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 H 2 O 2 and CH 3 OOH 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 CH 3 I concentrations; CH 3 I 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 CH 3 OOH in convective outflow at 8–12 km altitude are enhanced on average by a factor of 6 relative to background, while mixing ratios of H 2 O 2 are enhanced by less than a factor of 2. The scavenging efficiency of H 2 O 2 in the precipitation associated with deep convection is estimated to be 55–70%. Scavenging of CH 3 OOH is negligible. Photolysis of convected peroxides is a major source of the HO x radical family (OH + peroxy radicals) in convective outflow. The timescale for decay of the convective enhancement of peroxides in the upper troposphere is determined using CH 3 I as a chemical clock and is interpreted using photochemical model calculations. Decline of CH 3 OOH takes place on a timescale of a 1–2 days, but the resulting HO x converts to H 2 O 2 , so H 2 O 2 mixing ratios show no decline for ∼5 days following a convective event. The perturbation to HO x at 8–12 km altitude from deep convective injection of peroxides decays on a timescale of 2–3 days for the PEM‐Tropics A conditions.