Mineral dust in Namibia: new research on emissions, transport and properties

While the most extensive and active sources of mineral dust are found at mid-latitudes (Sahara, East Asia and the Arabic peninsula), source areas at high latitudes both in northern and the southern Hemispheres, are gaining attention because of their distinct characteristics and impacts at the appropriate regional and semi-hemispheric scales.

Southern Africa is estimated to account for approximately 5% of the global annual emissions of mineral dust and the long-range transport of dust emitted from these regions are shown to head towards the South Atlantic, the southern Oceans, and across the subcontinent by both observations and modelling.

In particular, hundreds distinct point sources have been identified in Namibia, including the topographical lows of the Etosha pan alluvial basin, the dry lands (Kalahari Desert, gravel plains bordering the Namib Deserts), but mostly the numerous ephemeral riverbeds, pans, wetlands and possibly mines along the coastline. By deposition, this windblown dust could impact to the productivity of the waters offshore, but also the formation and the chemical composition of the fog and low marine clouds. Through the fog, the dust emitted has the potential of redistributing nutrients not only to the marine but also to the continental ecosystems. Likewise, the strong and almost omnipresent southerly trade winds driven by the temperature contrast between the cold Benguela current and warm and dry continental desert air masses also provide an opportunity to transport sediment influenced by the nutrient rich aerosols of the ocean further onto land to nourish the nearshore ecosystems. To date, the majority of dust emissions observations from this region have relied heavily on the improved ability of satellite platforms to optically isolate dust aerosols over the ocean surface and despite the consistent high winds from the south, have excluded the potential for dust emission transport processes towards the interior.

This presentation illustrates the new results of recent coordinated research exploring the emissions, the transport, and properties of net transported mineral dust from Namibia sources. These new results rely on model simulations at different spatial resolution, on the analysis of local and regional wind regimes, on field observations, and on laboratory-based experiments on airborne dust generated from natural soils. Our results demonstrate that the frequency of emission might be higher than expected by only easterly berg winds. We also suggest that the Namibian dust may be transported to Antartica and that its processing by marine biogenic emissions could be responsible for the seasonal increase in the dust iron solubility observed in the Austral fall.