The chemical and mineralogical composition of southern African dust aerosols

Southern Africa (SAf) is a key region for dust emissions, characterised by a wide variety of natural and anthropogenic sources, but also a critical knowledge gap in the mineral dust budget of the Southern Hemisphere. Projected climate warming is expected to lead to an increase in mineral dust emissions, which are increasingly linked to human activity. Although the transport and deposition pathways of SAf dust suggest that it can directly affect the regional climate and nearby marine ecosystems through dust-aerosol interaction and indirectly through aerosol-cloud/ice interaction and nutrient deposition, the extent of this impact is highly uncertain due to significant uncertainties in atmospheric loads and climate-relevant properties.

This study provides the first comprehensive characterisation of the chemical and mineralogical composition of SAf dust aerosols. Aerosol samples were laboratory-generated using soils collected from key dust sources in southern Africa, including the Namib gravel plain, coastal ephemeral riverbeds, the Etosha salt pan, the Kalahari Desert, and anthropogenic sources such as agricultural soils from the Free State, savannah soils from the Kruger National Park, and a copper mine in Namibia.

A geographical distribution of the chemical and mineralogical properties of SAf dust was identified based on the elemental ratios Si/Al, (Ca + Mg)/Al, and K/Al. This is influenced by both the regional geology and rainfall distribution, which shows an increase in the Si/Al ratio and a decrease in the (Ca + Mg)/Al and K/Al ratios, in areas with higher rainfall inland compared to the arid coast, while the salt pans exhibit unique features with significantly higher (Ca+Mg)/Al and Si/Al ratios.

The SAf dust appears to be more enriched in Ca, Mg, and K than other dust sources in the Southern Hemisphere and northern African dust. Although Fe, a key micronutrient, occurs at similar levels in dust from both hemispheres, SAf dust contains more P, highlighting its potential significance in biogeochemical cycling. Despite limited mineralogical observations in the Southern Hemisphere, our results indicate that SAf dust contains more feldspar minerals than northern African dust, and may strongly influence the load of ice-nucleating particles over the Southern Ocean and, in turn, the regional radiative budget.