Aerosol properties of mineral dust and its mixtures in a regional background of north-central Iberian Peninsula

Abstract

To broaden the knowledge about desert dust (DD) aerosols in western Mediterranean Basin, their fingerprints on optical and microphysical properties are analyzed during DD episodes in the north-central plateau of the Iberian Peninsula between 2003 and 2014. Aerosol columnar properties obtained from the AErosol RObotic NETwork (AERONET), such as aerosol optical depth (AOD), Ångström exponent (AE), volume particle size distribution, volume concentration (VC), sphericity, single scattering albedo, among others, are analyzed in order to provide a general characterization, being some of them compared to particle mass surface concentrations PM10, PM2.5, and their ratio, data obtained from EMEP network. The mean intensity of DD episodes exhibits: AOD440nm = 0.27±0.12, PM10 = 24±18 μg/m3, AE=0.94±0.40 and PM2.5/PM10=0.54±0.16. The AOD and PM10 annual cycles show maximum intensity in March and summer and minima in winter. A customized threshold of AE=1 distinguishes two types of dusty days, those with a prevailing desert character and those of mixed type, which is corroborated by sphericity values. Three well established intervals are obtained with the fine mode volume fraction (VCF/VCT). Coarse-mode-dominated cases (VCF/VCT ≤ 0.2) present a mineral dust character: e.g., particle maximum concentration about 2 μm, non-sphericity, stronger absorption power at shorter wavelengths, among others. The relevance of the fine mode is noticeable in mixtures with a predominance of particles about 0.2-0.3 μm radii. Conditions characterized by 0.2 < VCF/VCT < 0.45 and VCF/VCT ≥ 0.45 present a larger variability in all investigated aerosol properties. Relationships between AOD and columnar particle volume concentration give volume extinction efficiencies between 1.7 and 3.7 μm2/μm3 depending on VCF/VCT. Aerosol scale height is obtained from relationships between surface and columnar concentrations displaying very large values up to 10 km. The uncertainty associated with the transformation between AOD and PM10 can be partially reduced when the aerosol microphysical properties are known.