Open AccessExperimental evidence for the importance of convected methylhydroperoxide as a source of hydrogen oxide (HO x ) radicals in the tropical upper troposphere
Author(s) -
Ravetta F.,
Jacob D. J.,
Brune W. H.,
Heikes B. G.,
Anderson B. E.,
Blake D. R.,
Gregory G. L.,
Sachse G. W.,
Sandholm S. T.,
Shetter R. E.,
Singh H. B.,
Talbot R. W.
Publication year - 2001
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2001jd900009
Subject(s) - outflow , troposphere , convection , radical , hydrogen , scavenging , atmospheric sciences , altitude (triangle) , reaction rate constant , atmosphere (unit) , boundary layer , chemistry , analytical chemistry (journal) , meteorology , physics , thermodynamics , kinetics , environmental chemistry , antioxidant , organic chemistry , geometry , mathematics , quantum mechanics
Concurrent measurements of OH, HO 2 , H 2 O 2 , and CH 3 OOH concentrations were made during an 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 HO 2 and CH 3 OOH in the convective outflow were elevated by 50 and 350% relative to background, respectively, while concentrations of OH and H 2 O 2 were not elevated. The high CH 3 OOH concentrations in the outflow were due to convective pumping from the marine boundary layer. In contrast to CH 3 OOH, H 2 O 2 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 HO 2 enhancement in the convective outflow and attributes it to the enhanced CH 3 OOH; the calculation also reproduces the lack of OH enhancement in the outflow and attributes it to OH loss from reaction with CH 3 OOH. Further analysis of model results shows substantial evidence that the rate constant used in standard mechanisms for the CH 3 O 2 + HO 2 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 cm 3 molecule −1 s −1 at 233 K for this rate constant yields better agreement with observed HO 2 concentrations and better closure of the chemical budgets for both CH 3 OOH and H 2 O 2 . The CH 3 O 2 + HO 2 reaction then becomes the single most important loss pathway for HO x radicals (HO x = OH + peroxy radicals) in the upper troposphere.