Ozone killer shows signs of slowing.

While chlorine from chlorofluorocarbons presents the greatest threat to the world's ozone layer, bromine chemicals also play an important role in destroying Earth's natural sunscreen. Atmospheric scientists report this month that levels of bromine-containing chemicals are increasing more slowly than they were a few years ago, a positive sign that nations are bringing this pollution under control.

Much of the bromine in the atmosphere comes from halons, a class of chemicals used primarily in extinguishing fires. The halons are unreactive in the lower atmosphere, giving them time to float up to the stratosphere, where they release their ozone-destroying bromine.

Since 1987, the National Oceanic and Atmospheric Administration (NOAA) has measured halon levels in the lower atmosphere by collecting air samples every one to two months in Alaska, Colorado, Hawaii, and American Samoa. Between 1987 and 1989, levels of one particular halon, H-1301, climbed at the steep rate of almost 3 percent per year. Since that time, however, the rate of increase has dropped to between 1 and 2 percent per year, reports a group of researchers from NOAA and the Cooperative Institute for Research in Environmental Sciences in Boulder, Colo. They discuss their findings in the Oct. 1 NATURE.

If nations continue to reduce their use of halons as they have in recent years, the gas concentrations could level off and start dropping as early as 1994, say the researchers.