Assessing the Origin of Sulfate Deposition at the Hubbard Brook Experimental Forest

Abstract

AbstractThe geographical and chemical origin of SO2−4 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 SO2−4 in bulk precipitation at the Hubbard Brook Experimental Forest (HBEF), New Hampshire. We analyzed 26 years of archived bulk precipitation samples for sulfur stable isotopes. We compared the δ34S values with anthropogenic SO2 emissions, the relative contribution of sea salt aerosols (as the SO−4/Na+ ratio in precipitation), and temperature and solar radiation effects on the long‐term patterns of δ34S values. The long‐term pattern of δ34S values in bulk precipitation could be explained partly by the relative contribution of marine SO2−4 or solar radiation but not by temperature variation or anthropogenic SO2 emissions. The high variability of the δ34S values of various fossil fuels makes it difficult to use stable S isotopes for identifying whether changing fossil fuel use is affecting the δ34S values in bulk precipitation. The seasonal pattern of δ34S values in bulk precipitation (significantly higher values in the winter than the summer) may be explained by the temperature dependence of the isotopic shift during SO2−4 formation in the atmosphere. A greater relative contribution of marine SO2−4 during the winter also may have contributed to the higher δ34S values in the winter. Previous investigations may have overestimated the role of biogenic emissions in affecting the δ34S pattern.