Removal of Stratospheric O 3 by Radicals: In Situ Measurements of OH, HO 2 , NO, NO 2 , ClO, and BrO

Abstract

Simultaneous in situ measurements of the concentrations of OH, HO₂, ClO, BrO, NO, and NO₂ demonstrate the predominance of odd-hydrogen and halogen free-radical catalysis in determining the rate of removal of ozone in the lower stratosphere during May 1993. A single catalytic cycle, in which the rate-limiting step is the reaction of HO₂ with ozone, accounted for nearly one-half of the total O₃ removal in this region of the atmosphere. Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O₃; reactions involving BrO account for one-half of this loss. Catalytic destruction by NO₂, which for two decades was considered to be the predominant loss process, accounted for less than 20 percent of the O₃ removal. The measurements demonstrate quantitatively the coupling that exists between the radical families. The concentrations of HO₂ and ClO are inversely correlated with those of NO and NO₂. The direct determination of the relative importance of the catalytic loss processes, combined with a demonstration of the reactions linking the hydrogen, halogen, and nitrogen radical concentrations, shows that in the air sampled the rate of O₃ removal was inversely correlated with total NO_x, loading.