Many commonly used atmospheric dispersion models cannot accurately simulate time-varying releases of hazardous chemicals in the atmosphere. A new version of the Phast Unified Dispersion Model (UDM) has been developed to more accurately simulate time-varying effects resulting from a decreasing discharge rate in a vessel or pipe and/or resulting from a time-varying pool (following rainout or from direct spill). The new model includes effects of along-wind diffusion which reduces concentrations in the far-field. This is particularly relevant for toxic releases. The new formulation presumes a number of "observers" to be released at successive times from the point of discharge or the upwind edge of the pool. The UDM first carries out pseudo steady-state calculations for each observer, where the release data correspond to the time at which the observer is released, and where the observer picks up vapor while traveling over the evaporating pool. Subsequently, effects of along-wind diffusion are included by means of Gaussian integration of observer concentrations over the downwind distance. The above new UDM formulation has been verified against the time-varying HGSYSTEM model HEGADAS-T for the specific case of dispersion directly from a pool. Furthermore, it has been tested for more general cases including elevated releases with rainout. (c) 2013 American Institute of Chemical Engineers