A conceptually simple two-stream model (TSM) of the convective planetary boundary layer (PBL) provides a surprisingly realistic description of basic boundary layer transport features. The model is empirically fitted to a variety of important passive tracer problems. This model, unlike previous eddy diffusion models of vertical transport, is constructed to have a basic similarity to the underlying physics of transport. One parameterization can describe sources at the top or the bottom boundary or at all points within the domain and thereby may be used in simulations describing chemical or radiative forcings thoughout the PBL. The TSM also allows estimates of concentration variations due to sources, chemical processes, and temporal variation in the boundary conditions. However, it is intended for use only in problems in which the relatively exact description of the PBL provided by a large-eddy model is not practical. The model is likely to be most useful in parameterizing vertical turbulent transfer in the convective boundary layer when the chemical time scale is comparable to or greater than the convective time scale zi/w*, around 15 min. We present one model set of parameters which appear to perform well in a variety of situations. A methodology is available which may be applied to fit the model to a variety of PBL transport situations as accurate tracer measurements or three-dimensional model data become available. ¿ American Geophysical Union 1987