An improved CeO _2 method for high-precision measurements of ~(17)O/ ~(16)O ratios for atmospheric carbon dioxide

摘要

The oxygen isotopic composition of carbon dioxide originating at the Earth's surface is modified in the stratosphere by interaction with ozone which has anomalous oxygen isotope ratio (Δ ~(17)O = 1000 * ln(1 + δ ~(17)O/1000) - 0.522 * 1000 * ln (1 + δ ~(18)O/1000) >0). The inherited anomaly provides a powerful tracer for studying biogeochemical cycles involving CO 2. However, the existing methods are either too imprecise or have difficulty in determining the small Δ ~(17)O variations found in the tropospheric CO _2. In this study an earlier published CeO _2 and CO _2 exchange method has been modified and improved for measuring the Δ ~(17)O values of atmospheric carbon dioxide with high precision. METHODS The CO 2 fraction from air samples was separated by cryogenic means and purified using gas chromatography. This CO _2 was first analyzed in an isotope ratio mass spectrometer, then artificially equilibrated with hot CeO _2 to alter its oxygen isotopes mass-dependently and re-analyzed. From these data the ~(17)O/ ~(16)O and ~(18)O/ ~(16)O ratios were calculated and the Δ ~(17)O value was determined. RESULTS The validity of the method was established in several tests by using artificially prepared CO _2 with zero and non-zero Δ ~(17)O values. The published value of the CO _2-H _2O equilibrium slope was also reproduced. CONCLUSIONS The CO _2-CeO _2 equilibration method has been improved to measure the oxygen isotope anomaly (Δ ~(17)O value) of atmospheric CO _2 with an analytical precision of ±0.12 ° (2σ).