In the stratosphere there is a natural balance between production of ozone (O3 ) and atomic oxygen (o) by photolysis of oxygen molecules (O2) and loss of O3 and O through reactions with with radical compounds like OH and HO2.
Aura MLS is making global measurements of both OH and HO2 for the first time. Earlier limited measurements of OH did not agree with expected values calculated from measurements of water vapor and ozone using photochemical reactions measured in the laboratory. These measurements of OH and HO2 agree with observations. Since profiles measured by MLS cover the globe and have already been made for more than one year, scientists can test their consistency with other measurements and the laboratory photochemical reactions at different altitudes, latitudes, time of day and seasons. Some fast photochemical reactions convert OH to HO2 and back again, and comparing the ratio of HO2/OH to its calculated value tests this very fast photochemistry. The calculated values and the ratio vary the same way as observed by MLS, showing that scientists understand and can model this important part of ozone chemistry.