First continuous measurements of δ<sup>18</sup>O-CO<sub>2</sub> in air with a Fourier transform infrared spectrometer

Abstract

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.

Evaluation of 4 years of continuous <i>δ</i><sup>13</sup>C(CO<sub>2</sub>) data using a moving Keeling plot method

“…In this work, we only use hourly CO 2 and δ 13 C(CO 2 ) values, since simulation runs often have an hourly resolution, and thus observations and simulations can directly be compared. However, from Allan standard deviation tests, we know that the intermediate measurement precision of hourly measurements is only slightly better than for 9-minutely measurements (Vardag et al, 2015a).…”

“…The dataset is available at http://www.iup.uni-heidelberg.de/institut/forschung/ groups/kk/Data/Hourly_FTIR.txt. The intermediate measurement precision of the FTIR is about 0.05 ppm for CO 2 and 0.04 ‰ for δ 13 C(CO 2 ) (both 9 min averages) as determined from the variation of daily target gas measurements (Vardag et al, 2014;Vardag et al, 2015a). In this work, we only use hourly CO 2 and δ 13 C(CO 2 ) values, since simulation runs often have an hourly resolution, and thus observations and simulations can directly be compared.…”

“…A description of the measurement principle can be found in Esler et al (2000) and Griffith et al (2010Griffith et al ( , 2012. Hammer et al (2013) describe the Heidelberg-specific instrumental setup in detail, and Vardag et al (2015a) describe modifications to this setup and the calibration strategy for the stable isotopologue measurements. The dataset is available at http://www.iup.uni-heidelberg.de/institut/forschung/ groups/kk/Data/Hourly_FTIR.txt.…”

“…Note that the background value is displayed for illustration, but it is not used in the moving Keeling plot method. ments. This has led to an increasing deployment of these instruments, thereby increasing the temporal and spatial resolution of 13 C(CO 2 ) and CO 2 data (Bowling et al, 2003;Tuzson et al, 2008;McManus et al, 2010;Griffith et al, 2012;Vogel et al, 2013;Vardag et al, 2015a;Eyer et al, 2016). These data records may lead to an improved understanding of regional CO 2 fluxes, allowing estimates of mean δ 13 C source signatures at high temporal resolution.…”

Long open path measurements of greenhouse gases in air using near infrared Fourier transform spectroscopy

“…At the IUP end of the open path, air from a roof-level inlet on the IUP building was sampled and analysed continuously with an in situ trace gas analyser described in detail elsewhere (Griffith et al, 2012;Hammer et al, 2013;Vardag et al, 2015). This analyser is based on an FTIR spectrometer operating in the mid-IR and provided simultaneous high precision measurements of CO 2 , CH 4 , CO, N 2 O, δ 13 C and δ …”