Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust

Abstract

Abstract. Mineral dust is a major fraction of global atmospheric aerosol, and the oxidation of SO 2 on mineral dust has implications for cloud formation, climate and the sulfur cycle. Stable sulfur isotopes can be used to understand the different oxidation processes occurring on mineral dust. This study presents measurements of the 34 S/ 32 S fractionation factor α 34 for oxidation of SO 2 on mineral dust surfaces and in the aqueous phase in mineral dust leachate. Sahara dust, which accounts for ∼60 % of global dust emissions and loading, was used for the experiments.The fractionation factor for aqueous oxidation in dust leachate is α leachate = 0.9917±0.0046, which is in agreement with previous measurements of aqueous SO 2 oxidation by iron solutions. This fractionation factor is representative of a radical chain reaction oxidation pathway initiated by transition metal ions. Oxidation on the dust surface at subsaturated relative humidity (RH) had an overall fractionation factor of α het = 1.0096 ± 0.0036 and was found to be almost an order of magnitude faster when the dust was simultaneously exposed to ozone, light and RH of ∼40 %. However, the presence of ozone, light and humidity did not influence isotope fractionation during oxidation on dust surfaces at subsaturated relative humidity. All the investigated reactions showed mass-dependent fractionation of 33 S relative to 34 S.A positive matrix factorization model was used to investigate surface oxidation on the different components of dust. Ilmenite, rutile and iron oxide were found to be the most reactive components, accounting for 85 % of sulfate production with a fractionation factor of α 34 = 1.012±0.010. This overlaps within the analytical uncertainty with the fractionation of other major atmospheric oxidation pathways such as the oxidation of SO 2 by H 2 O 2 and O 3 in the aqueous phase and OH in the gas phase. Clay minerals accounted for roughly 12 % of the sulfate production, and oxidation on clay minerals resulted in a very distinct fractionation factor of α 34 = 1.085±0.013. The fractionation factors measured in this study will be particularly useful in combination with field and modelling studies to understand the role of surface oxidation on clay minerals and aqueous oxidation by mineral dust and its leachate in global and regional sulfur cycles.

In-cloud sulfate addition to single particles resolved with sulfur isotope analysis during HCCT-2010

“…In contrast, coarse mineral dust became enriched in 32 S as it passed through the cloud, consistent with isotope fractionation during SO 2 removal. Transition metal ions from mineral dust leachate cause very fast oxidation of SO 2 (Tilly et al, 1991;Rani et al, 1992;Harris et al, 2012a), and the efficiency of oxidation by this pathway means that, despite the relatively low number concentration of these particles compared to, for example, mixed particles, they are able to account for the majority of in-cloud SO 2 oxidation at Schmücke (discussed further in Harris et al (2013).…”

“…The δ 34 S of the sulfate that could be added from each potential source was calculated from the upwind isotopic composition of SO 2 or H 2 SO 4 and the fractionation factors shown in Table 2. For SO 2 removal, the fractionation factors for SO 2 oxidation from Harris et al (2012a, b) were used with the Rayleigh fractionation laws to account for depletion of the SO 2 reservoir (Mariotti et al (1981); ; fraction of SO 2 oxidised determined in Harris et al, 2013). The δ 34 S values of the sulfate that would be added by each source are shown in Table 4 and Fig.…”

Investigation of aged aerosols in size-resolved Asian dust storm particles transported from Beijing, China, to Incheon, Korea, using low-<i>Z</i> particle EPMA

“…The reacted or aged mineral dust particles mainly included "reacted CaCO 3 / CaMg(CO 3 ) 2 " and "aluminosilicate + (N, S)", where the (N, S) notation represents compounds containing either nitrates, sulfates, or both. They were either produced when mineral dust particles (particularly Cacontaining species) react with airborne SO 2 and NO x in the presence of moisture or with "secondary acids" such as H 2 SO 4 , HNO 3 , and HCl (Harris et al, 2012;Wang et al, 2005), or were formed from the adsorption of NH 4 NO 3 and/or (NH 4 ) 2 SO 4 / NH 4 HSO 4 on the particle surface (Sullivan et al, 2007). In the present study, the overall relative abundance of mineral dust particles was more than 88 % of the particles analyzed for sample S1 collected in Beijing.…”

New mass spectrometry techniques for studying physical chemistry of atmospheric heterogeneous processes

“…Recently, Harris et al examined that sulphur isotope undergoes fractionation during oxidation of SO 2 in the gas phase, liquid phase and on mineral dust particle surfaces using NanoSIMS of collected products [122,123]. Oxidation of sulphur dioxide to sulphate is an important reaction in atmospheric chemistry [124].…”

“…It follows that isotope ratio measurements may be useful for distinguishing metal-catalysed oxidation of SO 2 from other competing pathways. Another study by Harris et al demonstrated that sulphur isotope fractionation can be used for understanding the kinetics and mechanisms of SO 2 oxidation by mineral dust [122]. Sulphur isotope fractionation during heterogeneous oxidation of SO 2 on Sahara dust particles was characterised by a substantially larger fractionation factor (α = 1.0096) than aqueous oxidation in Sahara dust leachate (α = 0.9917).…”