Visibility Reduction and Accompanying Aerosol Evolution Downwind of St. Louis

Abstract

Under the auspices of Project METROMEX, studies of visibility deterioration downwind of St. Louis were conducted during August 1976. Estimates of horizontal visibility and aerosol measurements were acquired upwind, over and downwind of the St. Louis metropolitan area, by means of airborne transects and standard meteorological data. Reductions amounting to 50% of regional upwind visibilities have been observed; during a weekend study when light, urban vehicular and industrial activity was observed, visibilities within the anomaly were reduced by only 20%. The anomalies were situated between distances corresponding to 2?4 h travel time downwind for an air parcel moving with the mean transport wind. Several case studies noted improvement in the visual range after 4?5 h downwind. The regions of minimum visibility do not coincide with locations of Aitken nucleus concentrations at 2?4 h downwind. Aerosol surface volume distributions indicate a general growth in the accumulation mode (0.1?1.0 ?m) with maximum values in a much narrower size range within the mode of 0.2?0.3 ?m diameter. These increases in the accumulation mode correspond to the size range in which particles are optically significant. Collaborative evidence from current literature suggests gas-to-particle reactions and/or gravitational sedimentation and coagulation as cause of the aerosol growth. The ramifications of visibility deterioration near large metropolitan areas are demonstrated by a conceptual model.