Research Group Prof. Dr. G. Friedrichs

Field Application of a cw-CRDS analyzer determining surface ocean fCO2 and δ13C(CO2)

The increasing amount of anthropogenic carbon dioxide (CO2) in the atmosphere, a major component of climate change, is also reflected in the world oceans' carbon budget. The oceans act as a net CO2 sink, which is driven by the CO2 partial pressure difference across the air-sea interface. Seasonal time-series data of surface-water-dissolved inorganic carbon (DIC) and its isotopic composition allow the separation of relative contributions of several processes to the surface layer carbon budget. Processes altering both DIC and its isotopic composition include photosynthesis, mixing, and the oceanic Suess effect. Isotope ratio mass spectrometry is the conventional method for accurately measuring isotope ratios, but sample collection, shipping, and processing is labor-intensive and costly. In this highly interdisciplinary project, the long-term stability, response time, susceptibility to external interferences, and precision of a CRDS based instrument for δ13C(CO2) isotope analysis of gaseous CO2 was thoroughly analyzed. A significant gas matrix effect, which is caused by subtle absorption line shape effects, was found and a correction procedure for field measurements of surface-water-dissolved CO2 has been worked out. For underway pCO2 measurements aboard a research vessel, the instrument was successfully operated aboard RV Polarstern during the ANT-XXVI/1 and ANT-XXVI/4 cruises (Bremerhaven - Punta Arenas (Chile)). For the first time, on-line data of oceanic δ13C(CO2) and pCO2 could be collected. By comparing the CRDS data with reference measurements performed by established methods (NDIR for pCO2, IRMS for isotope ratios) it turned out that the CRDS data were in good agreement with the reference data. The next step will be the installation aboard a voluntary observing ship (VOS) in order to obtain a high resolution dataset of pCO2 and δ13C(CO2) of the North Atlantic surface ocean.

allanAllan plot statistics of CRDS CO2 analyzer.


Schematic setup of the water supply, equilibration system and gas pathways aboard R/V Polarstern.


CO2 fugacity, fCO2, and its stable carbon isotope ratio, δ13C(CO2), during two Atlantic transects.


[1] G. Friedrichs, J. Bock. F. Temps, P. Fietzek, A. Körtzinger, D. Wallace, "Toward Continuous Monitoring of Seawater  13CO2/12CO2 Isotope Ratio and pCO2: Performance of a Cavity Ringdown Spectrometer and Gas Matrix Effects", Limnol. Oceanogr.: Methods 8 (2010), 539-551;doi:10.4319/lom.2010.8.539.

[2] M. Becker, A. Körtzinger, N. Andersen, B. Fiedler, P. Fietzek, T. Steinhoff, G. Friedrichs, "Using Cavity Ringdown Spectroscopy for Continuous Monitoring of δ13C(CO2) and fCO2 in the Surface Ocean", Limnol. Oceanogr.: Methods 10 (2012) 752-766; doi: 10.4319/lom.2012.10.752