This figure shows the amount of chlorine and bromine (expressed as the effective equivalent chlorine, or EECl) in the lower atmosphere. This quantity is a good indicator of the amount that is predicted to be released a few years hence in an inorganic form in the lower mid-latitude stratosphere, where it can contribute to ozone depletion. Estimates are made on a monthly basis for the Northern Hemisphere (open diamonds), Southern Hemisphere (open squares), and global mean (closed circles). The EECl is derived by considering the changing concentrations of about a dozen gases that can affect the stratospheric ozone concentration and developing an index based on their ability to catalyze the destruction of ozone relative to the ability of chlorine (the units of EECl are in parts per trillion by volume). As a result of controls instituted under the Montreal Protocol on Substances that Deplete the Ozone Layer and subsequent agreements and amendments, the stratospheric chlorine concentration is expected to start decreasing in the near future, allowing the stratospheric ozone concentration to begin what will be a multi- decadal recovery period. The figure is courtesy of S. Montzka of the NOAA Climate Monitoring and Diagnostics Laboratory in Boulder, CO, and appeared in Science, 272, pp. 1318-1322 (1996).