Elemental composition, iron mineralogy and solubility of anthropogenic and natural mineral dust aerosols in Namibia: a case study analysis from the AEROCLO-sA campaign

Abstract. This paper presents the results of three weeks of aerosol sampling at the Henties Bay coastal site in Namibia during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign in August–September 2017. The campaign coincided with a transition period between two synoptic regimes and corresponded to a significant change in the aerosol composition measured at the site and in particular of that of mineral dust. During August, the dust was natural windblown from the southerly gravel plains with a composition consistent with that previously observed in Namibia. In September, the dust was fugitive from anthropogenic mining and possibly minor contribution of smelting emissions in northern Namibia or as far as the Copper Belt in Zambia, one of the regional hotspot of pollution.

Chemical analysis of filter samples highlights the difference in elemental composition, in particular heavy metals, such as As, Cu, Cd, Pb, and Zn, but also silicon, in the anthropogenic dust. The metal solubility of the natural dust was higher, including that of iron. In addition to the higher content of iron oxides and the larger size of particles in the anthropogenic dust, we found that the iron solubility, and, more in general, the metals’ solubility, correlated to the high concentrations of fluoride ion which are attributed to marine emissions from the Namibian shelf. These results highlight in a renewed manner the importance of ocean-atmosphere exchanges affecting both the atmospheric composition and the marine biogeochemistry in the Benguela region.