Abstract

The convective injection and subsequent fate of the peroxides H₂O₂ and CH₃OOH in the upper troposphere is investigated using aircraft observations from the NASA Pacific Exploratory Mission - Tropics (A) (PEM-Tropics (A)) over the South Pacific up to 12-km altitude. Fresh convective outflow is identified by high CH₃I concentrations; CH₃I is an excellent tracer of marine convection because of its relatively uniform marine boundary layer concentration, relatively well-defined atmospheric lifetime against photolysis, and high sensitivity of measurement. We find that mixing ratios of CH₃OOH in convective outflow at 8-12 km altitude are enhanced on average by a factor of 6 relative to background, while concentrations of H₂O₂ are enhanced by less than a factor of 2. The scavenging efficiency of H₂O₂ in the precipitation associated with deep convection is estimated to be 55-70%. Scavenging of CH₃OOH is negligible. Photolysis of convected peroxides is major sources of the HO_x radical family (OH + peroxy radicals) in convective outflow. The time scale for decay of the convective enhancement of peroxides in the upper troposphere is determined using CH₃I as a chemical clock and is interpreted using photochemical model calculations. Decline of CH₃OOH takes place on a time scale of 1-2 days but the resulting HOx converts to H₂O₂, so that H₂O₂ concentrations show no decline for ~5 days following the convective event. The perturbation to HOx at 8-12 km altitude from deep convective injection of peroxides decays on a time scale of 2-3 days for the PEM-Tropics (A) conditions.