DECEMBER 2000: Hydrogen oxide radical chemistry in the upper troposphere

Concentrations of HO_x radicals (HO_x = OH + HO₂) and peroxide reservoirs (H₂O₂, CH₃OOH) observed at 10 km altitude on a flight from the NASA PEM-Tropics B aircraft mission over the South Pacific in March 1999 (Jacob was mission scientist for PEM-Tropics B). The aircraft repeated a back-and-forth pattern NW of Tahiti for five hours across sunrise and into mid-morning. One end of the pattern (between the blue lines) sampled air freshly convected from the lower troposphere, as indicated by high concentrations of CH₃OOH and other chemical tracers. The rest of the pattern sampled typical background conditions for the upper troposphere. Measurements are shown as black lines; for HO_x they were made by W.H. Brune from Penn State, and for peroxides they were made by Brian Heikes from U. Rhode Island. Photochemical model calculations constrained with aircraft observations for species other than HO_x were made by Francois Ravetta from our group and results are shown as red lines (standard calculation) and green lines (calculation with the rate constant for the CH₃O₂+HO₂ reaction tripled). The model reproduces the large observed enhancements of HO₂ in the convective outflow, and the lack of enhancement of OH. Analysis of model results shows that the convective enhancement of HO₂ is due to convective injection of CH₃OOH. This is the first experimental demonstration of the importance of deep convection of CH₃OOH as a source of HO_x radicals to the upper troposphere; this supply of radicals to the upper troposphere has a number of implications including the environmental effects of aircraft [Jaegle et al., 1999]. Further analysis of model results also shows that increasing the magnitude of the CH₃O₂+HO₂ reaction rate constant at low temperatures (where it is highly uncertain) provides a better match of model results with observations for both HO_x radicals (green lines) and peroxides; the CH₃O₂+HO₂ reaction is then the most important sink of HO_x in the upper troposphere. Further details of this work are presented by Ravetta et al. [2000].