The role of HOx in super‐ and subsonic aircraft exhaust plumes

The generation of sulfuric acid aerosols in aircraft exhaust has emerged as a critical issue in determining the impact of supersonic aircraft on stratospheric ozone. It has long been held that the first step in the mechanism of aerosol formation is the oxidation of SO 2 emitted from the engine by OH in the exhaust plume. We report in situ measurements of OH and HO 2 in the exhaust plumes of a supersonic (Air France Concorde) and a subsonic (NASA ER‐2) aircraft in the lower stratosphere. These measurements imply that reactions with OH are responsible for oxidizing only a small fraction of SO 2 (2%), and thus cannot explain the large number of particles observed in the exhaust wake of the Concorde.