Satellite observations of upper stratospheric and mesospheric OH: The HOx dilemma

We report the first observations of the vertical distribution of hydroxyl ( OH) from the upper stratosphere to the mesopause, The Middle Atmosphere Hig h Resolution Spectrograph Investigation (MAHRSI) made these measurements in August 1997. The data confirm the results from the earlier November 1994 M AHRSI mission that were confined to altitudes above 50 km, namely that meso spheric OH densities are 25 to 35% lower than predicted by standard photoch emical theory, However, the new observations show that below 50 km the OH d ensity increases rapidly and at 43 km altitude it is larger than that expec ted from standard theory. This represents a serious dilemma for our underst anding of odd-hydrogen chemistry because the same key reactions are thought to dominate OH/HO2 partitioning in both regions. We show that neither stan dard photochemical theory nor any previously proposed changes are adequate to explain the OH observations in both the upper stratosphere and mesospher e.