A DECREASE IN THE GROWTH-RATES OF ATMOSPHERIC HALON CONCENTRATIONS

HALONS H-1301 (CBrF3) and H-1211 (CBrClF2) have been introduced into the atmosphere, mainly through use in fire extinguishers, for almost three decades. Although each is now present in the troposphere at a concentration of only 2 parts per 10(12), these gases have long atmospheric lifetimes (65-77 yr for H-1301 and 11-16 yr for H-1211)1,2 and carry significant amounts of bromine to the stratosphere2,3, where it can destroy ozone catalytically4,5. For this reason, the halons have high ozone depletion potentials6. The manufacture of both gases is to be discontinued globally by the year 2000, according to the Montreal Protocol, and perhaps sooner, as a result of unilateral action by users, manufacturers and producing countries7,8. Here we present a six-year record of tropospheric halon mixing ratios which shows that the growth rates of H-1301 and H-1211 have already begun to decrease substantially. This recent decrease in growth rates is consistent with industry emission estimates (although these have greater uncertainties), and supports current appraisals of atmospheric lifetimes. Our results suggest that, even though these halons are relatively long-lived species, their atmospheric mixing ratios may stabilize or begin to decrease within the next few years.