Elemental ratios as tracers of the sources of mineral dust in north-eastern Sahara

Abstract

The objective of this study is to confirm the possibility of tracing sources of airborne mineral dust using elemental ratios. The region of study is the north-eastern part of the Sahara where dust compositional data were lacking; 272 mineral dust samples collected at 3 experimental stations of Egypt were analyzed by X-ray fluorescence. Based on the mineral dust concentration, 65% of the samples correspond to moderate (3–10 μg m⁻³), 22% to medium (10–25 μg m⁻³), and 13% to intense (> 25 μg m⁻³) dust events. The Ca/Al and (Mg + Ca)/Fe ratios were found to be strongly correlated and their ranges of variation to cover are more than one order of magnitude, which confirms their interest for the tracing of the dust sources. Using a combination of MODIS satellite observations, HYSPLIT back-trajectory analysis, and simulations with dust emission models, the large (> 6) and intermediate (2–6) Ca/Al ratios were shown to correspond to dust originating from saline playas (chotts) and calcareous surfaces, respectively. Conversely, the lowest (< 2) ratios correspond to dust emitted by siliceous sand seas (ergs). Therefore, a classification of the dust in 3 categories (erg-like, calcareous, and chott-like) based on Ca/Al is proposed. The events of moderate to medium intensity are dominated by chott-like and calcareous dust, which suggests that these sources are activated during erosion events of limited (local) extension. Conversely, the fact that the dust composition is more likely to be of the erg-like type during intense events underlines the dominant role played by sand dune fields during major erosion events.