Temporal Trends in the Isotope Signature of Air-Borne Sulfur in Central Europe

Abstract

In various parts of the Northern hemisphere air-borne S exhibits a seasonality, with isotopically light (i.e., 32S-rich) sulfur predominating in the warm summer months. Such seasonality has been reported from the United States, Canada, Japan, and China. Elevated biological emissions of isotopically light S in summer, a temperature-dependent isotope fractionation accompanying the oxidation of SO2, and heavy rains in winter bringing 34S-rich marine S have been suggested as the controlling mechanisms. In the atmosphere of Central Europe, one of the most severely polluted regions of the world, we have found an opposite seasonal trend: Isotopically light SO2-S predominates in the cold winter months, whereas isotopically heavy SO2-S is typical of the summer. The low delta34S values of air-borne SO2 in winter are influenced by low-delta34S emissions from local coal-burning power plants. The coal contains isotopically light S (mean delta34S of 1.6/1000). Higher demand for electricity during the heating season leads to higher anthropogenic S emission rates in winter. On a yearly basis, atmospheric sulfate S in Central Europe is isotopically heavier than atmospheric SO2-S by 4/1000. Atmospheric oxidation of SO2 is accompanied by an isotope fractionation resulting in 34S-enriched sulfate. In addition to the seasonality in air-borne delta34S(SO2), we report also an interannual trend of 1/1000 yr(-1) toward isotopically light sulfate S in atmospheric deposition. This interannual trend cannot be explained by a change in pollution sources accompanying the present massive environmental cleanup. To investigate the role of biological S emissions from the soil of heavily polluted ecosystems, we conducted a series of laboratory experiments using repacked soil columns and 34S-enriched precipitation under summer and winter temperatures. These experiments indicate that, under summer temperatures, the 34S-labeled precipitation is largely captured by the upper organic-rich soil horizons, a high proportion (53-74%) of S input is revolatilized, and the biologically reemitted S is isotopically light. Under winter temperatures more precipitation S is leached to the bottom of the soil columns. Our experiments have shown that biological emissions in Central Europe can be sizable. Yet, they cannot be singled out in the overall SO2 isotope pattern in the atmosphere. The main reason is continuous, variable (0-4/1000), open-system depletion in 34S in the residual SO2 during the isotopically selective SO2-to-SO4(2-) conversion.

Sulfur isotope fractionation during oxidation of sulfur dioxide: gas-phase oxidation by OH radicals and aqueous oxidation by H<sub>2</sub>O<sub>2</sub>, O<sub>3</sub> and iron catalysis

“…Even a fractionation factor of 1.07 rather than 1.14 is significantly larger than the variation observed in atmospheric samples (e.g. Norman et al (2006); Novak et al (2001)), so it is likely that RRKM theory can accurately predict only the direction and not the magnitude of this isotope effect. This is in agreement with recent results from Lin et al (2011) and Hattori et al (2011), which found a similar overprediction of the sulfur isotopic fractionation during the photolysis of OCS by RRKM theory (Leung et al, 2002).…”

“…Seasonality in data, with lower δ 34 S values measured for sulfate in summer, could show that the gas-phase fractionation factor is less than the heterogeneous fractionation factor and probably less than 1 (Saltzman et al, 1983;Sinha et al, 2008a). However, seasonality may also be explained by changing sources or the temperature-dependence of fractionation factors (Caron et al, 1986;Novak et al, 2001;Ohizumi et al, 1997). The study of 17 O of sulfate trapped in ice cores showed that the ratio of gas-phase to aqueous-phase oxidation was higher and the δ 34 S was lower during the last glacial maximum than the preceeding and subsequent interglacials (Alexander et al, 2002(Alexander et al, , 2003.…”

“…However, for field studies measuring the isotopic composition of both ambient SO 2 and sulfate, the major limitation to interpreting atmospheric isotope measurements is the lack of laboratory studies of the isotopic fractionation factors involved in the most common atmospheric reactions of sulfur (Tanaka et al, 1994;Novak et al, 2001;Tichomirowa et al, 2007). For heterogeneous oxidation, equilibrium fractionation of 34 S/ 32 S during the uptake of SO 2 into solution and the subsequent acid-base equilibria has been measured in several studies.…”

“…Sulfur dioxide is traditionally collected on filters impregnated with alkaline solutions such as Na 2 CO 3 (Novak et al, 2001;Huygen, 1963). A variety of solutions were tested with varying amounts of Na 2 CO 3 , BaCl 2 , triethanolamine, glycerol and H 2 O 2 , and the average fractionation factor was measured as α 34 = 1.007±0.003 for all methods tested.…”

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

“…[4] Sulfur isotopes have been traditionally used to identify and characterize the sources of sulfur in the atmosphere because sources such as anthropogenic SO 2 , biogenic sulfur, and sulfur from airborne particulate matter (dust and mineral matter) often possesses different d 34 S values [Nielsen, 1974;Kawamura et al, 2001;Norman et al, 1999;Novak et al, 2001;Ono et al, 2009;Lyons, 2009;Szynkiewicz et al, 2009]. 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].…”