The records from pollutant sensors abroad two aircraft are compared. The aircraft flew along arcs of either 80 or 120 km radius from the Gateway Arch in St. Louis. One aircraft contained a light-scattering instrument which determined the concentrations of particles with radii between 0.15 and 0.30 μm and between 0.30 and 1.3 μm. The other airplane contained an integrating nephelometer, a condensation nucleus counter, and an ozone monitor. It appears that neither the concentration of the condensation nuclei nor the ozone concentration are as reliable indicators of the location of the St. Louis plume at these distances as are data from the light-scattering particle counter or the nephelometer.