Sulfur isotope analyses of individual aerosol particles in the urban aerosol at a central European site (Mainz, Germany)

Abstract

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 −9‰ for homogeneous and +16.5‰ for heterogeneous oxidation. The sulfur isotope ratio of secondary sulfate particles can therefore be used to identify the oxidation pathway by which this sulfate was formed. 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.

Identification of sources and formation processes of atmospheric sulfate by sulfur isotope and scanning electron microscope measurements

“…[25] As mentioned earlier, d 34 S values may change if SO 2 is oxidized into secondary aerosol sulfates in the atmosphere via different homogeneous and heterogeneous reactions. The d 34 S compositions of precursor SO 2 can be predicted with knowledge of the d 34 S value of secondary sulfate and SO 2 oxidation pathways [Winterholler et al, 2008]. Assuming 50% of the summer aerosol sulfates are produced via heterogeneous oxidation and another half by homogenous oxidation of SO 2 , the increase in d 34 S value of summer aerosol sulfates should be ∼3.75‰, and the actual d 34 S value increase is expected to be >3.75‰ due to the dominant heterogeneous oxidation of SO 2 in summer sulfate formation.…”

“…The sulfur isotopes in atmospheric aerosols over China have been observed to be similar to those in coal combusted in the region [Mukai et al, 2001]. It has also been recognized that sulfur isotope data may be a useful tool for evaluating the oxidation reaction pathways of SO 2 [Norman et al, 2006;Winterholler et al, 2008] and in analyzing the contribution of atmospheric pollutants into groundwater and the transport of contaminants in the environment [Torfs and Van Grieken, 1997;Toyama et al, 2007;Li et al, 2006].…”

Nature and Climate Effects of Individual Tropospheric Aerosol Particles

“…The S isotopic composition of sulfate particles depends on both the source of the precursor SO 2 and the homogeneous or heterogeneous oxidation processes that led to sulfate formation, and thus can shed light on origin. Winterholler et al (2008) combined secondary ion mass spectrometry at the nanoscale (NanoSIMS) with SEM/EDS for the analysis of urban aerosol particles. All secondary sulfate particles were isotopically homogeneous, suggesting cloud processing.…”