Size distribution and optical properties of long-range transported African dust
The number size distribution of mineral dust, measured between 10 nm and 20 µm by a combination of Scanning Mobility Particle Sizer and an optical particle counter, showed that dust was generally characterized by three modes centered at 180 nm, 700 nm and 2 µm. On one occasion, an additional mode centered at 6 µm was observed. The particle fraction below 100 nm showed the highest variability as a consequence of mixing which anthropogenic pollution which occurred any time the trade winds blown from the south east before reaching the sampling site. In these cases, the air masses travelled over the populated Virgin Islands.
Mass scattering and absorption efficiency, and single scattering albedo have been calculated based on measurements of scattering and absorption coefficients and mass concentrations. For particles in the PM10 fraction, the mass scattering efficiency at 450 nm was of the order of 1.4 m2 g-1 the mass scattering efficiency at 467 nm of the order of 0.05 m2 g-1. The single scattering albedo at 450 nm was of the order of 0.97.