Microwave Limb Sounder
Hydrogen Chemistry
SummaryThe odd-hydrogen radicals, OH and HO2 (collectively known as HOx), play fundamental roles in the middle atmospheric photochemistry due to their ability to destroy odd oxygen (O3 and O). In particular, at altitudes above ~40 km, the major catalytic O3 loss is controlled by reactions involving HOx species. A better understanding of the trend in HOx thus helps to properly identify the recovery of the O3 layer. In this region of the atmosphere, HOx mainly come from the photolysis of O3 (i.e., the break down of O3 by sunlight) followed by reaction with H2O. In addition, chemical destruction of H2 and direct photolysis of H2O are also sources of HOx, especially above 80 km. The distribution of HOx species and the chemistry involved are sensitive to solar forcing such as the diurnal cycle, the seasonal cycle, the solar 11-year and 27-day cycles, and solar proton events (SPEs).Before the launch of Aura, the investigation of hydrogen chemistry had been greatly limited due to the lack of multi-year systematic observations on a global scale. The only available long-term records were a number of ground-based measurements of OH total column abundance. Large discrepancies among measurements and between model predictions and observations were reported. MLS provides the first global daily measurements of HOx vertical profiles in the stratosphere and the mesosphere, making it possible to resolve some of these discrepancies, and significantly improve our understanding of the chemistry involved and the response to external forcing. In particular, MLS observations will help to investigate the discrepancy between modeled and observed O3 in the upper stratosphere, known as the "O3 deficit", which implies that there could be a gap in the current understanding of hydrogen chemistry. In addition, the magnitude of the solar effects on hydrogen chemistry is currently unclear, given the large discrepancies between observed and modeled response of O3 and OH to the solar cycle. |
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MLS-related publications concerning stratospheric and mesospheric hydrogen chemistry2011
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