Observed Downwash Concentrations Compared to ISCST Predictions in Urban Core

Abstract

An atmospheric tracer (SF 6 ) was released from an industrial stack in an urban core during nocturnal neutral to stable atmospheric conditions in an effort to determine the dispersive characteristics and the atmospheric transport of stack effluent under these conditions. Tracer concentration data are compared to predictions made by the Environmental Protection Agency's Industrial Source Complex Short Term (ISCST) model. ISCST is the regulatory air-pollution model typically recommended in situations where structure-induced plume downwash is a possibility. Under the meteorological conditions and stack parameters sampled in the present study, the downwash algorithm causes the ISCST model to overpredict surface concentrations below 3.5 m/s or below a wind speed/exit velocity ( u /V e) ratio of 0.25. Model performance was better in neutral than in stable atmospheric conditions. The stack effluent temperature was >450°K, and the model overprediction may be due in part to the treatment of thermal buoyancy in the model treatment of building wake downwash.