The Lagrangian Operational Dispersion Integrator, LODI, is an atmospheric dispersion model developed for
operational emergency response within the U.S. Department of Energy’s National Atmospheric Release
Advisory Center (NARAC). It solves the 3-D advection diffusion equation using a Lagrangian stochastic,
Monte Carlo method which calculates possible trajectories of fluid “particles” in a turbulent flow. Particles are marked at the source of a contaminant with an appropriate amount of contaminant mass based upon prescribed mass emission rates. These computational particles can also be given total density and diameters, sampled from an input aerosol size distribution, which are used to calculate gravitational settling and deposition. Initial particle
positions are assigned by sampling the spatial distribution based on the geometry of the source. A large number
of independent particle trajectories are calculated by moving particles in response to the various processes, such as mean wind advection, gravitational settling and turbulent dispersion, represented within the simulation. The mean contaminant air concentration is estimated from the spatial distribution of the particles at a particular time.