New Observational Constraints for Atmospheric Hydroxyl on

Dramatic declines in emissions of methyl chloroform (1,1,1-trichloroethane) resulting from the Montreal Protocol provide an unprecedented opportunity to improve our understanding of the oxidizing power of EarthÕs atmosphere. Atmospheric observations of this industrial gas during the late 1990s yield new insights into the global burden and distribution of the hydroxyl radical. Our results set Þrm upper limits on the global and Southern Hemispheric lifetimes of methyl chloroform and conÞrm the predominance of hydroxyl in the tropics. Our analysis suggests a global lifetime for methyl chloroform of 5.2 (10.2, 20.3) years, a Southern Hemispheric lifetime of 4.9 (10.2, 20.3) years, and mean annual concentrations of OH that are 15 6 10 % higher south of the intertropical convergence zone than those north of this natural mixing bound-ary between the hemispheres. The hydroxyl radical plays a central role in the chemistry of Earth’s atmosphere (1). This powerful oxidant initiates photochemical re