Mineral dust is a major fraction of global atmospheric aerosol, andthe oxidation of SO on mineral dust has implications forcloud formation, climate and the sulfur cycle. Stable sulfur isotopescan be used to understand the different oxidation processes occurringon mineral dust. This study presents measurements of theS/S fractionation factor α foroxidation of SO on mineral dust surfaces and in the aqueousphase in mineral dust leachate. Sahara dust, which accounts for~60% of global dust emissions and loading, was used for theexperiments.The fractionation factor for aqueous oxidation in dust leachate isα = 0.9917±0.0046, which is in agreementwith previous measurements of aqueous SO oxidation by iron solutions.This fractionation factor is representative of a radical chain reactionoxidation pathway initiated by transition metal ions. Oxidation on the dustsurface at subsaturated relative humidity (RH) had an overall fractionationfactor of α = 1.0096±0.0036 and was found to bealmost an order of magnitude faster when the dust was simultaneously exposedto ozone, light and RH of ~40%. However, the presence of ozone,light and humidity did not influence isotope fractionation during oxidationon dust surfaces at subsaturated relative humidity. All the investigatedreactions showed mass-dependent fractionation of S relative toS.A positive matrix factorization model was used to investigate surfaceoxidation on the different components of dust. Ilmenite, rutile andiron oxide were found to be the most reactive components, accountingfor 85% of sulfate production with a fractionation factor ofα = 1.012±0.010. This overlaps within the analyticaluncertainty with the fractionation of other major atmosphericoxidation pathways such as the oxidation of SO byHO and O in the aqueous phase and OH in the gasphase. Clay minerals accounted for roughly 12% of the sulfateproduction, and oxidation on clay minerals resulted in a very distinctfractionation factor of α = 1.085±0.013. Thefractionation factors measured in this study will be particularlyuseful in combination with field and modelling studies to understandthe role of surface oxidation on clay minerals and aqueous oxidationby mineral dust and its leachate in global and regional sulfur cycles.
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