Abstract. Sulfur isotope analysis of atmospheric aerosols is a well established tool for identifying sources of sulfur in the atmosphere, estimating emission factors, and tracing the spread of sulfur from anthropogenic sources through ecosystems. Conventional gas mass spectrometry averages the isotopic compositions of several different types of sulfur aerosol particles, and therefore masks the individual isotopic signatures. In contrast, the new single particle technique presented here determines the isotopic signature of the individual particles.

Primary aerosol particles retain the original isotopic signature of their source. The isotopic composition of secondary sulfates depends on the isotopic composition of precursor SO₂ and the oxidation process. The fractionation with respect to the source SO₂ is poorly characterized. In the absence of conclusive laboratory experiments, we consider the kinetic fractionation of −9‰ during the gas phase oxidation of SO₂ by OH as suggested by Saltzman et al. (1983) and Tanaka et al. (1994) to be the most reasonable estimate for the isotope fractionation during gas phase oxidation of SO₂ (α_hom=0.991) and the equilibrium fractionation for the uptake of SO₂ (g) into the aqueous phase and the dissociation to HSO₃^- of +16.5‰ measured by Eriksen (1972a) to be the best approximation for the fractionation during oxidation in the aqueous phase (α_het=1.0165). The sulfur isotope ratio of secondary sulfate particles can therefore be used to identify the oxidation pathway by which this sulfate was formed. However, the fraction of heterogeneous and homogeneous oxidation pathway calculated is very sensitive to the isotope fractionation assumed for both pathways. With the new single particle technique, different types of primary and secondary sulfates were first identified based on their chemical composition, and then their individual isotopic signature was measured separately. Our samples were collected in Mainz, Germany, in an urban environment. Secondary sulfates (ammonium sulfate, gypsum, mixed sulfates) and coatings on silicates or organic aerosol dominated sulfate loadings in our samples. Comparison of the chemical and isotopic composition of secondary sulfates showed that the isotopic composition was homogeneous, independent of the chemical composition. This is typical for particles that derive from in-cloud processing. The isotopic composition of the source SO₂ of secondary sulfates was calculated based on the isotopic composition of particles with known oxidation pathway and showed a strong dependence on wind direction. The contribution of heterogeneous oxidation to the formation of secondary sulfate was highly variable (35%–75%) on day-to-day basis and depended on meteorological conditions.