Convective injection and photochemical decay of peroxides in the tropical upper troposphere: Methyl iodide as a tracer of marine convection

The convective injection and subsequent fate of the peroxides H2O2 and CH3O OH in the upper troposphere, is investigated using aircraft observations fr om 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 convecti on because of its relatively uniform marine boundary layer concentration, r elatively well-defined atmospheric lifetime against photolysis, and high se nsitivity of measurement, We find that mixing ratios of CH3OOH in convectiv e outflow at 8-12 lan altitude are enhanced on average by a factor of 6 rel ative to background, while mixing ratios of H2O2 are enhanced by less than a factor of 2. The scavenging efficiency of H2O2 in the precipitation assoc iated with deep convection is estimated to be 55-70%. Scavenging of CH3OOH is negligible. Photolysis of convected peroxides is a major source of the H Ox radical family (OH + peroxy radicals) in convective outflow. The timesca le for decay of the convective enhancement of peroxides in the upper tropos phere is determined using CH3I as a chemical clock and is interpreted using photochemical model calculations. Decline of CH3OOH takes place on a times cale of a 1-2 days, but the resulting HOx converts to H2O2, so H2O2 mixing ratios show no decline for similar to 5 days following a convective event. The perturbation to HOx at 8-12 km altitude from deep convective injection of peroxides decays on a timescale of 2-3 days for the PEM-Tropics A condit ions.