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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/amt-2017-120
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
11 May 2017
Review status
This discussion paper is under review for the journal Atmospheric Measurement Techniques (AMT).
A new instrument for stable isotope measurements of 13C and 18O in CO2 – Instrument performance and ecological application of the Delta Ray IRIS analyzer
Jelka Braden-Behrens1, Yuan Yan1, and Alexander Knohl1,2 1University of Goettingen, Bioclimatology, Faculty of Forest Sciences and Forest Ecology, Germany
2University of Goettingen, Centre of Biodiversity and Sustainable Land Use (CBL), Germany
Abstract. We used the recently developed Delta Ray Isotope Ratio Infrared Spectrometer (IRIS) to continuously measure the CO2 concentration c and its isotopic composition δ13C and δ18O in a managed beech forest in Central Germany. Our objectives are (a) to characterize the Delta Ray IRIS and its performance under field conditions and (b) to quantify the seasonal variability of c, δ13C, δ18O and the isotopic composition of CO2 exchange (Reco13C and Reco18O) derived from nighttime Keeling-Plot intercepts. The minimal Allan deviation (as a measure of precision) was below 0.01 ppm for the CO2 concentration and below 0.03 ‰ for both delta values. For repeated measurements of a target gas, the long-term standard deviation from the mean value was 0.2 ppm for c and below 0.3 ‰ for both delta values. The accuracy based on our calibration setup was approximately 0.45 ppm for c, 0.1 ‰ for δ13C and 0.6 ‰ for delta 18O. We used measurements of nine different inlet heights in the beech forest, to evaluate the isotopic compositions of respiration Reco13C and Reco18O in a three months measurement campaign in autumn 2015. During this period, an early snow and frost event occurred, coinciding with a change in the observed characteristics of both Reco13C and Reco18O . Before the first snow, R_eco 13C correlated significantly (Pearson correlation coefficient rpear = 0.55 and corresponding critical value rcrit ≈ 0.38 for α = 0.005) with time-lagged net radiation Rn, a driver of photosynthesis and photosynthetic discrimination against 13C. This correlation became insignificant after the first snow. For 18O, we measured a decrease of 30 ‰ within 10 days in Reco18O after the snow event potentially reflecting the influence of 18O depleted snow on soil moisture. This decrease was ten times larger than the corresponding decrease in delta 18O in ambient CO2 (below 3 ‰) and took three times longer to recover (three weeks vs. one week). In summary, we conclude that 1) the new Delta Ray IRIS with its internal calibration procedure provides an opportunity to precisely and accurately measure c, delta 13C and delta 18O at field sites and 2) even short snow or frost events could have strong effects on the isotopic composition of CO2 exchange at ecosystem scale.

Citation: Braden-Behrens, J., Yan, Y., and Knohl, A.: A new instrument for stable isotope measurements of 13C and 18O in CO2 – Instrument performance and ecological application of the Delta Ray IRIS analyzer, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-120, in review, 2017.
Jelka Braden-Behrens et al.
Jelka Braden-Behrens et al.
Jelka Braden-Behrens et al.

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Short summary
Here we present instrument characteristics and field applicability of the newly developed 'Delta Ray' analyzer for stable isotope measurements in CO2. We used this analyzer to measure the concentration and the isotopic composition of CO2 exchange in a managed beech forest for three months in autumn 2015. During this period an early frost event occured and our measurements suggest that this short event strongly influenced the measured isotopic composition of CO2 exchange.
Here we present instrument characteristics and field applicability of the newly developed...
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