Temporal trends in the isotope signature of air-borne sulfur in Central Europe

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.