Effects of atmospheric aerosols on solar radiation

Abstract Calculations of the spectral distribution of visible radiation (270–710 nm) in the direct solar beam were made for three aerosol size distributions at “thin” and “dense” concentrations and for solar zenith angles of 30 and 60°. Direct irradiance (270–2800 nm) was tabulated for each aerosol and diffuse and total irradiance were estimated from an empirical relationship. Direct and diffuse irradiance and the fraction ϵ 1 , of visible to all-wavelength radiation in the direct solar beam were strongly dependent on aerosol size distribution, but total irradiance was much less dependent. Calculations showed that increasing aerosol concentration reduced the photosynthetically active radiation in the direct solar beam by up to 50 per cent but the depletion of photosynthetically active total radiation would be markedly less. Radiation measurements at three sites in Britain showed that turbidity depended mainly on air mass origin; local sources of aerosol were relatively unimportant. In the Sudan, ϵ 1 , was greater than in Britain although turbidity was similar. Mean monthly values of a turbidity coefficient τ a calculated for summer months at five Meteorological Office sites showed that turbidity in Britain was greatest in south east England.