Abstract. The continuous in situ measurement of δ 18 O in atmospheric CO 2 opens a new door to differentiating between CO 2 source and sink components with high temporal resolution. Continuous 13 C-CO 2 measurement systems have already been commercially available for some time, but until now, only few instruments have been able to provide a continuous measurement of the oxygen isotope ratio in CO 2 . Besides precise 13 C/ 12 C observations, the Fourier transform infrared (FTIR) spectrometer is also able to measure the 18 O / 16 O ratio in CO 2 , but the precision and accuracy of the measurements have not yet been evaluated. Here we present a first analysis of δ 18 O-CO 2 (and δ 13 C-CO 2 ) measurements with the FTIR analyser in Heidelberg. We used Allan deviation to determine the repeatability of δ 18 O-CO 2 measurements and found that it decreases from 0.25 ‰ for 10 min averages to about 0.1 ‰ after 2 h and remains at that value up to 24 h. We evaluated the measurement precision over a 10-month period (intermediate measurement precision) using daily working gas measurements and found that our spectrometer measured δ 18 O-CO 2 to better than 0.3 ‰ at a temporal resolution of less than 10 min. The compatibility of our FTIR-spectrometric measurements to isotoperatio mass-spectrometric (IRMS) measurements was determined by comparing FTIR measurements of cylinder gases and ambient air with IRMS measurements of flask samples, filled with gases of the same cylinders or collected from the same ambient air intake. Two-sample t tests revealed that, at the 0.01 significance level, the FTIR and the IRMS measurements do not differ significantly from each other and are thus compatible. We describe two weekly episodes of ambient air measurements, one in winter and one in summer, and discuss what potential insights and new challenges combined highly resolved CO 2 , δ 13 C-CO 2 and δ 18 O-CO 2 records may provide in terms of better understanding regional scale continental carbon exchange processes.

Abstract. The continuous in-situ measurement of δ18O in atmospheric CO2 opens a new door to differentiating between CO2 source and sink components with high temporal resolution. Continuous 13C-CO2 measurement systems have been commercially available already for some time, but until now, only few instruments have been able to provide a continuous measurement of the oxygen isotope ratio in CO2. Besides precise 13C/12C observations, the Fourier Transform InfraRed (FTIR) spectrometer also measures the 18O/16O ratio of CO2, but the precision and accuracy of the measurements has not been evaluated yet. Here we present a first analysis of δ18O-CO2 (and δ13C-CO2) measurements with the FTIR in Heidelberg. We find that our spectrometer measures 18O in CO2 with a reproducibility of better than 0.3‰ at a temporal resolution of less than 10 min, as determined from surveillance gas measurements over a period of ten months. An Allan deviation test shows that the δ18O repeatability reaches 0.15‰ for half-hourly means. The compatibility of our spectroscopic measurements was determined by comparing FTIR measurements of calibration gases and ambient air to mass-spectrometric measurements of flask samples, filled with the cylinder gases or episodically collected over a diurnal cycle (event). We found that direct cylinder gas measurements agree to 0.01 ± 0.04‰ (mean and standard deviation) for δ13C-CO2 and 0.01 ± 0.11‰ for δ18O. Two weekly episodes of recent ambient air measurements, one in winter and one in summer, are discussed in view of the question, which potential insights and new challenges combined highly resolved δ18O-CO2 and δ13C-CO2 records may provide in terms of better understanding regional scale continental carbon exchange processes.