Many commonly used atmospheric dispersion models are limited to continuous or instantaneous releases only, and cannot accurately simulate time-varying releases. The current paper discusses a new enhanced dispersion formulation accounting for time-varying effects resulting from a pressure drop in a vessel or pipe, and presuming no rainout. This new formulation is implemented in the Unified Dispersion Model (UDM), and is planned to be included in a future version of Phast. First existing methods are summarised for modelling finite-duration and time-varying releases, and limitations of these methods are identified. Secondly the new mathematical model is summarised. The new formulation presumes a number of 'observers' to be released at successive times from the point of discharge. The UDM carries out pseudo steady-state calculations for each observer, where the release data correspond to the time at which the observer is released. Subsequently the model applies a correction to the observer concentrations to ensure mass conservation when observers move with different velocities. Finally effects of along-wind diffusion (due to ambient turbulence) are included by means of Gaussian integration over the downwind distance. This results in reduced concentrations while the cloud travels in the downwind direction. The benefits of the new UDM methodology are illustrated for the case of a H2S toxic release from a long, pipeline representative of some extremely sour fields in the Middle East that are now being developed. Using corrected observer concentrations and along-wind diffusion significantly reduces toxic effect distances when compared to the current Phast 7.1 approach. (c) 2014 Elsevier Ltd. All rights reserved.