Thermodynamic Properties and Aerosol Patterns in the Plume Downwind of St. Louis

Abstract

An instrumented aircraft equipped to measure temperature, humidity, turbulence, Aitken nuclei, and wind speed and direction was used to examine the thermal and aerosol plumes downwind of St. Louis, Mo., for four case studies with moderate winds, under the auspices of Project METROMEX. Cross-wind patterns out to 140 km were flown upwind, over and downwind of the city. Meteorologically instrumented vans, one of which was equipped to launch radiosondes, were also employed. Thermodynamic perturbations in the mixing layer were limited to regions no more than 40 km (1.3 h travel time) downwind of the intense surface anomalies in the metropolitan area. The magnitude of the thermodynamic perturbations and their vertical extent are inversely related to the mixing layer wind speed. In all cases, St. Louis and the adjacent Alton-Wood River industrial complex produced distinct aerosol plumes out to 100 km or more, but never more than 4 h travel time. Forty-five percent reductions in visibility were consistently detected in regions 2.5–3.0 h downwind of St. Louis. These locations did not coincide with those of maximum concentrations of Aitken nuclei. A conceptual model describing various urban influences in the mid-day mixing layer downwind of a metropolitan area has been synthesized.