Understanding the absorption Angstrom exponent provided in the AERONET database

Abstract:

Recently, some authors have suggested that the absorption Angstrom exponent (AAE) can be used to deduce the component aerosol absorption optical depths (AAOD) of dust, brown carbon, and soot carbon in the atmosphere. The premise behind this AAE approach is that AAE is a species-dependent aerosol property that does not depend upon particle size or mass, that absorbing aerosol species are externally mixed with one another, and that AAE is much less than 1 for black carbon. Other authors have found that AAE does not contain enough information to unambiguously speciate the absorbing aerosols. Thus, we explore this topic here, and point out some theoretical inconsistencies associated with using the AAE approach to deduce component AAODs from the AERONET retrievals.
For instance, Level 2.0 retrievals at 15 West African sites subsampled for AAE < 1.0 indicate that 86% of the fine volume fractions are less than 0.2, 56% of the depolarization ratios are greater than 0.2, and 94% of the Angstrom exponents are less than 1.0. This indicates that most of the West African data with AAE < 1 are dominated by coarse mode dust, and that low AAE does not indicate pure BC, and that therefore AAE can not be used to separate carbonaceous aerosols from dust. We obtained similar results at five Middle East dust sites subsampled for AAE < 1.0, with 59% of the fine volume fractions less than 0.2, 88% of the depolarization ratios greater than 0.2, and 73% of the Angstrom exponents less than 1.0.
Additionally, we find that AAE << 1 is very unlikely to occur for size distributions with fine volume fractions greater than 0.5 at nine southern Africa and South America sites, unless the imaginary refractive index at the 440 nm wavelength is less than the imaginary refractive index at the red and near infrared wavelengths (i.e., k(440) < k(rnir)). Since black carbon has a spectrally invariant imaginary refractive index at these wavelengths, it is unlikely to be the cause of k(440) < k(rnir) and AAE < 1 when the fine mode dominates. We conclude that AAE < 1 is not caused by pure BC, and that the AAE approach can not be used to separate carbonaceous aerosols from dust.