MESOSPHERIC HOX PHOTOCHEMISTRY - CONSTRAINTS FROM RECENT SATELLITE MEASUREMENTS OF OH AND H2O

We use recent measurements of OH made with the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) along with satellite ob servations of water vapor abundances, to study odd-hydrogen photochemi stry 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 mis sion. Our approach is to use a photochemical model, with input H2O abu ndances fixed to observed values, to simulate the diurnal variation of mesospheric HOx species. Models of OH using standard recommended HOx chemical rate coefficients are found to substantially overpredict OH b etween 50-65 km. Proposed modifications to HOx chemistry that produce lower OH and higher HO2 and O-3, yield better agreement in the lower m esosphere but worsen the agreement at the observed OH peak near simila r to 70 km. We find that neither standard HOx chemistry, nor models in corporating proposed HOx modifications, adequately reproduce the obser ved OH density profile over the entire 50-75 km altitude range for any of the observed early to mid morning local times.