Influence of convection and biomass burning outflow on tropospheric chemistry over the tropical Pacific

Observations over the tropics from the Pacific Exploratory Mission-Tropics A Experiment are analyzed using a one-dimensional model with an explicit fo rmulation for convective transport. Adopting tropical convective mass fluxe s from a general circulation model (GCM) yields a large discrepancy between observed and simulated CH3I concentrations. Observations of CH3I imply the convective mass outflux to be more evenly distributed with altitude over t he tropical ocean than suggested by the GCM. We: find that using a uniform convective turnover lifetime of 20 days in the upper and middle troposphere enables the model to reproduce CH3I observations. The model reproduces obs erved concentrations of H2O2 and CH3OOH. Convective transport of CH3OOH fro m the lower troposphere is estimated to account for 40-80% of CH3OOH concen trations in the upper troposphere. Photolysis of CH3OOH transported by conv ection more than doubles the primary HOx, source and increases OH concentra tions and O-3 production by 10-50% and 0.4 ppbv d(-1), respectively, above II km. Its effect on the OH concentration and O-3 production integrated ove r the tropospheric column is, however, small. The effects of pollutant impo rt from biomass burning regions are much more dominant. Using C2H2 as a tra cer, we estimate that biomass burning outflow enhances O-3 concentrations, O-3 production, and concentrations of NOx and OH by 60%, 45%, 75%, and 7%, respectively. The model overestimates HNO3 concentrations by about a factor of 2 above 4 km for the upper one third quantile of C2H2 data while it gen erally reproduces HNO3 concentrations for the lower and middle one-third qu antiles of C2H2 data.