Characteristics of desert dust optical and physical properties from AERONET: spectral absorption, size distributions and spectral AOD

Abstract. The AERONET multi-decade global database is examined to investigate long-term averages of retrieved physical and optical properties and spectral aerosol optical depth (AOD) of significant events at sites in major global dust source and receptor regions. After examples are shown of retrieved particle size and properties such as absorption, attention is paid to average spectral single scattering albedo (SSA) with comparisons to the laboratory measurements of dust from soil samples by Di Biagio et al. (2019). Overall, the AERONET retrieved and laboratory SSA agree very well for specific dust source regions in the Sahara and Sahel, with some regions showing annual cycles at 440 nm, an indicator of changes in iron oxide content. The strongest dust absorption was measured at an Australia desert site, and the least absorption was in dust transported to the Caribbean from the Sahara/Sahel regions. Also notable is regional and seasonal variability in the prevalence of ~0.3 to 1 μm radius dust, especially in the Sahel and Sahara. Such middle mode dust can often account for ~40 % of AOD at 440 nm. These ‘middle mode’ sized particles are a probable significant contributor for the observed relatively large spectral drop-off in AOD in the wavelengths beyond the visible, at 1020 nm and especially 1640 nm, in most dust events where Angstrom Exponent is low. Increased understanding of absorption, size distributions and AOD spectra of dust aerosol may help in improving remote sensing retrievals plus in more accurately characterizing the radiative effects of desert dust.