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

Abstract

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.