Sources of upper tropospheric HO_<i>x</i> 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 = HO 2 + 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( 1 D) + H 2 O 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, CH 3 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.