Solar almucantar radiance and aerosol scattering: Comparative data from one tropical station and one mid-latitude station

Abstract

Solar, sky, and aureole radiance data from Kensington, Australia, and Lagos, Nigeria, are analysed. The values of the aerosol optical thickness at Lagos are comparable to measured values elsewhere in the subregion (Abidjan) under light haze. The values at Kensington are close to those of the “background aerosol.” Identical and anomalous values of aerosol optical thickness occurred at 1.67 µm at both stations as less than 15% of the values can be accounted for by aerosols. The high optical depths are therefore explained in terms of absorption, possibly mainly by CO2 (59–65%) and, to a lesser extent, by H2O (20–26%). Following correction for molecular multiple scattering, the aerosol scattering phase function increases substantially at 0.5 µm in Kensington and decreases considerably at 0.76 µm in Lagos when compared to the uncorrected function, confirming the relative fineness of the Kensington aerosol. The Lagos aerosol size distribution for moderately dusty air is reasonably bimodally log-normal with modal radiusr m=0.49 µm (1.36 µm) for the moder≤1 µm (r>1 µm). The Kensington aerosol resembles one withr m=0.05 µm forr<1 µm. There the Born approximation appears appropriate, and the weighted mean square radiusp 2=0.13 µm2. In the near forward direction the aerosol scattering phase funciton for Lagos resembles Deirmendjian’s simulations for a silicate-L haze and, for Kensington, a silicate-H haze.