Abstract
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 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 CO2. Besides precise 13C/12C observations, the Fourier transform infrared (FTIR) spectrometer is also able to measure the 18O / 16O ratio in CO2, but the precision and accuracy of the measurements have not yet been evaluated. Here we present a first analysis of δ18O-CO2 (and δ13C-CO2) measurements with the FTIR analyser in Heidelberg. We used Allan deviation to determine the repeatability of δ18O-CO2 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 δ18O-CO2 to better than 0.3‰ at a temporal resolution of less than 10 min. The compatibility of our FTIR-spectrometric measurements to isotope-ratio 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 CO2, δ13C-CO2 and δ18O-CO2 records may provide in terms of better understanding regional scale continental carbon exchange processes.
Highlights
Quantitative understanding of the processes governing the carbon cycle is vital in order to assess the impact and fate of increasing anthropogenic CO2 emissions to the atmosphere
The stable isotopes in CO2 can provide information about the fluxes between the different carbon reservoirs, such as the atmosphere, the biosphere and the oceans. 13CO2 measurements can be used to distinguish between terrestrial biosphere and marine fluxes (Keeling et al, 1989; Ciais et al, 1995), and are used as a tracer for anthropogenic emissions, as most fossil fuel CO2 emissions are depleted in 13C relative to those of the biosphere (Tans, 1981)
Given improvements in the instrumentation and spectral analysis methods since that time, we have revisited the practicality of continuous measurements of δ18O in CO2 using Fourier transform infrared (FTIR) spectroscopy. The scope of this manuscript is to answer two important questions: first, is it possible to measure δ18O-CO2 using FTIR spectroscopy, and if yes, how well can we measure it in terms of precision, accuracy and compatibility to conventional isotoperatio mass-spectrometric (IRMS) observations? Second, what insight into regional scale carbon exchange processes can one gain from a highly resolved δ18O-CO2 record in the catchment area of our measurement site?
Summary
Quantitative understanding of the processes governing the carbon cycle is vital in order to assess the impact and fate of increasing anthropogenic CO2 emissions to the atmosphere. Still et al (2009), Welp et al (2011) and Buenning et al (2014) have studied the susceptibility of atmospheric δ18O-CO2 to environmental parameters, such as precipitation, relative humidity, temperature, solar radiation and cloud cover, and estimated the influences of these parameters on the atmospheric δ18O-CO2 using regional and global scale models They assessed the effect of the isotopic composition of precipitation and water vapour. In order to understand atmospheric δ18O-CO2 measurements in all their complexity, information about the regional isotopic composition of precipitation, environmental parameters such as temperature and water vapour deficit and a comprehensive land-surface model are necessary (Yakir and Wang, 1996; Ciais et al, 1997; Langendörfer et al, 2002; Cuntz et al., 2003a; Buenning et al, 2014). The scope of this manuscript is to answer two important questions: first, is it possible to measure δ18O-CO2 using FTIR spectroscopy, and if yes, how well can we measure it in terms of precision, accuracy and compatibility to conventional IRMS observations? Second, what insight into regional scale carbon exchange processes can one gain from a highly resolved δ18O-CO2 record (along with the continuous CO2, CO and δ13C-CO2 records) in the catchment area of our measurement site?
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
