Airborne in-situ OH and HO₂ observations in the cloud-free troposphere and lower stratosphere during SUCCESS

The hydroxyl (OH) and hydroperoxyl (HO₂) radicals were measured for the first time throughout the troposphere and in the lower stratosphere with a new instrument aboard the NASA DC-8 aircraft during the 1996 SUCCESS mission. Typically midday OH was 0.1-0.5 pptv and HO₂ was 3-15 pptv. Comparisons with a steady-state model yield the following conclusions. First, even in the lower stratosphere OH was sensitive to the albedo of low clouds and distant high clouds. Second, although sometimes in agreement with models, observed OH and HO₂ were more than 4 times larger at other times. Evidence suggests that for the California upper troposphere on 10 May this discrepancy was due to unmeasured HO_x sources from Asia. Third, observed HO₂/OH had the expected inverse dependence with NO, but was inexplicably higher than modeled HO₂/OH by an average of 30%. Finally, small-scale, midday OH and HO₂ features were strongly linked to NO variations.