Abstract

The burden of ozone-depleting chemicals in the lower atmosphere has been decreasing since 1994 as a result of the Montreal Protocol1, 3. Here we show how individual chemicals have influenced this decline, in order to estimate how the burden could change in the near future. Our measurements of atmospheric concentrations of the persistent, anthropogenic chemicals that account for most ozone-depleting halogens in today's stratosphere show that the decline stems predominantly from the decrease in the atmospheric load of trichloroethane (CH3CCl3), a previously common cleaning solvent. The influence of this chemical on the decline has now peaked, however, and will become much smaller over the next five to ten years. As this influence lessens, a decrease in the burden of ozone-depleting halogen will be sustained only if emissions of other halocarbons fall. Although emissions of most gases regulated by the Montreal Protocol have decreased substantially over the past ten years (refs 4), emissions of the potent ozone-depleting gas CBrClF2 (halon-1211) have remained fairly constant during this period12, 29, despite stringent limits on production in developed countries since 1994. The consequent atmospheric accumulation of this halon is retarding the decline of ozone-depleting halogens in the atmosphere more than any other persistent gas.