Observations of Saharan Aerosols: Results of ECLATS Field Experiment. Part I: Optical Thicknesses and Aerosol Size Distributions

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 ?m) 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 ?m), whereas the concentration in very (r<0.2 ?m) and large (r>1 ?m) particles remained roughly constant.