Alewell, C; Mitchell, M; Likens, GE; Krouse, RH: Assessing the origin of sulfate deposition at the Hubbard Brook Experimental Forest, J. Environmental Quality, 29, 759-767 (2000)
Abstract: The geographical and chemical origin of SO42-deposition has become a concern, because anthropogenic S emissions have influenced the biogeochemistry of forested ecosystems and surface waters. Our aim was to evaluate the origin of SO42- in bulk precipitation at the Hubbard Brook Experimental Forest (HBEF), New Hampshire. We analyzed 26 years of archived bulk pricipitation samples for sulfur stable isotopes. We compared the d34S values with anthropogenic SO2 emissions, the relative contribution of sea salt aerosols (as the SO42-/Na+ ratio in precipitation), and temperature and solar radiation effects on the long-term patterns of d34S values. The long-term pattern of d34S values in bulk precipitation could be explained partly by the relative contribution of marine SO42- or solar radiation but not by temperature variation or anthropogenic SO2 emissions. The high variability of the d34S values of various fossil fuels makes it difficult to use stable S isotopes for identifying whether changing fossil fuel use is affecting the d34S values in bulk precipitation. The seasonal pattern of d34S values in bulk precipitation (significantly higher values in the winter than the summer) may b explained by the temperature dependece of the isotopic shift during SO42- formation in the atmosphere. A greater relative contribution of marine SO42- during the winter also may have contributed to the higher d34S values in the winter. Previous investigations may have overestimated the role of biogenic emissions in affecting the d34S pattern.