Sulfur isotope analysis of individual aerosol particles – a new tool for studying heterogeneous oxidation processes in the marine environment

Abstract. Understanding the importance of the different oxidation pathways of sulfur dioxide (SO₂) to sulfate is crucial for an interpretation of the climate effects of sulfate aerosols. Sulfur isotope analysis of atmospheric aerosol is a well established tool for identifying sources of sulfur in the atmosphere and assessment of anthropogenic influence. The power of this tool is enhanced by a new ion microprobe technique that permits isotope analysis of individual aerosol particles as small as 0.5 μm diameter. With this new single particle technique, different types of primary and secondary sulfates are first identified based on their chemical composition, and then their individual isotopic signature is measured. Our samples were collected at Mace Head, Ireland, a remote coastal station on the North Atlantic Ocean. Sea-salt-sulfate (10–60%), ammonium sulfate/sulfuric acid particles (15–65%), and non-sea-salt-sulfate (nss-sulfate) on aged salt particles all contributed significantly to sulfate loadings in our samples.

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. Particles with known oxidation pathway (fine mode ammonium sulfate) are used to estimate the isotopic composition of the source SO₂. It ranged from δ³⁴S_VCDT=0±3‰ to δ³⁴S_VCDT=(14±3)‰ under clean conditions and δ³⁴S_VCDT=(3±1)‰ under polluted condition. Condensation of H₂SO₄(g) onto sea salt aerosol produces an isotopic ratio that, when plotted against the sea-salt-sulfate content of the sample, lies on a mixing line between sea salt and ammonium sulfate. The contribution of heterogeneous oxidation is estimated based on the deviation of non-sea-salt-sulfate from this isotopic mixing line.

The contribution of heterogeneous oxidation to nss-sulfate formation on aged sea salt sodium sulfate, magnesium sulfate gypsum and mixed sulfate particles under clean conditions is on average 10% for coarse and 25% for fine mode particles. Under polluted conditions, the contribution of heterogeneous oxidation to nss-sulfate formation increased to 60% on coarse mode and 75% on fine mode particles. However, large day-to-day variations in the contribution of heterogeneous oxidation to nss-sulfate formation occurred. Our results suggest that a~significant portion of SO₂ in coastal regions is converted to fine mode ammonium sulfate/sulfuric acid particles (40–80% of nss-sulfate) and that condensation of H₂SO₄(g) contributes significantly even to the nss-sulfate in aged sea salt particles (20–85%).