Sources of upper tropospheric HO_x over the South Pacific Convergence Zone: A case study

A zero-dimensional (0-D) model has been applied to study the sources of hydrogen oxide radicals (HO_x = HO₂ + OH in the tropical upper troposphere during the Pacific Exploratory Mission in the tropics (PEM-Tropics B) aircraft mission over the South Pacific in March-April 1999. Observations made across the Southern Pacific Convergence Zone (SPCZ) and the southern branch of the Intertropical Convergence Zone (ITCZ) provided the opportunity to contrast the relative contributions of different sources of HO_x, in a nitrogen oxide radical (NO_x)-limited regime, in relatively pristine tropical air. The primary sources of HO_x vary significantly along the flight track, in correlation with the supply of water vapor. The latitudinal variation of HO_x sources is found to be controlled also by the levels of NO_x and primary HO_x production rates P(HO_x). Budget calculations in the 8- to 12-km altitude range show that the reaction O(¹D + H₂O is a major HO_x source in the cloud region traversed by the aircraft, including SPCZ and the southern branch of the ITCZ. Production from acetone becomes significant in drier region south of 20°S and can become dominant where water vapor mixing ratios lie under 200 ppmv. Over the SPCZ region, in the cloud outflow, CH₃ OOH transported by convection accounts for 22% to 64% of the total primary source. Oxidation of methane amplifies the primary HO_x source by 1-1.8 in the dry regions.