Polarising properties of the aerosols in the north-eastern tropical Atlantic Ocean, with emphasis on the ACE-2 period

Measurements of the polarisation state of the atmosphere were performed at Tenerife in June-July 1997, in the framework of ACE-2 (second Aerosol Chara cterization Experiment), by 2 ground-based instruments: RefPol (a LOA proto type) which took measurements at 445, 665, 870, 1610 nm in the solar princi pal plane: and an automatic CIMEL (CE 318) sun/sky-photometer which measure d polarised radiation at 870 nm in the same observational geometry. Measure ments acquired during the campaign, as well as AERONET (AErosol RObotic NET work) measurements acquired at the sites of Cape Verde and M'Bour, are proc essed with an algorithm determining the polarised single-scattering sky-rad iance due to aerosols, directly proportional to the aerosol polarised phase function (representing the probability to scatter polarised radiation in t he direction of the scattering angle). A good correlation between the Angst rom exponent alpha, representing the spectral dependence of the extinction measurements, and the polarised phase function is observed on each set of d ata. The uncertainty of retrievals at 445 nm makes the determination of the spectral dependence of polarisation inconclusive but does not prevent conf irming the dependence of the aerosol polarised phase function on alpha, at all wavelengths. An Angstrom exponent of 1 corresponds to a polarised phase function of around 0.1 (+/-0.04), at 870 nm and at a scattering angle of 6 0 degrees. For alpha between 0 and 0.4, the average value of the polarised phase function is 0.05. The correlation shows that polarisation is more sen sitive to small particles than to large particles. The discrepancy between retrievals and Mie calculations from an AERONET size distribution, inverted from Izana measurements acquired during a dust event, suggests the presenc e of small particles, not detected by total sky-radiance measurements.