ observations. These combined results demonstrate that the Montreal Protocol and subsequent strengthening amendments are currently having the desired effect of reducing stratospheric chlorine.