An identifiable plume of contaminated air forms downwind of metropolitan St. Louis under certain conditions. The contribution of this urban plume to tropospheric ozone and aerosol loadings is calculated from concentration and wind fields measured during Project MISTT. On all three summer days selected for study, net ozone exports reached 2-3¿106 g mol/h, or about 3/2 mole per mole of emitted nitrogen, within hours of emission. Conversion of sulfur dioxide to sulfate aerosol was incomplete at the distances sampled: it is estimated that haze exports ultimately reached 500--600 km2/h, or 8--9 m2 per gram of emitted sulfur. Simple extrapolation of the yields observed at St. Louis indicates that anthropogenic inputs overwhelm natural contributions to ozone and aerosols over eastern North America.