Fumigation and Plume Trapping on the Shores of Lake Michigan During Stable Onshore Flow

Abstract

Previous studies have shown that the lake breeze circulation cell which develops along the western shore of Lake Michigan during almost half of the warm season days has detrimental effects upon the air quality of the Gray-Chicago-Milwaukee area. However, stable onshore flow associated with a synoptic-scale pressure gradient occurs for an additional 15% of warm season days. This study examines the dispersion patterns during gradient, onshore flow. Fumigation and plume trapping, in particular, appear to cause serious degradation of air quality. Continuous fumigation of elevated plumes develops on days with strong insulation. Plume trapping occurs when a plume is emitted into a shallow layer of unstable air capped by a deep lid of stable air. This condition is frequent on overcast spring days. Two days characterized by easterly gradient winds were studied: 27 May 1970, overcast; 25 June 1970, predominately sunny. The studies utilized meteorological data obtained from ground observers, ship's records, a wiresonde, and from aircraft photography. A computer diffusion model incorporating the mesoscale meteorological characteristics for each day predicted ground level concentrations from several sources including a large coat burning power plant. The model for the fumigating power plant plume (25 June) yielded estimates in excess of 1.0 ppm SO2 7 km downwind of the plant. Limited air monitoring data appear to confirm the diffusion model estimates and observations of plume behavior.