A total of 40 soils were selected for experiments (Fig. 1 and Table S1) to represent major natural dust-source areas across southern Africa, as reported in the literature (Ginoux et al., 2012; Prospero et al., 2002; Vickery et al., 2013) and subject to sample availability. A particular focus is on the Namib Desert, with its extensive ephemeral riverbed system, which is a recognised dust source area (Ginoux et al., 2012; Vickery et al., 2013; von Holdt et al., 2017). To characterise sources across the Namib Desert, we used two soil samples from the Central Namib Gravel Plain and the Kuiseb riverbed, collected at the Gobabeb Namib Research Centre. In addition, we collected soil samples from the terminal reaches of the Huab, Kuiseb, and Omaruru riverbeds along 1 km transects at 100 m intervals, resulting in 12, 8, and 13 points, respectively. A sample from the Etosha Pan was included in our study as another significant dust source (Buch and Rose, 1996; Vickery and Eckardt, 2021), though it is more variable and intermittent than the Namib Desert (Bryant, 2003; Bryant et al., 2007; Haustein et al., 2015). Dust sources in the Kalahari Desert tend to emit less than those in the Namib Desert. We included one sample from the southwestern Kalahari in Botswana, which is a significant emerging source (Bhattachan et al., 2012, 2013; Vickery et al., 2013), as well as two samples from areas of the Kalahari Desert north and south of the Okavango Delta in northwestern Botswana, which is an important but more local source (Bullard, 2004; Vickery et al., 2013).

This study also provides the first characterisation of anthropogenic dust sources (four source-soil samples in total, Fig. 1 and Table S1) that are emerging in southern Africa as a result of anthropogenic pressure and climate change (Eckardt et al., 2020; Ginoux et al., 2012; Vickery et al., 2013; Vos et al., 2025). These include an agricultural soil in the Free State, identified as a key area of agricultural dust emissions (Eckardt et al., 2020; Trisos et al., 2022; Vickery et al., 2013). In addition, two savannah soils were collected at Skukuza in the Kruger National Park, South Africa, from an area periodically subjected to prescribed fires, representing bare soils that can generate dust emissions after vegetation is removed by fire, a condition frequent in northeastern and eastern South Africa (Strydom and Savage, 2016). Finally, we used one sample from an abandoned copper mine, Hope Mine in Namibia, to explore the composition of mineral dust from mining activities, which are widespread across Southern Africa and have also been identified as emerging sources of mineral dust (Vos et al., 2025)

Previous studies have documented the primary sources of mineral dust in Southern Africa (Eckardt et al., 2020; Ginoux et al., 2012; Vickery et al., 2013; Vos et al., 2025), which are summarised herein. The Namib Desert extends more than 2000 km from Angola to South Africa, over a ∼150-km-wide area along the Atlantic Ocean coast (Fig. 1). Inland, this coastal desert slopes ∼0.5° from sea level to the west to approximately 1000 ma.s.l. to the east, where it is bounded by the Great Escarpment. This escarpment is a major topographic step of several 100s-m-high that separates the coastal areas surrounding Southern Africa from a high plateau, the South African Plateau, in the centre of the continent, with an average elevation of ∼1700m. On the Atlantic side, the aridity of the Namib Desert is due to a combination of the southern subtropical high-pressure belt, where cold and dry air masses of the Hadley and Ferrel atmospheric convection cells subside to the ground, and the cold Benguela current, which causes upwelling of cold water along the coast and limits sea surface evaporation (Diester-Haass et al., 1988; Mendelsohn et al., 2022; Stuut et al., 2004). Consequently, the desert experiences irregular precipitation, receiving less than 100 mm of rain per year, with the 100 mm isohyet running parallel to the Great Escarpment. Near the coast, frequent fogs supply moisture, supporting terrestrial ecosystems (Desmet, 2007; Desmet and Cowling, 1999; Olivier, 1995). Local geomorphology is shaped by aeolian, fluvial, and marine erosion, sediment transport, and sedimentation, with mechanical weathering prevailing over chemical weathering (Scholz, 1972; Watson and Lemon, 1985).

On the Great Escarpment, a series of watersheds feed ephemeral rivers that flow westward across the desert toward the ocean. The Kuiseb River divides the southern Namib Sand Sea from the Central Namib Desert Gravel Plain (hereafter referred to as Namib GP, Fig. 1). Silty deposits are widespread in this gravel plain, which extends from the Skeleton Coast erg, a dune belt running along the coastline, up the edge of the Great Escarpment. These loess deposits are supplied by ephemeral rivers and by wind-blown dust from the western Kalahari, located on the South African Plateau to the east of the Namib Desert (Eitel et al., 2001; Scholz, 1972; Watson and Lemon, 1985). They cover the Protero-Paleozoic crystalline basement, which is composed of igneous and metamorphic rocks of the Damara Supergroup, forming inselberg-like topography characterised by isolated rocky outcrops (Watson and Lemon, 1985). These loess deposits are a significant source of mineral dust, with numerous point sources on the Namib GP, including riverbeds and coastal pans (Ginoux et al., 2012; Vickery et al., 2013). In particular, the Huab, Kuiseb, and Omaruru rivers were recognised as the three dustiest river systems within the Namib Desert (von Holdt et al., 2017).

In the centre of the continent, the South African Plateau receives over 300 mm of rainfall per year and supplies tributaries of the Orange River flowing southwards, as well as ephemeral rivers that drain eastwards and northwards into large endorheic depressions of the Kalahari Basin (Watson and Lemon, 1985). In these depressions, the Makgadikgadi Pan in northeastern Botswana and Etosha Pan in northeastern Namibia are two of the largest paleolakes in Africa. Their highly dynamic surfaces are characterised by a crust of evaporite minerals and clays, which serve as major sources of dust globally (Buch and Rose, 1996; Vickery and Eckardt, 2021), although emissions are intermittent due to precipitation and complex salt-basin dynamics (Bryant, 2003; Bryant et al., 2007; Haustein et al., 2015). In addition, upstream of the Makgadikgadi Pan, the fluvial Okavango Delta in northwestern Botswana is also considered an important local dust source (Bullard, 2004; Vickery et al., 2013).

The Kalahari Basin is covered by the sediments of the Kalahari Desert that, at the surface, primarily consist of quartz-rich sands, forming an extensive, continuous sand sea, the largest on Earth. The northern and central parts of the Kalahari contain pure quartzose sand and clay minerals, indicating a strong weathering regime under a humid past climate (Garzanti et al., 2022). However, sediment compositions vary notably at the western and eastern edges of the desert, where rivers flow across different crystalline basement lithologies in Southern Africa. In the western Kalahari, dune fields in southeastern Namibia are distinguished by their enrichment in feldspar, which is thought to be the result of the influence of the Damara Supergroup rocks (Garzanti et al., 2022). The Kalahari Desert is also characterised by duricrusts – chemical precipitates of carbonates or silica, which are commonly found in pans and river valleys (Garzanti et al., 2022; Nash and McLaren, 2003). In addition to the large endorheic watersheds in the north, the southern Kalahari is an important dust source area due to numerous active pans in the southwestern Kalahari across Namibia and South Africa (Vickery and Eckardt, 2013). Furthermore, interdunes in the southern Kalahari, enriched with fine sediments, are emerging as a dust source as savannah-covered dune fields are being remobilised by overgrazing and drought (Bhattachan et al., 2012, 2013).

Several areas in South Africa have also been identified as sources of dust. Mineral dust emissions are increasingly affected by human activities, especially in the Free State (Eckardt et al., 2020; Trisos et al., 2022; Vickery et al., 2013), characterised by intensive agriculture, and along the west coast, with vulnerable semi-arid shrubland environments and extensive mining (Vos et al., 2025). Furthermore, frequent fires in the grasslands and savannahs of northeastern and eastern South Africa (Strydom and Savage, 2016) contribute to vegetation loss, land degradation, and the emergence of new mineral dust sources.

The “Générateur d'Aérosol Minéral En Laboratoire” (GAMEL, Lafon et al., 2014) was used to generate dust aerosols from parent soil samples. The GAMEL apparatus consists of three main components: a generation system, a PM10 cyclone for size selection of particles with aerodynamic diameters less than 10 µm, and a collection system for dust aerosols on filters connected to a pump via tubing.

For the soil samples collected at the terminal parts of the Huab, Kuiseb, and Omaruru riverbeds, dust aerosols were generated using the Multiphase Atmospheric Experimental Simulation Chamber (CESAM, Wang et al., 2011). This facility has been widely used to characterise the chemical and physical properties of natural mineral dust aerosols (Baldo et al., 2023, 2020; Caponi et al., 2017; Di Biagio et al., 2014, 2019, 2017). An external dust-generation system is attached to the 4.2 m3 multi-instrumented environmental stainless-steel reactor. Stainless-steel tubes connect the chamber body to custom-made samplers, which are linked to a pump that extracts dust aerosol from the chamber onto filters.

We compiled an integrated dataset from this study and the existing literature on the chemical and mineralogical composition of mineral dust aerosols in Southern Africa, Australia, and Southern America. These three source regions are important for the mineral dust budget of the Southern Hemisphere. Additionally, we included Northern Africa, the main contributor to global mineral dust emissions (Kok et al., 2021). Table S2 in the Supplement provides key references for each dust source region, including details of the study area, aerosol collection method, particle size, available data (i.e. elemental and/or mineralogical composition), and the analytical technique used in each study.

To the best of our knowledge, literature on the mineralogical composition of SAf dust and more broadly on mineral dust in the Southern Hemisphere is limited. Consequently, we could only integrate our data with laboratory observations from Di Biagio et al. (2017). However, numerous studies report on the mineralogical composition of dust from Northern Africa (e.g. Di Biagio et al., 2017; Engelbrecht et al., 2016; Formenti et al., 2014, 2008; Klaver et al., 2011).

Available data on the chemical composition of SAf dust include studies on the Namib GP (Di Biagio et al., 2017; Eltayeb et al., 1993; Qu, 2016), the western Kalahari in central Namibia (hereafter referred to as Kalahari NAM, Qu, 2016), the Etosha and Makgadikgadi Pans (Engelbrecht et al., 2016), and the Okavango Delta (Humphries et al., 2014). In Southern America, most dust observations come from Patagonia (Demasy et al., 2024; Di Biagio et al., 2017; Gaiero et al., 2007, 2003; Qu, 2016), although there is also some data from the Atacama Desert in northern Chile (Di Biagio et al., 2017; Engelbrecht et al., 2016; Rojas et al., 1990), and from a few remote locations in Argentina, Bolivia, and Brazil (Rojas et al., 1990). In Australia, most observations originate from the arid interior of the continent, with main dust source areas located within the Lake Eyre Basin (Box et al., 2010; Di Biagio et al., 2017; Engelbrecht et al., 2016; Karlson et al., 2014; Moreno et al., 2008; Radhi et al., 2010a, b). These studies focus on ground-based observations of dust aerosols or laboratory-generated aerosols derived from parent soils (Table S2).

To represent Northern African dust, we selected studies on laboratory-generated aerosols or aerosols collected in source areas. These include laboratory measurements by Di Biagio et al. (2017) and Engelbrecht et al. (2016), as well as ground based and airborne observations from key aircraft campaigns: the African Monsoon Multidisciplinary Analysis (AMMA, Rajot et al., 2008; Reeves et al., 2010); the Dust And Biomass–burning Experiment (DABEX, Haywood et al., 2008); the Dust Outflow and Deposition to the Oceans (DODO, McConnell et al., 2008); the FENNEC – Saharan Climate System (Washington et al., 2012); and the Geostationary Earth Radiation Budget Intercomparisons of Longwave and Short-wave radiation (GERBLIS, Haywood et al., 2011).

Figure 2 shows the percentage of the major crustal elements found in different types of SAf dust grouped by source region. Note that the Kalahari samples collected across Botswana have been grouped into the Kalahari BOT dataset. The most abundant element is Si, which varies significantly, ranging from around 35 wt % in the Kalahari BOT samples to 18 wt % at Etosha Pan. Al content is highest in the riverbed dust, reaching up to 13 wt %. The riverbed dust also has the highest Fe content (up to 9 wt %) along with dust from savannah soils at Skukuza, whereas the Etosha Pan dust contains less than 1 wt % of Al and Fe. On the other hand, mineral dust from the Etosha Pan has the highest concentrations of Ca and Mg, each around 17 wt %, and about 4 wt % Na. In the riverbed, Namib GP, and Hope Mine samples, the levels of Ca, Mg, and Na are lower but still significant, ranging from 4.5 wt %–17 wt % overall. Samples from the Free State, Kalahari BOT, and Skukuza show the lowest total contents, ranging from 0.6 wt % to 3.7 wt %.

The riverbed dust contains the highest K concentrations, ranging from 2.4 wt %–3.6 wt %, while in the Kalahari BOT samples, K varies from 0–3 wt %, with the highest level observed in the sample from south of the Okavango Delta.

Hope Mine dust is notably rich in Cu, approximately 8 wt % (not shown), and also contains a high level of P at 0.3 wt %. P varies from 0.02 wt %–0.2 wt % in the other dust samples, but was not detected in the Etosha Pan dust. The riverbed dust is slightly richer in Ti (up to 0.9 wt %) compared to the other dust samples, while Mn shows no significant variability across the samples. Neither of these elements was detected in the Etosha Pan dust.

The main minerals in SAf dust aerosols are quartz and clays, while carbonates and feldspars are only present in some samples. The limited aerosol mass results in low diffractogram intensity relative to the baseline, which complicates the identification of minor mineral species. For instance, Fe oxide minerals, gypsum (CaSO4⋅2H2O), and halite (NaCl) have been identified in a few samples, mostly in small quantities. The fact that these minor phases were not detected by XRD can have two reasons: (1) the phases contain crystalline defects or are even amorphous (like certain iron oxides), (2) the very low number of crystals present reduces the likelihood of planes oriented in a way that allows their diffraction. The detection limit for X-ray diffraction is generally estimated to be a few per cent by mass. Therefore, the presence or absence of these minerals cannot be definitively confirmed, and further characterisation using alternative techniques is recommended. The same consideration applies to apatite (Ca5(PO4)3(F,Cl,OH)), which could be present given the relatively good correlation between Ca and P (Figs. S3–S5 in the Supplement), although it was not detected by XRD.

Figure 3 reports key mineral components of SAf dust aerosols. Dust aerosol samples generated from sandy soils collected across the Kalahari BOT and in the Free State have the highest quartz content, ranging from 26 wt %–37 wt %. The remaining samples contain less than 15 wt % quartz, while quartz was not detected in mineral dust from the Etosha Pan, where carbonates (as the total contribution of calcite, CaCO3, and dolomite, CaMg(CO3)2) dominate at 78 wt %. A carbonate content of up to 20 wt % was also observed in mineral dust from arid coastal regions, including the Namib GP and ephemeral riverbeds. The feldspar content, calculated as the sum of albite (Na(AlSi3O8)) and microcline (K(AlSi3O8)), is highest in dust from savannah soils (over 30 wt %), followed by riverbed dust (10–30 wt %), the Kalahari BOT sample from the south of the Okavango Delta (20 wt %), Namib GP dust (over 10 wt %), Free State dust (2 wt %), and not detected in other samples.

Dust aerosols are mostly composed of clay minerals, ranging from 20 wt % (Etosha Pan) to 50-90 wt % (other SAf dust). The clay minerals observed include illite (K0.6-0.85(Al,Mg)2(Si,Al)4O10(OH)2), kaolinite (Al2(Si2O5)(OH)4), Mg-rich clays (i.e. sepiolite, Mg4(Si6O15)(OH)2⋅6H2O, and palygorskite, (Mg,Al)2Si4O10(OH)⋅4H2O), chlorite ((Mg,Fe)3(Si,Al)4O10(OH)2⋅(Mg,Fe)3(OH)6(Fe,Mg)2Al4O2(SiO4)2(OH)4), chrysocolla (Cu4,H4Si4O10(OH)8), and montmorillonite ((Al,Mg)8(Si4O10)4(OH)8⋅12H2O), all of which reveal specific regional features.

The abundance of kaolinite indicates highly weathered soils and tends to follow rainfall patterns (Fig. 4). Arid areas close to the Namibian coast and the Kalahari BOT sample from the southwestern Kalahari have the lowest kaolinite content, while illite-like clays (represented by the sum of illite and muscovite, KAl2(AlSi3O10)(OH)2) dominate due to low chemical weathering. Kaolinite content increases from south to north in the Kalahari BOT samples, reaching over 40 wt % of the total clay in the sample collected north of the Okavango Delta, an area that receives the highest levels of precipitation. Agricultural soils in the Free State, inland South Africa, and savannah soils in eastern South Africa receive more precipitation than coastal Namibia, with kaolinite accounting for 20 wt %–50 wt % of the total clay fraction in dust. Lastly, kaolinite accounts for over 30 wt % of the clay fraction in the mineral dust from the copper mine, which also contains a significant amount of chrysocolla, a Cu-rich clay, and around 8 % Fe oxide minerals.

Mg-rich clays such as sepiolite and palygorskite can form in arid and semi-arid environments with alkaline soils, high Mg activity, and evaporation, like in lacustrine and peri-marine zones (Singer, 2002). They can also form through meteoric or hydrothermal alteration of basic rocks, or as a result of pedogenic processes in soils, such as the formation of calcareous crusts (Singer, 2002). The presence of sepiolite and palygorskite is documented in sediments of Etosha Pan (Buch and Rose, 1996) and in sediments of the Central Namib Gravel Plain, including riverbeds (Heine and Völkel, 2010). Here, we observed that dust from Etosha Pan is characterised by a high Mg-rich clay content, accounting for around 50 % of its total clay composition. Mg-rich clays are also present in smaller amounts in dust from riverbeds, Namib GP, and savannah soils (Fig. 3). Lastly, samples from the Namib GP and the Kuiseb riverbed contain around 10 wt % chlorite and 1 wt % montmorillonite (not shown).

These results align with potential mineral associations indicated by the correlation between crustal elements (Figs. S3–S5). A strong correlation was observed between Al and Si, as well as between Al and Si with Fe, K, Ti, P, and Mn (r>0.7), suggesting an association with aluminosilicate minerals, such as feldspars, clays, and Fe–Ti oxides. The concentrations of Ca and Mg are strongly correlated (r≈0.8), while their correlations with Fe, Si, and Al are weaker (r≤0.4). The weak correlation of Ca and Mg with Al suggests that these elements can be associated with different minerals depending on the source. For instance, they are mainly associated with calcite and palygorskite in Namib GP and riverbed samples, with dolomite and sepiolite in Etosha Pan, and with other aluminosilicate minerals in the remaining dust samples. This indicates that (Ca+Mg)/Al is a good discriminating indicator of SAf dust sources. Additionally, Na shows a strong correlation with Mg (r=0.8) and a moderate correlation with Ca (r=0.6), indicating a common source and/or similar geochemical processes. This may suggest that some Ca and Mg derive from minor evaporitic salts.

The abundance of P is often associated with phosphate minerals such as apatite. Our findings show that P is moderately correlated with Ca (r≈0.5), thus suggesting the presence of apatite. However, its strong correlations with Si, Al, and Fe suggest that P is probably associated with clay minerals and Fe and Al oxides.

The elemental ratios, primarily Si/Al, (Ca+Mg)/Al, and K/Al, are critical tracers of mineral dust that can be used to distinguish source areas across Southern Africa. Significant associations were identified and are reported in Fig. S6 in the Supplement based on comparisons of these chemical ratios with the dust mineralogy. The K/Al ratio shows a direct correlation with K-feldspar (microcline) and an inverse correlation with kaolinite, a mineral produced by the chemical weathering of aluminosilicate minerals, including feldspars. The Si/Al ratio increases with quartz and decreases with increasing primary aluminosilicate mineral contents (feldspar and muscovite). The (Ca+Mg)/Al ratio is directly correlated with carbonate mineral content and Mg-rich clay content, which formed in similar geochemical environments.

Figure 5 illustrates the variability in the chemical composition of mineral dust across Southern Africa, presenting the Si/Al, (Ca+Mg)/Al, K/Al, and Fe/Al ratios from this study and laboratory-generated dust aerosols from the literature (Di Biagio et al., 2017; Engelbrecht et al., 2016; Qu, 2016). The data were grouped into spatial-environmental categories based on their geographic location and geomorphological environment (Table S3 in the Supplement). Additionally, we compared the elemental ratios of laboratory-generated mineral dust aerosols with atmospheric observations over coastal Namibia, which is representative of dust mixed aerosols in a marine coastal environment (Desboeufs et al., 2024; Formenti et al., 2025; Klopper et al., 2020), and the South African Highveld, representative of dust mixed aerosols in an inland urban-industrial environment (Baldo et al., 2025). Both areas are influenced by a range of natural and anthropogenic emissions.

Mineral dust found in the salt pans in Namibia and Botswana exhibits highly variable compositions, but can be distinguished by its elevated (Ca+Mg)/Al ratios (from 3–43) and Si/Al ratios (from 5–70), reflecting the predominance of carbonates and Mg-rich clays. The K/Al ratios (from 0.9–3.6) are also the highest reported in the literature. However, K was not detected in the Etosha Pan sample analysed in this study, indicating variability in the composition of dynamic pan surfaces.

In the case of the Namib GP, the Si/Al ratio is restricted to between 2.8 and 4.8, while the (Ca+Mg)/Al ratio varies between 0.5 and 7.7. In the areas of the Namib GP that are closer to the west coast, the mineral dust (Ca+Mg)/Al ratio is usually greater than 2.5. However, in the eastern part of the Namib GP and in the Kalahari NAM, this ratio falls below 2.5, while the Si/Al ratio decreases to 1.7–3.2. Aerosol samples collected near areas characterised by carbonate-rich rocks and sediments, such as the Etosha Pan and the Otavi Mountain Land in northern Namibia, show higher ratios. The (Ca+Mg)/Al and Si/Al ratios measured in mineral dust from ephemeral riverbeds are closer to those observed in the eastern Namib GP and Kalahari NAM. However, these riverbeds fall within the lower end of the K/Al ratio range observed in these areas: 0.3–0.7.

Mineral dust from the Kalahari BOT, agricultural soils in the Free State, and savannah soils at Skukuza differ from other SAf dust due to their low (Ca+Mg)/Al ratios (0.1–0.4). This is consistent with the absence of carbonate minerals, as indicated by XRD analysis. These samples also exhibit relatively higher Si/Al ratios of 3.8–4.9, reflecting an increase in quartz and a decrease in primary aluminosilicate minerals compared to the Namibian dust (Fig. 3). Overall, mineral dust originating from inland eastern sources (the Botswana section of the Kalahari Desert, the Free State, and Skukuza) has lower K/Al values than dust from Namibia (below 0.4). This is consistent with the observed increase in kaolinite content, which is attributed to increased chemical weathering in the parent sediments. Mineral dust from the copper mine has similar (Ca+Mg)/Al, Si/Al, and K/Al ratios to those of the inland eastern sources, but can be readily distinguished by its high Cu and P contents (8 wt % and 0.3 wt %, respectively).

These three crustal element ratios alone are insufficient to distinguish inland eastern sources. However, mineral dust from Skukuza and the Free State can be distinguished by their high Fe content (Fig. 2), as reflected in elevated Fe/Al ratios of 0.8–1.2 (Fig. 5e–f). Additionally, differences in kaolinite content help to distinguish between regions from north to south within the Kalahari BOT area (Figs. 3 and 4).

This study analysed only a limited number of mineral dust samples from anthropogenic sources, so it may not reflect the variability across different anthropogenic dust source types. However, our results show distinct features, such as high Cu and P in mining dust and high Fe in savannah and agricultural dust, which may have important implications for marine biogeochemical cycles in the present and especially in the long term, as these sources are emerging due to climate change and anthropogenic pressure.

Figure 4 shows the spatial distribution of kaolinite, (Ca+Mg)/Al, Si/Al, and K/Al ratios overlaid on a mean annual precipitation map of Southern Africa. This map is based on the annual average precipitation data for 2015–2024 from the Climate Hazards Center Infrared Precipitation with Stations dataset version 3 (CHIRPS3, 2025). Additional details on the rainfall source data are provided in Sect. S2. Overall, mineral dust sources in Namibia exposed to more arid environments tend to have lower kaolinite and Si/Al ratios, but higher (Mg+Ca)/Al and K/Al ratios compared to sources farther inland and to the east, which receive more precipitation. These results are further corroborated by a Principal Component Analysis (PCA) of key elemental ratios, conducted using the prcomp() function from core CRAN R software on our integrated dataset. This dataset included the elemental ratios (Ca+Mg)/Al, K/Al, Si/Al, and Fe/Al measured in this study, as well as data from the literature. The PCA results are discussed in Sect. S3 in the Supplement. The Etosha and Makgadikgadi Pans stand out for their very distinctive compositions. Variations between source areas are also affected by differences in geological and geomorphological settings.

Ground-based observations of SAf aerosols are influenced by sources other than mineral dust, such as marine emissions along the Namibian coast (Desboeufs et al., 2024; Formenti et al., 2025; Klopper et al., 2020). To remove the marine aerosol contribution from data reported at the Henties Bay site (Desboeufs et al., 2024; Formenti et al., 2025; Klopper et al., 2020), the concentrations of K, Ca, and Mg were apportioned into sea salt (ss) and non-sea salt (nss) fractions using the nominal mass ratio to Na+ in seawater, as in Klopper et al. (2020). Thus, the ratios shown in Fig. 5 for Henties Bay are the nss-element fractions relative to Al. No significant influence of marine emissions was observed in aerosols collected from the Southern African Highveld (Baldo et al., 2025). The chemical composition of aerosols from both Henties Bay and the South African Highveld remains affected by anthropogenic emissions (Baldo et al., 2025; Desboeufs et al., 2024; Formenti et al., 2025; Klopper et al., 2020).

The (Ca+Mg)/Al and Si/Al ratios from ground-based observations are consistent with those of mineral dust aerosols analysed in this study and previous research (Table S3). Atmospheric aerosols over the Namibian coast consist of 20 % mineral dust, with (Ca+Mg)/Al ratios ranging from 0–5 and Si/Al ratios between 0.5 and 6.8 (Desboeufs et al., 2024; Formenti et al., 2025; Klopper et al., 2020). This indicates that a variety of mineral dust sources, including the eastern Namib GP, riverbeds, and more inland areas, influence the Namibian coastal environment. In contrast, mineral dust accounts for around 60 % of aerosol mass over the South African Highveld, with (Ca+Mg)/Al and Si/Al ratios of 0.3-1.5 and 1.5-2.7, respectively (Baldo et al., 2025), values that are closer to those of mineral dust from the eastern Namib GP and the Kalahari NAM areas.

Other crustal element ratios reported for atmospheric aerosols exhibit greater variability than those of mineral dust sources, likely due to anthropogenic influences. The K/Al ratio, in particular, is affected by biomass burning emissions (Baldo et al., 2025; Desboeufs et al., 2024; Formenti et al., 2025; Klopper et al., 2020), varying up to 1 in atmospheric aerosols over the Namibian coast and up to 1.8 in aerosols over the Highveld. The Fe/Al ratio in atmospheric aerosols over the Namibian coast is consistent with that observed in dust source areas. However, additional sources appear to influence the Fe/Al ratios over the South African Highveld, where the ratios are lower (Fig. 5e and f).

Enrichment factors (EFs) were used to analyse differences in elemental enrichment across key mineral dust source regions in the Southern Hemisphere, including Southern Africa, Australia, and Southern America, compared with Northern Africa, the primary global dust source. The reference source data used to compile the integrated dataset of the chemical composition and mineralogy of mineral dust are presented in Table S2.

The availability of data varies unevenly across global source regions. Northern African (NAf) dust accounts for the highest proportion (64 %), followed by Southern American (SAm) dust (20 %), SAf dust (9 %), and Australian (Aus) dust (7 %). This reflects the limited availability of mineral dust observations in the Southern Hemisphere. The availability of measured elements in Aus dust is particularly limited. Although Fe, Ti, and Mn were consistently reported in 85 % of the compiled measurements, other crustal elements were available for only 16 %–37 %. For SAm, SAf, and NAf dust, crustal element coverage was extensive, ranging from 85 %–100 %, except for Mn, which was not quantified in most of the selected studies on NAf dust.

Figure 6 shows the EF values by global dust source region, while Table S4 in the Supplement summarises the relevant statistics. The most abundant element in the Earth's crust, Si, has EF interquartile ranges (IQRs) close to the crustal ratio across all regions, except for NAf dust, which is slightly depleted (Table S4). All dust sources are depleted in Na, ranging from slightly depleted in SAm dust (EF median 0.8) to strongly depleted in NAf dust (EF median 0.02).SAf dust is enriched in Ca, Mg and K, which are key tracers of carbonate (and Mg-rich clays) and K-feldspar minerals. EF median values in Aus and NAf dust vary from slight depletion to near-crustal levels. In contrast, SAf and SAm dust show slight to moderate enrichment. SAf dust exhibits the broadest range and the highest maximum values of 96, 110, and 12, respectively.

SAf dust is from slightly to moderately enriched in the essential micronutrients Fe, P, and Mn. Median EF values range from 1.6–2, with IQRs spanning from 0.95–3.3. However, SAm dust shows the highest enrichment in P (median EF: 5.8; IQR: 4.9–7), whereas Aus dust is slightly enriched in Fe (median EF: 1.8; IQR: 1.4–2.2) and Mn (median EF: 1.7; IQR: 1.2–3.2), and NAf dust is only slightly enriched in Mn (median EF: 1.7; IQR: 1.6–2.5).

SAf dust could be important for atmospheric heterogeneous reactions. In addition to being rich in Ca, SAf dust exhibits a wide range of EF values for Ti, with slight to moderate enrichment (median EF: 1.5; IQR: 1.4–2.9). However, SAm dust is the most enriched in Ti (median EF: 4.1; IQR: 3.4–4.9). NAf dust also shows moderate enrichment (median EF: 1.9; IQR: 1.6–2.2).

Figure 7 shows the variability of key mineral components in dust particles, including quartz, clays, carbonates, and feldspars, in global dust sources. The integrated mineralogical dataset is still dominated by NAf dust observations; SAf dust accounts for about 22 % of the total, and only one observation of Aus dust and two of SAm dust are included. Due to the limited data on Aus and SAm dust, a comprehensive comparison is not possible. Table S5 in the Supplement provides the summary statistics.

Aus and SAm dust sources tend to have higher quartz content than SAf and NAf dust, but the very limited data available do not allow for comparisons that are more meaningful. Notably, there are distinct differences between SAf and NAf dust. SAf dust shows significant enrichment in feldspars, with a median of around 16 % and a broad IQR of 2 %–26 %, whereas Naf dust has a median feldspar content of 0.6 % and an IQR of 0.4 %–1 %. Conversely, NAf dust exhibits the highest clay content, with a median of 87 % and an IQR of 84 %–91 %, compared to SAf dust, which has a median of 60 % and an IQR of 67 %–70 %. SAf dust is also more enriched in carbonates (median: 4; IQR: 0 %–4 %) than NAf dust (median: 1; IQR: 0.3–1.9). These findings are consistent with the enrichment of K, Ca, and Mg in SAf dust relative to their depletion in NAf dust (Fig. 6).