Resolving the Model?￠????Observation Discrepancy in the Mesospheric and Stratospheric HO x Chemistry

摘要

We examine the middle atmospheric odd?￠????hydrogen (HO x ) chemistry by comparing the Aura Microwave Limb Sounder (MLS) OH and HO 2 measurements with a photochemical model simulation. The model underestimates mesospheric OH and HO 2 concentrations if the standard chemical kinetic rates are used, whether the model H 2 O and O 3 are constrained with observations or not. To resolve the discrepancies, we adjust the kinetic rate coefficients of three key reactions (O????+????OH?????￠????????O 2 ????+????H, OH????+????HO 2 ?????￠????????H 2 O????+????O 2 , and H????+????O 2 ????+????M?????￠????????HO 2 ????+????M) and the O 2 photo absorption cross section at Lyman ???± (121.57????nm) using the Bayesian optimal estimation. A much better model?￠????observation agreement can be achieved if the kinetic rate coefficients for H????+????O 2 ????+????M?????￠????????HO 2 ????+????M is increased by 134?￠????310%, and the O 2 photo absorption cross section at Lyman ???± is reduced by 33?￠????54%, while the kinetic rate coefficients for O????+????OH?????￠????????O 2 ????+????H and OH????+????HO 2 ?????￠????????H 2 O????+????O 2 remain consistent with the current laboratory values. The kinetic rate coefficient for H????+????O 2 ????+????M?????￠????????HO 2 ????+????M requires a very large adjustment beyond the uncertainty limits recommended in the NASA Data Evaluation, suggesting the need for future laboratory measurements. An alternative explanation is that the radiative association reaction, H????+????O 2 ?????￠????????HO 2 ????+???? h ????, plays a significant role, which has never been measured. Our results demonstrate that high?￠????quality satellite observations can be used to constrain photochemical parameters and help improve our understanding of atmospheric chemistry.