Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave spectrum: a simulation chamber study

This paper presents new laboratory measurements of the mass absorption efficiency (MAE) between 375 and 850 nm for 12 individual samples of mineral dust from different source areas worldwide and in two size classes: PM_10.6 (mass fraction of particles of aerodynamic diameter lower than 10.6 μm) and PM_2.5 (mass fraction of particles of aerodynamic diameter lower than 2.5 μm). The experiments were performed in the CESAM simulation chamber using mineral dust generated from natural parent soils and included optical and gravimetric analyses. The results show that the MAE values are lower for the PM_10.6 mass fraction (range 37-135 x 10³ m² g^-1 at 375 nm) than for the PM_2.5 (range 95-711 x 10³ m² g^-1 at 375 nm) and decrease with increasing wavelength as λ^-AAE, where the Angstrom absorption exponent (AAE) averages between 3.3 and 3.5, regardless of size. The size independence of AAE suggests that, for a given size distribution, the absorption properties and elemental iron rather than the iron oxide fraction, which could ease the application and the validation of climate models that now start to include the representation of the dust composition, as well as for remote sensing of dust absorption in the UV-vis spectral region. (authors)