Fractional solubility of iron in mineral dust aerosols over coastal Namibia: a link with marine biogenic emissions?

Mineral dust is the largest contributor to elemental iron in the atmosphere, and, by deposition, to the oceans, where elemental iron is the main limiting nutrient. Southern Africa is an important source at the regional scale, and for the Southern Ocean, however limited knowledge is currently available about the fractional solubility of iron from those sources, as well as on the atmospheric processes conditioning its dissolution during deposition. This paper presents the first investigation of the solubility of iron in mineral dust aerosols from 176 filter samples collected at the Henties Bay Aerosol Observatory (HBAO), in Namibia, from April to December 2017. During the study period, 10 intense dust events occurred. Elemental iron reached peak concentrations as high as 1.5 µg m^-3, significantly higher than background levels. These events are attributed to wind erosion of natural soils from the surrounding gravel plains of the Namib desert. The composition of the sampled dust is found to be overall similar to that of aerosols from northern Africa, but characterised by persistent and high concentrations of fluorine, which are attributed to fugi-tive dust from mining activities and soil labouring for construction. The fractional solubility of Fe (%SFe) for both the identified dust episodes and background conditions ranged between 1.3 to 20 %, in the range of values previously observed in the remote Southern Ocean. Even in background conditions, the iron fractional solubility was correlated to aluminium and silicon solubility. The solubility was lower between June and August, and increased from September onwards, during the austral spring months. The relation with measured concentrations of particulate MSA (methanesulfonic acid), solar irradiance and wind speed suggests a possible two-way interac-tion whereby marine biogenic emissions from the coastal Benguela upwelling to the atmosphere would increase the solubility of iron-bearing dust, according to the photo-reduction processes pro-posed by Johansen and Key (2006). The subsequent deposition of soluble iron could act to further enhance marine biogenic emissions. This first investigation points to the west coast of southern Africa as a complex and dynamic environment with multiple processes and active exchanges between the atmosphere and the Atlantic Ocean, requiring further research. [ABSTRACT FROM AUTHOR]