Soluble iron dust export in the high altitude Saharan Air Layer

Every summer huge amounts of desert dust particles are exported from the hyperarid subtropical Sahara to the North Atlantic the so-called Saharan Air Layer (SAL), a dry, warm and dust-laden corridor that expands from the North African coast (1-5 km.a.s.l.) to the Americas above the marine boundary layer. Because of the potential impact of the dust deposited on the ocean on marine biogeochemistry and climate, we studied the Fe solubility (in seawater) of atmospheric aerosols samples directly collected in the SAL off the North African coast, i.e. the fresh aerosols recently exported from the Sahara in the SAL. The aerosol sampling was performed at ∼2400 m.a.s.l. in Izaña observatory in Tenerife island. In the total aerosols, we found low Fe concentrations and high fractional Fe solubility (FFS ∼2%) in the North Atlantic free troposphere airflows and high Fe concentrations and low FFS (∼0.7%) within the SAL; the resulting FFS versus total dust (or total Fe) plot shows a hyperbolic trend attributed to the conservative mixing of 'fine combustion aerosols' and 'lithogenic mineral dust'. We then focused on the soluble Fe in the SAL. Our results indicate that ∼70% of soluble Fe is associated with the dissolution of submicron dust particles, probably involving Fe-bearing clays. We found a FFS of submicron dust (∼6%) higher than that typically observed in submicron particles of soil dust samples (<1%). The correlation between FFS and the ammonium-sulphate/dust ratio and the low variability in the Fe/Al ratio in the dust samples, suggests that the increase in the FFS of submicron dust aerosols (with respect to soil dust particles) may be related to the presence of acid pollutants mixed with dust. Previous studies had focused on dust processing and changes of Fe solubility during the trans-Atlantic transport of dust in the SAL. We found that submicron dust exported off the coast of North Africa may have already experienced acid processing over the Sahara, i.e. before dust export to the Atlantic. Export of soluble submicron Fe dust and deposition of coarse and depleted in soluble Fe dust particles during the trans-Atlantic transport may account for the observed variability in dust, soluble Fe and FFS.