Triple Oxygen Isotope Composition of CO2: Analytical System Improvements and Seasonal Variability

Abstract

The triple oxygen isotope composition (∆′17O) of troposphere CO2 is considered as a potential tracer for quantifying gross primary production (GPP) at an ecosystem scale. In this project, air samples from two different ecosystems, Loobos (evergreen coniferous forest ecosystem) and Cabauw (grassland ecosystem) were taken from December 2020 to April 2022 in order to quantify the seasonal variation in ∆′17O values of tropospheric CO2 with the focus of quantifying gross primary production at an ecosystem scale. The ∆′17O was measured using a CO2-O2 isotope exchange technique coupled with an isotope ratio mass spectrometer (IRMS). We characterized and investigated the stability of CO2-O2 the isotope exchange system using different CO2 standards. The results showed that larger thermal diffusion (larger temperature gradients in the CO2-O2 isotope exchange reactor) will lead to smaller fractionation both for reactor temperature of 750 °C and 1000 °C. Tropospheric CO2 sampled in Cabauw has an average ∆′17O value of 0.221 ± 0.006‰(SE) from December 2020 to April 2022. The temporal evaluation of ∆′17O values does not show a clear seasonal cycle with slight upward trend toward summer but more scattered in the winter, with an amplitude of approximately 0.08‰. The seasonal variation of ∆′17O value of tropospheric CO2 and its upward trend toward summer indicate the activity of terrestrial vegetation and may be used as a potential tracer of GPP.