Assessment of upper tropospheric HOx sources over the tropical Pacific based on NASA GTE/PEM data: Net effect on HOx and other photochemical parameters

Data for the tropical upper troposphere (8-12 km, 20 degrees N-20 degrees S ) collected during NASA's Pacific Exploratory Missions have been used to ca rry out a detailed examination of the photochemical processes controlling H Ox (OH+HO2). Of particular significance is the availability of measurements of nonmethane hydrocarbons, oxygenated hydrocarbons (i.e., acetone, methan ol, and ethanol) and peroxides (i.e., H2O2 and CH3OOH). These observations have provided constraints on model calculations permitting an assessment of the potential impact of these species on the levels of HO2, CH3O2 CH2O, as well as ozone budget parameters. Sensitivity calculations using a time-dep endent photochemical box model show that when constrained by measured value s of the above oxygenated species, model estimated HOx levels are elevated relative to unconstrained calculations. The impact of constraining these sp ecies was found to increase with altitude, reflecting the systematic roll-o ff in water vapor mixing ratios with altitude. At 11-12 km, overall increas es in HOx approached a factor of 2 with somewhat larger increases being fou nd for gross and net photochemical production of ozone. While significant, the impact on HOx due to peroxides appears to be less than previously estim ated. In particular, observations of elevated H2O2 levels may be more influ enced by local photochemistry than by convective transport. Issues related to the uncertainty in high-altitude water vapor levels and the possibility of other contributing sources of HOx are discussed. Finally, it is noted th at the uncertainties in gas kinetic rate coefficients at the low temperatur es of the upper troposphere and as well as OH sensor calibrations should be areas of continued investigation.