Measurements of solar radiation and estimation of optical depth in the high Andes of Peru

Abstract

During an expedition to the high Andes of Southern Peru in June–July 1977, measurements of direct solar radiation in four spectral bands (0.270–0.530–0.630–0.695–2.900 μ) were conducted at six sites in elevations ranging from sea level to 5645 m. These measurements were evaluated in Langley plots to determine total optical depths (τ) and irradiances at the top of the atmosphere. In addition, water vapor optical depths (τwv) were calculated from the mean radiosounding over Lima during the expedition, and Rayleigh (τray) and ozone (τoz) optical depths were obtained from published tabulations. Subtracting τray, τoz, and τwv from τ yielded estimates of aerosol optical depth τaer. The components τray and τoz decrease from the shorter towards the longer wavelength bands and from the lower towards the higher elevation sites; τaer also decreases towards the higher elevations. Particularly pronounced is the decrease of τaer and τ from the lowlands of the Pacific coast to the highlands of the interior, reflecting the effect of a persistent lower-tropospheric inversion and the contrast from the marine boundary layer to the clear atmosphere of the high Andes.