## 4 Discussion
### Surface Chemical Composition  
Due to the heterogeneity of the mineral surfaces in atmospheric dust, the XPS technique allows a close approximation to study their atomic composition. Si and Al were detected in abundant to intermediate proportions (Si>>Al) (Table 7 and Figure 5). The binding energy values were compatible with SiO\({}_{2}\) and aluminosilicates, being consistent with the proportions of both quartz and clay minerals (Table 6) [46]. Additionally, although K and Mg were found in trace amounts, both showed a binding energy in agreement with muscovite and montmorillonite [47]. The low proportions of these elements would indicate that they belong to the internal crystalline lattice.  
C was also dominant in the dust surface, in contrast to the percentage of carbonates found by XRD (Inorganic C). The proportion of inorganic C (0.63%), estimated from calcite (CaCO\({}_{3}\)) and dolomite (MgCaCO\({}_{3}\)) quantities, was extremely low compared to total C (XPS) (15.67%). The presence of organic matter in the dust surface would justify the prevalence of organic C (96%) over inorganic C from carbonates (4%). Organic matter has a great affinity for adsorption on mineral surfaces such as aluminosilicates, quartz and iron oxides. In addition, the Si/Al ratio obtained by XPS would support this affinity for clay minerals [48].  
The binding energy of Fe corresponded with iron oxides (Fe\({}_{2}\)O\({}_{3}\)) [49] and, therefore, this would be corroborated by the presence of haematites. However, differences between total Fe (XPS) and haematites (XRD) were observed. The proportion of Fe (XRD) (2.8%) from haematites (Fe\({}_{2}\)O\({}_{3}\)) was lower than the total Fe (XPS) (4.4%), which also included free iron. In this case, free iron (63.64%) would predominate over haematites (36.36%) in the dust surface. Aggregates of iron oxides are often in association with aluminosilicate platelets, indicating the existence of iron films over mineral particles, owing to the affinity of their reactive surfaces [50].  
The presence of Ca and Na revealed important information. The binding energy of Ca might correspond to calcium phosphate and calcium carbonate [51], with the latter being consistent with calcite (Table 6). Similarly, Na and Cl had a binding energy in agreement with sodium chloride (NaCl) [46; 52]. Halite (NaCl) was not detected by XRD, but this is not surprising as it is not commonly found in North Africa. Thus, the origin of this compound might be due to the enrichment of mineral dust with sea salt particles [42].  
The presence of N and several minor elements is also noteworthy. N and P had binding energy values compatible with the presence of ammonium and phosphate, respectively [53]. The binding energy of S was in agreement with sulphates, namely gypsum, as previously detected by XRD (Table 6) [46]. The interaction of the mineral particles with anthropogenic compounds (NO\({}_{\text{X}}\) and SO\({}_{\text{X}}\)) might increase the amount of secondary aerosols during Saharan dust intrusions in the Canary Islands [54]. This process could also result in the neoformation of sulphates such as gypsum and alunite (KAl\({}_{3}\)(SO\({}_{4}\))\({}_{2}\)(OH)\({}_{6}\)) [17].  
Trace amounts of Zn were also detected in dust, showing a binding energy that might correspond to ZnO [55]. Although Zn has previously been detected during Saharan dust intrusions, it is mainly associated with road traffic and non-exhaust vehicle emissions [56].