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 CH 3 I concentrations. Observations of CH 3 I 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 CH 3 I observations. The model reproduces observed concentrations of H 2 O 2 and CH 3 OOH. Convective transport of CH 3 OOH from the lower troposphere is estimated to account for 40–80% of CH 3 OOH concentrations in the upper troposphere. Photolysis of CH 3 OOH transported by convection more than doubles the primary HO x source and increases OH concentrations and O 3 production by 10–50% and 0.4 ppbv d −1 , respectively, above 11 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 C 2 H 2 as a tracer, we estimate that biomass burning outflow enhances O 3 concentrations, O 3 production, and concentrations of NO x and OH by 60%, 45%, 75%, and 7%, respectively. The model overestimates HNO 3 concentrations by about a factor of 2 above 4 km for the upper one‐third quantile of C 2 H 2 data while it generally reproduces HNO 3 concentrations for the lower and middle one‐third quantiles of C 2 H 2 data.