Mesospheric HO_<i>x</i> photochemistry: Constraints from recent satellite measurements of OH and H₂O

Abstract

We use recent measurements of OH made with the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) along with satellite observations of water vapor abundances, to study odd‐hydrogen photochemistry in the mesosphere. The MAHRSI data sampled primarily the morning part of the diurnal variation of OH, and here we focus on one orbit of data that is representative of the MAHRSI observations during the mission. Our approach is to use a photochemical model, with input H 2 O abundances fixed to observed values, to simulate the diurnal variation of mesospheric HO x species. Models of OH using standard recommended HO x chemical rate coefficients are found to substantially overpredict OH between 5065 km. Proposed modifications to HO x chemistry that produce lower OH and higher HO 2 and O 3 , yield better agreement in the lower mesophere but worsen the agreement at the observed OH peak near∼70 km. We find that neither standard HO x chemistry, nor models incorporating proposed HO x modifications, adequately reproduce the observed OH density profile over the entire 50–75 km altitude range for any of the observed early to mid morning local times.