During a ''locally occurring system'' (LOS) in Amazonian forest characterized by an active mixing layer from surface to 1000 m capped by a fossil mixed layer between 1000 and 1500 m, the vertical mixing effects of a shallow cumulus are examined. The explicit redistribution of CO and O3 has been studied with a two-dimensional convective cloud model coupled with a chemical model including gas and aqueous phases, for a shallow convective situation taken from the GTE/ABLE 2B campaign. The chemistry describes the main oxidation chains of CH4 and CO in presence of low NOx concentration in a remote troposphere. Model results compare favorably with observations obtained after development of the shallow convection. The analysis of results explains how a growing and decaying cloud field allows exchanges between a mixing layer, a fossil mixed layer and the free troposphere. An inert tracer study has shown that the layer lying between surface to 500 m is transported up to 2000 m. Even small clouds are responsible for the transport and the transformation of chemical species. Sensitivity tests are performed to evaluate the relative importance of dynamical, microphysical and chemical processes. The cumulus venting is the main process which modifies the trace species redistribution. The O3 and CO amounts which are transported by a fair-weather cumulus through the boundary layer and free troposphere in the wet season, over a tropical rain forest, are respectively 1.2¿1023 molecules km-2 h-1 and 7.7¿1023 molecules km-2 h-1. In tropical regions, over rain forest, even with low NOx concentration, several cumulus exist every day and the vertical fluxes of some chemical species (like O3) cannot be neglected. ¿ American Geophysical Union 1996