PROJECT MISTT: MESOSCALE PLUME MODELING OF THE DISPERSION, TRANSFORMATION AND GROUND REMOVAL OF SO2

Abstract

A diagnostic β-mesoscale (25–250 km) plume model is developed using an existing steady-state model as a building block. This quasi-steady, Lagrangian model incorporates the diurnal variability of the planetary boundary layer (PBL) structure and of the parameters governing the chemical conversion and ground removal of SO2. The vertical inhomogeneity of atmospheric dispersion is simulated by the use of an assumed height- and stability-dependent profile of the eddy diffusion coefficient. Two important dimensionless system parameters are identified which govern pollutant dilution and ground removal. Model inputs are derived from Project MISTT aircraft data and the ground monitoring data of the St. Louis Regional Air Pollution Study (RAPS). On 9 and 18 July 1976, the plume of the 2400 MW, coal-fired Labadie power plant near St. Louis was sampled from aircraft out to 300 km. Model application is considered specifically for the data of these two days, and corresponding quantitative information about the dispersion, transformation and ground removal of SO2 is extracted. The results show that peak daytime SO2 conversion rates reached 1.8 and 3.0% h–1 on 9 and 18 July, respectively; the corresponding peak dry deposition velocities were between 1.5 and 2.0 cm s–1. The model is used to investigate the effects of source height, time of SO2 release and eddy diffusion on the overall sulfur budget of the plume. The mid- and late-afternoon plumes appear to have the highest potential for long range transport and sulfate formation. Ground removal is strongly influenced by the profile of vertical eddy diffusion in the surface layer, and much less by the profile shape and magnitude higher up.