Experimental evidence for the importance of convected methylhydroperoxide as a source of hydrogen oxide (HOx) radicals in the tropical upper troposphere

F. Ravetta, D. J. Jacob, W. H. Brune, B. G. Heikes, B. E. Anderson, D. R. Blake, G. L. Gregory, G. W. Sachse, S. T. Sandholm, R. E. Shetter, H. B. Singh, R. W. Talbot

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Concurrent measurements of OH, HO2, H2O2, and CH3OOH concentrations were made during ari aircraft flight over the tropical South Pacific that followed a back-and-forth pattern at constant 10 km altitude for 4 hours. One end of the pattern sampled an aged convective outflow, while the other end sampled the background atmosphere. Concentrations of HO2 and CH3OOH in the convective outflow were elevated by 50 and 350% relative to background, respectively, while concentrations of OH and H2O2 were not elevated. The high CH3OOH concentrations in the outflow were due to convective pumping from the marine boundary layer. In contrast to CH3OOH, H2O2 was not enhanced in the outflow because its high water solubility allows efficient scavenging in the convective updraft. A photochemical model calculation constrained with the ensemble of aircraft observations reproduces the HO2 enhancement in the convective outflow and attributes it to the enhanced CH3OOH; the calculation also reproduces the lack of OH enhancement in the outflow and attributes it to OH loss from reaction with CH3OOH. Further analysis of model results shows substantial evidence that the rate constant used in standard mechanisms for the CH3O2 + HO2 reaction is about a factor of 3 too low at the low temperatures of the upper troposphere. A sensitivity simulation using a value of 3.4 × 10-11 cm3 molecule-1 s-1 at 233 K for this rate constant yields better agreement with observed HO2 concentrations and better closure of the chemical budgets for both CH3OOH and H2O2. The CH3O2 + HO2 reaction then becomes the single most important loss pathway for HOx radicals (HOx = OH+ peroxy radicals) in the upper troposphere.

Original languageEnglish (US)
Article number2001JD900009
Pages (from-to)32709-32716
Number of pages8
JournalJournal of Geophysical Research Atmospheres
Issue numberD23
StatePublished - Dec 16 2001

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Oceanography
  • Forestry
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Space and Planetary Science
  • Earth and Planetary Sciences (miscellaneous)
  • Palaeontology


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