Spectral solar radiation measurements and turbidity: Comparative studies within a tropical and a sub-arctic environment

The effects of atmospheric turbidity due to aerosols on the direct solar irradiance at the surface at a low latitude (Ile-Ife (7 degrees 30'N, 4 degrees 31'E), south-western Nigeria) and a high latitude station (Abisko (68 degrees 21'N, 18 degrees 49'E), northern Sweden) before and after the Pinatubo eruption are compared. The Pinatubo aerosols increased the total atmospheric optical depths by a factor between 2 and 4 at Abisko and also had a drastic effect on spectral distribution of the optical depths. Under intense harmattan conditions, the atmospheric turbidity at Ile-Ife is about 7 times higher than that in the Arctic region, even with the significant effect of the Pinatubo volcanic eruption on the latter. The application of the Angstrom formula for approximating the aerosol spectral extinction is discussed for the cases concerned. (C) 1997 Elsevier Science Ltd.