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 formulation for convective transport. Adopting tropical convective mass fluxes 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 the 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 observed concentrations of H2O2 and CH3OOH. Convective transport of CH3OOH from the lower troposphere is estimated to account for 40-80% of CH3OOH concentrations in the upper troposphere. Photolysis of CH3OOH transported by convection more than doubles the primary HOx, source and increases OH concentrations 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 over the tropospheric column is, however, small. The effects of pollutant import from biomass burning regions are much more dominant. Using C2H2 as a tracer, 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 generally reproduces HNO3 concentrations for the lower and middle one-third quantiles of C2H2 data.