Interim Results

This research shows that perchlorate (ClO₄^-) is produced in the atmosphere likely from reactions of Cl^- or ClO_x with O₃ and perhaps with ultraviolet radiation. These reactions have been extensively evaluated, including the impact of reactant concentration, reaction rates, and distribution of final products. This information has important implications for both atmospheric chemistry as well as numerous oxidation processes involved in water treatment and industrial applications. The project data clearly indicate that deposition of perchlorate is ubiquitous across North America and apparently the world. This relatively unexplored source may help to explain the growing reports of perchlorate in produce, milk, and other food items and should be considered when evaluating overall source contributions or drinking water standards. Significant progress has also been made in characterizing the stable isotope signatures of Cl and O in natural perchlorate and the occurrence of the long-lived radioactive isotope ³⁶Cl in natural perchlorate. The current data indicate that natural perchlorate indigenous to the southwestern United States has at least two unique stable isotope signatures (when considering δ¹⁸O, δ¹⁷O, and δ³⁷Cl together) and that much of this material has elevated ³⁶Cl, suggesting formation in the stratosphere. Based on isotopic composition, natural perchlorate indigenous to the United States can be distinguished from both synthetic perchlorate and perchlorate derived from Chilean fertilizers.