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

  • Author(s): Cohan, DS
  • Schultz, MG
  • Jacob, DJ
  • Heikes, BG
  • Blake, DR
  • et al.

Published Web Location Commons 'BY' version 4.0 license

The convective injection and subsequent fate of the peroxides H O and CH 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 I concentrations; CH 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 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 O are enhanced by less than a factor of 2. The scavenging efficiency of H O in the precipitation associated with deep convection is estimated to be 55-70%. Scavenging of CH OOH is negligible. Photolysis of convected peroxides is a major source of the HO 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 I as a chemical clock and is interpreted using photochemical model calculations. Decline of CH OOH takes place on a timescale of a 1-2 days, but the resulting HO converts to H O , so H O mixing ratios show no decline for ∼5 days following a convective event. The perturbation to HO 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. Copyright 1999 by the American Geophysical Union. 2 2 3 3 3 3 2 2 2 2 3 x 3 3 x 2 2 2 2 x

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