Radiative heating rates and some optical properties of the St. Louis aerosol,as inferred from Aircraft measurements

Radiative heating rates were determined from measurements made aboard an aircraft flying within and above the St. Louis boundary layer during the Regional Air Pollution Study (RAPS; 1975–1976). In the mean, heating rates tor both solar and infrared radiation were found to diminish steadily from a maximum near the surface to nearly zero above the mixing layer. Similarly, there was a mean decrease in large particle aerosol concentrations and in extinction coefficient with height to insignificant values above the mixing layer. Over the entire mixing depth, there was an average net solar warming and an average net longwave cooling due to the presence of aerosol. When normalized for a 24 h period the solar warming was found to be very slightly less than the magnitude of the long-wave cooling, although the former somewhat exceeded the latter during midday. Derived values of the asymmetry parameter and single scattered albedo were large, the value for each averaging close to 0.9. The ratio of backscatter to absorption for the aerosol was approximately unity.Thus the arosol was not a strongly absorbing material and. consequently, the aerosol healing or cooling rates were not very large within the St. Louis mixing layer and the total (solar + IR) effect of the temperature changes on the atmospheric motions were likely to have been insignificant.