Late autumn aerosol trace element composition and source tracking over the southern Mozambique Channel

Abstract

The southern Mozambique Channel (20–30° S) receives a range of atmospheric influences, from desert dust and fire emissions through to industrial, mining and agricultural emissions, emitted from both Madagascar and southeastern Africa. Our study characterises the trace element composition of aerosols collected between the south of Madagascar and Durban, South Africa during the low dust season. Dust deposition fluxes (40–263 mg m−2 yr−1), calculated based on Al measurement in aerosols, fell within the lower range of modelled fluxes estimates, confirming the absence of major dust or fire events during the study. While prevailing air-masses affecting our samples were modelled to originate from long-range particulate transport over the Southern Ocean, a holistic understanding of our sample composition could only be obtained when accounting for sporadic aeolian inputs from the two local landmasses. Notably, we found surprising high levels of Cr (4 ± 2 ng m−3) and Cd (0.02 ± 0.01 ng m−3) in the atmosphere over the southern Channel which could be, at least in part, attributed to emissions from mining (chromite and gold, respectively) and smelting activities (Cu, Zn and Cd co-emission) on both neighbouring landmasses. Our results emphasise the difficulty to track such specific and overlooked atmospheric sources in the absence of known atmospheric tracers. We also stress the need for multi-elemental studies and encourage the use of detailed (cluster) air-mass transport model analysis in regions dominated by the long-range atmospheric transport as complex atmospheric circulation and minor (sporadic) inputs from terrestrial air-masses may have disproportionate impact on the atmospheric composition.