A Lagrangian model has been developed to treat the global-scale transport, transformation, and removal of trace species in the atmosphere. The model uses prescribed velocity fields (either from observations or from Eulerian dynamical models) to advect constant-mass parcels of air. These constant-mass air parcels can contain varying amounts of aerosols, such as dust and smoke, or varying amounts of trace gases, such as ozone methane, or nitrogen dioxide. Because advection operates on the air parcels, rather than on the trace species, the computational efficiency of the approach improves, in contrast to Eulerian approaches, as the number of trace species increases. Numerical diffusion is also much less than in typical Eulerian models, permitting a wide range of spatial distributions of species to be advected with minimal dispersion. The Lagrangian method is particularly well suited to species that are chemically active because deviations from chemical equilibrium associated with advection are greatly reduced. Additional processes presently represented in the model include precipitation scavenging, dry deposition, aerosol coagulation, gravitational settling, local mixing with nearby parcels, vertical mixing by moist convection, and horizontal and vertical mixing associated with eddy mixing on scales finer than the prescribed velocity field grid. The model can operate in either a passive or an interactive mode. In the passive mode the meteorological flow is assumed to be unaffected by the trace species. In this mode a wide variety of sensitivity experiments using the same meteorology can readily be conducted. In this paper we consider several such experiments using submicron and supermicron aerosol particles, including an assessment of the relative importance of convective versus dynamical mixing, an assessment of the relative importance of various removal mechanisms, and a sample application to investigate the deposition to the surface of locally injected radionuclides. Use of this model in the interactive mode is considerd in a companion paper (Ghan et al., this issue). ¿ American Geophysical Union 1988