Convection in the tropics turns over the upper troposphere at rates (0.08 d-1) comparable to photochemical processes controlling the absolute abundance of HOx (OH+HO2) and the abundance of NOx (NO+NO2) relative to HNO3. Here we identify convection of boundary-layer CH3OOH as a primary source of HOx to the upper troposphere. Turnover of NOx leads to NO/HNO3 ratios much higher than predicted for local photochemical steady-state. Through convective transport the upper troposphere is more photochemically active in producing O3, an important greenhouse gas. ¿ 1997 American Geophysical Union