Measured and Calculated Solar Radiation in an Aerosol Atmosphere

Abstract

<p>Profiles of upwelling and downwelling solar radiation, aerosol number size distribution and meteorological variables were obtained in the planetary boundary layer over Lake Ontario. These data and simultaneous surface measurements of downwelling, normal incidence direct beam solar radiation and scattering coefficient were the bases of a comparison between independently calculated and measured radiation fluxes.</p> <p>Collected aerosol samples were analyzed using energy dispersive X-ray analysis. A three component refractive index for each profile level was then estimated. Combining these with cubic spline interpolated aerosol number size distributions, the Mie extinction coefficient, single scatter albedo and asymmetry factor were calculated.</p> <p>Using the measured temperature and humidity profiles, climatic profiles of ozone, sulphur dioxide and nitrogen dioxide and the aerosol optical properties, solar radiation fluxes were calculated from a δ-Eddington approximation using 83 spectral intervals. The modelled downwelling fluxes were within 4% of the measured fluxes at profile levels above the surface and within 6% at the surface.</p> <p>Tests using Chandrasekhar mean optical properties for the aerosol showed excellent agreement with the full model calculations. Similarly calculations using tabulated refractive indices (Shettle and Fenn, 1979) were in good agreement with the full model. Model estimates using aerosol optical properties inferred from surface visibility, however, were found to be in serious error.</p> <p>Calculations indicated that the regional aerosol over western Lake Ontario doubled the solar heating rates throughout the lower boundary layer. Such increases could affect regional climatic processes.</p>