Rise of moist plumes from tall stacks in turbulent and stratified atmospheres

This paper, developed as a part of the European project RECAPMA (REgional Cycles of Air Pollution in the Mediterranean Area), presents a plume rise model capable of dealing with complex atmospheric profiles-wind shear and thermal stratification-since it performs a numerical integration, along the plume trajectory, of a set of differential equations derived from the balance of mass, momentum and energy in the plume. The classical parametrization for entrainment of air into plume due to self-generated turbulence has been completed with entrainment-detrainment processes in turbulent winds. The model deals with a mixture of four components-dry combustion gas, dry air, water vapor and liquid water-to allow for condensation and/or re-evaporation within the plume. Profile shapes of temperature and velocity of the plume cross sections are assumed to have axisymmetrical distributions and they can vary with the longitudinal distance to stack. Twenty field observations of plume trajectories and final rise in two power plants were compared to the numerical model predictions and a set of classical formulations used for regulatory models: effects like plume merger in a multiple source, condensation and re-evaporation, rise in turbulent winds, rise in light winds and in stratified atmosphere with wind shear, are discussed and tested separately. The results show that much of the scatter between classical model prediction and experimental results is due to the presence of complexities not allowed for in that formulation: some of the discrepancies are favorable because models underestimate plume rise, but caution has to be exercised, especially under negative wind shear-wind decreasing with height-combined with the presence of high inversions since plume rise is systematically overestimated. Our numerical model performs better under all those circumstances and can be used to calculate impacts downwind, by using the three-dimensional trajectories coupled with a dispersion model for the pollutant to be traced. A case study is presented in which impacts at long distances in turbulent winds and in a quasi-adiabatic atmosphere can be explained by using a model of uniform vertical distribution of pollutants under a subsidence inversion.