The aim of this work is to study the local impact of deep convection on the upper troposphere/lower stratosphere air composition. For this purpose, we performed a 42-h simulation of a severe convective event near Bauru, in the central State of São Paulo (Brazil), with the 3-D mesoscale model RAMS coupled on-line with a chemistry model. The meteorological results of the simulation are evaluated using comparisons with near surface measurements of wind and temperature and with surface rainfall rates derived from radar observations. These comparisons show that the model produces meteorological fields consistent with the observations. <br><br> This present paper (Part I) is devoted to the analysis of the ozone precursors in the upper troposphere/lower stratosphere: CO, NO<sub>x</sub> (=NO+NO<sub>2</sub>) and non-methane volatile organic compounds. The simulation results show that the distribution of CO with altitude is closely related to the upward convective motions and consecutive outflow at the top of the convective cells leading to a bulge of CO between 7 km altitude and the cold point tropopause (around 17km altitude). The model results for CO are consistent with satellite-borne measurements in the 700–500 hPa layer. The simulation also indicates enhanced amounts of NO<sub>x</sub> up to 2 ppbv in the 7–17 km altitude layer. These NO<sub>x</sub> concentrations are mainly produced by the lightning associated with the intense convective activity. Stratospheric NO<sub>x</sub> are not affected by the tropospheric NO<sub>x</sub> since there is, on average, no significant upward NO<sub>x</sub> flux through the tropopause. For non-methane volatile organic compounds, the convective activity tends to significantly increase the amount of ozone precursors in the 7–17 km layer by dynamical effects as for CO. During daytime, this bulge is largely reduced in the upper part of the layer for reactive species, such as isoprene, ethene and propene, since they undergo chemical loss. This loss is mainly due to their reactions with OH, OH mixing ratio being significantly increased during the daytime by the production of NO<sub>x</sub> by lightning. The bulges of ozone precursors in the upper troposphere are likely to be of importance in the ozone budget in the upper troposphere and lower stratosphere. This issue is discussed in Part II of this series of papers.