We have developed a detailed parametrization scheme to represent the effects of subgrid-scale convective transport in a three-dimensional chemistry-transport model (CTM). The CTM utilizes the meteorological fields generated by a general-circulation model (GCM) to redistribute over 70 chemical species. The convective transport is implemented using the convective mass fluxes, entrainment rates and detrainment rates from the GCM.We compare the modelled distributions of 222Rn with observations. This shows that the vertical profile of this species is affected by the choice of convective-transpo rt parametrization. The new parametrization is found to improve significantly the simulation of 222Rn over the summertime continents. © Crown copyright, 2002. Royal Meteorological Society.