Tropospheric lifetimes of three compounds for possible replacement of CFC and halons

Abstract

CHLORINE and bromine have been implicated in the massive springtime depletion of stratospheric ozone over Antarctica¹. As the source of these halogens is anthropogenic emissions of halo-alkanes, the problem has acquired political and economic significance². The chemical industry has been forced to consider urgently possible replacements for conventional halocarbons, and the potential effects on the environment of proposed alternative compounds have been evaluated recently in the Alternative Fluorocarbon Environmental Acceptability Study (AFEAS)³. Three such compounds that are not included in the AFEAS report are CF₂BrH (halon 1201). CF₃CFBrH (halon 2401) and CF₃CF₂CC1₂H (HCFC 225ca). Removal of these compounds from the atmosphere will occur primarily by reaction with the hydroxyl radical, OH (ref. 4). Here we determine, from laboratory studies, the absolute rate of reaction between these three species and OH. Such kinetic data are vital for assessing their viability as replacement compounds. We use these data, along with our measurements of ultraviolet absorption cross-sections, to estimate the tropospheric lifetimes of the halons and HCFC 225ca against removal by OH, and their potential for destroying ozone in the stratosphere. Our approach shows how laboratory measurements can provide a useful first estimate of the environmental acceptability of compounds of this sort.