Comparing atmospheric [HO₂]/[OH] to modeled [HO₂]/[OH]: Identifying discrepancies with reaction rates

Reactions that inter‐convert OH and HO 2 are directly involved in the catalytic removal of O 3 in the lower stratosphere and in the catalytic production of O 3 in the upper troposphere. The agreement between the measured and modeled [HO 2 ]/[OH] tests our current understanding of this important chemistry. Recent changes to the recommended rate constants for OH+O 3 and HO 2 +O 3 call into question how accurately the chemistry of the stratosphere is understood. [HO 2 ]/[OH] calculated with the new recommendations is 48% higher than the observations throughout the lower stratosphere, exceeding the uncertainty limits of the observations (20%). The extensive atmospheric data set allows tests of the rates of the individual processes that couple these free radicals. This work shows that the discrepancy is largest when the ratio is controlled by the reactions of OH and HO 2 with ozone.