Rapid changes of dust geochemistry in the Saharan Air Layer linked to sources and meteorology

摘要

Based at Izafia Observatory (-2400 m a.s.l. in Tenerife), we performed 1-h resolution measurements of elemental composition of dust in the Saharan Air Layer (SAL) and studied the variability of the ratios of these elements to aluminium (elemental ratios). In a period (-1 week) of continuous dust presence (50-200 pg/m3), we observed rapid variations of dust composition; some elemental ratios changed by a factor 2 in a few (5-8) hours. The lowest variability (Normalized Variability Range, %) was found for Si/A1 (9%) and Fe/A1 (9%), followed by the ratios of K, Ti, Mg, Mn, Ca and Sr to Al (20-80%), and the highest for S/Al, Na/Al and Cl/A1 (110-160%) and a number of trace metals (Cr, Cu, Ni, Zn, Zr) and Br (>200%). This variability was induced by the alternating impacts of three of the large North African dust sources: NE Algeria (rich in evaporite minerals bearing Ca, S, Sr, K and Mg and in illite mineral), Western Sahara to Bechar region (containing Na, S and Cl rich Yermosol soils) and SW Sahara Western Sahel (rich in illite and hematite). We traced the variability in large-scale meteorology using the so-called North African Dipole Intensity (NAFDI: strength of the subtropical Saharan high -Morocco-to the monsoon tropical low -Nigeria-). The mobilization of dust from the different sources was associated with westward propagating Harmattan pulses linked to the change of phase of NAFDI (- to +), the associated westward shifts of the Saharan Heat Low and convective monsoon inflow. We found a correlation between dust composition in the SAL and NAFDI: moderate NAFDI values (0 to +2.5) were associated with Ca, K, Na, Mg and S rich dust linked to dust sources in NE Algeria, whereas higher NAFDI values (+2.5 to +4) were linked to Fe rich dust (Ca, Na and S depleted) linked to dust sources in SW Sahara - Western Sahel. The results of this study also show that some trace elements (Br, Cr, Ni, Zn and Zr) are influenced by industrial emissions into North Africa.