Evidence of a weakly absorbing intermediate mode of aerosols in AERONET data from Saharan and Sahelian sites

Abstract

Accurate retrievals of aerosol size distribution are necessary to estimate aerosols' impact on climate and human health. The inversions of the Aerosol Robotic Network (AERONET) usually retrieve bimodal distributions. However, when the inversion is applied to Saharan and Sahelian dust, an additional mode of intermediate size between the coarse and fine modes is sometimes seen. This mode explains peculiarities in the behavior of the Ångström exponent, along with the fine mode fraction retrieved using the spectral deconvolution algorithm, observed in a March 2006 dust storm. For this study, 15 AERONET sites in northern Africa and on the Atlantic are examined to determine the frequency and properties of the intermediate mode. The mode is observed most frequently at Ilorin in Nigeria. It is also observed at Capo Verde and multiple sites located within the Sahel but much less frequently at sites in the northern Sahara and the Canary Islands. The presence of the intermediate mode coincides with increases in Ångström exponent, fine mode fraction, single‐scattering albedo, and to a lesser extent percent sphericity. The Ångström exponent decreases with increasing optical depth at most sites when the intermediate mode is present, but the fine mode fraction does not. Single‐scattering albedo does not steadily decrease with fine mode fraction when the intermediate mode is present, as it does in typical mixtures of dust and biomass‐burning aerosols. Continued investigation is needed to further define the intermediate mode's properties, determine why it differs from most Saharan dust, and identify its climate and health effects.

The role of cloud contamination, aerosol layer height and aerosol model in the assessment of the OMI near-UV retrievals over the ocean

“…OMAERUV particle size distributions are static, for example in the case of dust, the bi-lognormal size distribution is fixed and with a constant Ångström exponent (AE) of 0.6 based on the distributions reported by Duvobik et al (2002). While this assumption seems to work in most cases, it is known that dust AE can fluctuate (values ranging from −0.5 to 1.0 have been reported in dust, Kim et al, 2011) because of variability in the size distribution (Toledano et al, 2007;Eck et al, 2010) or the distribution may not be bi-lognormal (Gianelli et al, 2013). Radiative transfer simulations were carried out using the OMAERUV dust size distributions and varied the coarse-mode concentration such that the respective AE ranged between −0.5 and 0.6.…”