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The continuous in situ measurement of δ18O in atmospheric CO2opens a new door to differentiating between CO2 source and sinkcomponents with high temporal resolution. Continuous13C–CO2 measurement systems have already been commercially available for some time,but until now, only few instruments have been able to provide a continuousmeasurement of the oxygen isotope ratio in CO2. Besides precise13C/12C observations, the Fourier transform infrared (FTIR)spectrometer is also able to measure the 18O / 16O ratio inCO2, but the precision and accuracy of the measurements have not yetbeen evaluated. Here we present a first analysis of δ18O-CO2 (and δ13C-CO2) measurements with theFTIR analyser in Heidelberg. We used Allan deviation to determine therepeatability of δ18O-CO2 measurements and found that itdecreases from 0.25‰ for 10 min averages to about0.1‰ after 2 h and remains at that value up to24 h. We evaluated the measurement precision over a 10-month period(intermediate measurement precision) using daily working gas measurementsand found that our spectrometer measured δ18O-CO2 tobetter than 0.3‰ at a temporal resolution of less than10 min. The compatibility of our FTIR-spectrometric measurements toisotope-ratio mass-spectrometric (IRMS) measurements was determined bycomparing FTIR measurements of cylinder gases and ambient air with IRMSmeasurements of flask samples, filled with gases of the same cylinders orcollected from the same ambient air intake. Two-sample t tests revealedthat, at the 0.01 significance level, the FTIR and the IRMS measurements donot differ significantly from each other and are thus compatible. Wedescribe two weekly episodes of ambient air measurements, one in winter andone in summer, and discuss what potential insights and new challengescombined highly resolved CO2, δ13C-CO2 and δ18O-CO2 records may provide in terms of better understandingregional scale continental carbon exchange processes

atmospheric measurement techniques

First continuous measurements of δ&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O-CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in air with a Fourier transform infrared spectrometer