[M-TT37] Frontiers in Geochemistry

[MTT37-02] Development of a high-precision measurement system for the triple oxygen isotopic compositions of atmospheric carbon dioxide

*Tsunekawa Shun¹, Urumu Tsunogai¹, Fumiko Nakagawa¹ (1.Graduate School of Environmental Studies, Nagoya University)

Keywords:carbon dioxide, high precision, triple oxygen isotope

The triple oxygen isotopic compositions (Δ¹⁷O) of tropospheric carbon dioxide (CO₂) can be a useful tracer to quantify carbon cycle in terrestrial environments. Traditionally, CO₂ must be converted to O₂ to determine Δ¹⁷O precisely. However, toxic and dangerous reagents such as fluoride must be needed for the reactions to convert CO₂ to O₂. Alternative safer, more simple, and more easy techniques should be needed for the routine measurements on the Δ¹⁷O of tropospheric CO₂. In this study, I developed an alternative new method for the high precision measurements on the Δ¹⁷O of CO₂ using Cavity Ring-Down Spectroscopy (CRDS) for H₂O. First of all, atmospheric sample with CO₂ was introduced into pre-evacuated line and separated from atmospheric nitrogen (N₂), oxygen (O₂) and water (H₂O) using the differences in the boiling points. Then, N₂O having the similar molecular weight and the similar boiling point with CO₂ was separated from CO₂ using a PorapakPS column (3m long, 0.31cm i.d.) under -70 °C. By reacting the purified CO₂ with H₂ at 600 °C under the existence of the nickel catalysts, CO₂ was converted to methane (CH₄) and water (H₂O). Subsequent to purify H₂O from CH₄, Δ¹⁷O of H₂O was measured in CRDS. In this study, I developed this new vacuum line to purify atmospheric CO₂ and to convert CO₂ into H₂O. Then, I determined the optimum conditions for the purification of CO₂, the reduction of CO₂, and the analysis of H₂O.The new system developed in this study has several advantages over the conventional methods, such as (1) safe and easy operations, (2) less than 50 min for the analysis, and (3) comparable precisions with previous methods. By using the new system developed in this study, we can determine Δ¹⁷O of tropospheric CO₂ with precision better than 0.015 ‰ (1σ). By using the new system developed in this study, I determined the difference in the Δ¹⁷O values between tropospheric CO₂ in Nagoya and that in car exhaust, and found that tropospheric CO₂ in Nagoya was 0.22‰ higher in Δ¹⁷O than that in car exhaust.