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Y. Fouquart, B. Bonnel, M. Chaoui Roquai, R. Santer, and A. Cerf

Abstract

A series of ground-based and airborne observations of desert aerosols, the ECLATS experiment was carried out in December 1980 in the vicinity of Niamey (Niger). This paper deals with aerosol optical thicknesses and size distributions derived from (i) in situ measurements using singe particle optical counters (a Kratel and a Knollenberg FSSP), (ii) a ground-based cascade impactor, and (iii) ground-based measurements of the spectral variation of the sober extinction.

During the experiment, aerosol optical thicknesses (at 550 nm) varied from 0.20 on very clear days to 1.5 during a so-called “dry haze” episode.

Comparisons between size distributions derived from in situ measurements from ground-based cascade impactor, and from inversion of the spectral optical thicknesses, showed that the optical counters drastically underestimated the concentration of small (r<0.5 μm) particles It was shown that the occurrence of a “dry haze” episode was characterized by a large increase (an order of magnitude in this particular case) of the intermediate particles (r≅0.5 μm), whereas the concentration in very (r<0.2 μm) and large (r>1 μm) particles remained roughly constant.

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Y. Fouquart, B. Bonnel, G. Brogniez, J. C. Buriez, L. Smith, J. J. Morcrette, and A. Cerf

Abstract

The results presented in this paper are a part of those obtained during the ECLATS experiment The broadband radiative characteristics of the Sahelian aerosol layer and the vertical radiative flux divergence within the dust layer were determined both from in situ measurements and Mie calculations.

In situ measurements of the aerosol layer's reflectances and transmittances of solar radiation led to aerosol single-scattering albedos close to ωA∼0.95. Measurements of the 8–14 μm radiances led to an optied depth by unit of volume of dust in a vertical column C A∼0.34 μm−1. Mie calculations assuming the aerosol refractive index published by Carlson and Benjamin for solar radiation and that measured by Volz for the atmospheric window, showed good agreement with observations. The ratio of infrared to visible optical thickness was δA(8–14 μm)/δA (0.55 μm)∼0.1, instead of 0.3 as calculated by Carlson and Benjamin. This discrepancy is attributable to differences in size distributions assumed.

The radiative budget of the Sahelian aerosol layer was determined for clear and dusty conditions. The additional aerosol shortwave heating was as much as 5 K day−1 for δA(0.55 μm) = 1.5 and with the sun overhead, whereas the additional cooling was close to 1 K day−1. As a consequence of the large temperature discontinuity at the surface, important infrared heating at the surface layer was observed.

The rather large differences between the aerosol optical properties reported here and those previously reported in the literature are due to different aerosol size distributions; therefore the present paper stresses the importance of careful determination of the size distributions.

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