Observed OH and HO₂ in the upper troposphere suggest a major source from convective injection of peroxides

Abstract

ER‐2 aircraft observations of OH and HO 2 concentrations in the upper troposphere during the NASA/STRAT campaign are interpreted using a photochemical model constrained by local observations of O 3 , H 2 O, NO, CO, hydrocarbons, albedo and overhead ozone column. We find that the reaction Q(¹D) + H 2 O is minor compared to acetone photolysis as a primary source of HO x (= OH + peroxy radicals) in the upper troposphere. Calculations using a diel steady state model agree with observed HO x concentrations in the lower stratosphere and, for some flights, in the upper troposphere. However, for other flights in the upper troposphere, the steady state model underestimates observations by a factor of 2 or more. These model underestimates are found to be related to a recent (< 1 week) convective origin of the air. By conducting time‐dependent model calculations along air trajectories determined for the STRAT flights, we show that convective injection of CH 3 OOH and H 2 O 2 from the boundary layer to the upper troposphere could resolve the discrepancy. These injections of HO x reservoirs cause large HO x increases in the tropical upper troposphere for over a week downwind of the convective activity. We propose that this mechanism provides a major source of HO x in the upper troposphere. Simultaneous measurements of peroxides, formaldehyde and acetone along with OH and HO 2 are needed to test our hypothesis.