A new instrument for stable isotope measurements of <sup>13</sup>C and <sup>18</sup>O in CO<sub>2</sub> – instrument performance and ecological application of the Delta Ray IRIS analyzer

Abstract

Abstract. We used the recently developed commercially available Delta Ray isotope ratio infrared spectrometer (IRIS) to continuously measure the CO 2 concentration c and its isotopic composition δ 13 C and δ 18 O in a managed beech forest in central Germany. Our objectives are (a) to characterize the Delta Ray IRIS and evaluate its internal calibration procedure and (b) to quantify the seasonal variability of c, δ 13 C, δ 18 O and the isotopic composition of nighttime net ecosystem CO 2 exchange (respiration) R 13 eco C and R 18 eco O derived from Keeling plot intercepts. The analyzer's minimal Allan deviation (as a measure of precision) was below 0.01 ppm for the CO 2 concentration and below 0.03 ‰ for both δ values. The potential accuracy (defined as the 1σ deviation from the respective linear regression that was used for calibration) was approximately 0.45 ppm for c, 0.24 ‰ for 13 C and 0.3 ‰ for 18 O. For repeated measurements of a target gas in the field, the long-term standard deviation from the mean was 0.3 ppm for c and below 0.3 ‰ for both δ values. We used measurements of nine different inlet heights to evaluate the isotopic compositions of nighttime net ecosystem CO 2 exchange R 13 eco C and R 18 eco O in a 3-month measurement campaign in a beech forest in autumn 2015. During this period, an early snow and frost event occurred, coinciding with a change in the observed characteristics of both R 13 eco C and R 18 eco O. Before the first snow, R 13 eco C correlated significantly (p < 10 −4 ) with time-lagged net radiation R n , a driver of photosynthesis and photosynthetic discrimination against 13 C. This correlation became insignificant (p > 0.1) for the period after the first snow, indicating a decoupling of δ 13 C of respiration from recent assimilates. For 18 O, we measured a decrease of 30 ‰ within 10 days in R 18 eco O after the snow event, potentially reflecting the influence of 18 O depleted snow on soil moisture. This decrease was 10 times larger than the corresponding decrease in δ 18 O in ambient CO 2 (below 3 ‰) and took 3 times longer to recover (3 weeks vs. 1 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, δ 13 C and δ 18 O at field sites and (2) even short snow or frost events might have strong effects on the isotopic composition (in particular 18 O) of CO 2 exchange on an ecosystem scale.

Calibration of isotopologue-specific optical trace gas analysers: a practical guide

“…Isotope ratios for standard or reference materials are assigned by the isotope metrology community, (e.g. Allison et al, 1995;Brand et al, 2010;Werner and Brand, 2001). Isotope ratios are commonly expressed as delta values relative to a standard or reference material:…”

“…For a calibration or reference gas, δ 13 C and δ 18 O are usually provided by calibration laboratories, and δ 17 O can normally be deduced from δ 18 O, assuming mass dependent fractionation of oxygen isotopes with negligible error (Brand et al, 2010):…”

Combining a Quantum Cascade Laser Spectrometer with an Automated Closed-Chamber System for δ13C Measurements of Forest Soil, Tree Stem and Tree Root CO2 Fluxes

“…Direct comparison of our results with previous studies is not possible because no one else has previously combined an Aerodyne CO 2 isotope QCL system with the LI8100A/LI8150 system for automated closed-chamber measurements. However, the issues of dependence of water vapour and especially CO 2 concentration on measured δ 13 C have been addressed in a few studies using laser spectroscopy methods, including quantum cascade lasers [19,[23][24][25]34]. In a study where δ 13 C was measured with a quantum cascade laser spectrometer, [21] performed 90 closed-chamber measurements and determined the CO 2 concentration dependency but did not report the effect of this calibration on the raw δ 13 C values or the effect on the δ 13 C determined by the Keeling plots.…”

Calibration of isotopologue-specific optical trace gas analysers: A practical guide

“…Several studies have addressed isotopic calibration (e.g. Esler et al, 2000;Bowling et al, 2003;Griffis et al, 2005;Mohn et al, 2008;Loh et al, 2011;Tuzson et al, 2011;Griffith et al, 2012;Wen et al, 2013;Rella et al, 2015;Vardag et al, 2015;Pang et al, 2016;Flores et al, 2017;Tans et al, 2017;Braden-Behrens et al, 2017) and compared calibration approaches (Wen et al, 2013), but until recently most studies made some level of approximation in dealing with the calculations required to properly include the contributions of all possible isotopologues of the target species in the calculation scheme. Most recently Griffith et al (2012), Flores et al (2017) and Tans et al (2017) have published isotopic calibration strategies which are equivalent and which correctly and completely account for the full isotopic composition of the target gas (CO2 in these studies, but applicable in principle to any species).…”

“…Several groups have reported on ratio calibration schemes and the consequent concentration dependence (e.g. Griffith et al, 2012;Wen et al, 2013;Rella et al, 2015;Pang et al, 2016;Braden-Behrens et al, 2017;Flores et al, 2017). The concentration dependence inevitably follows if the actual calibration relationships between measured and true amounts of individual isotopologues (section 4.1, Eq.…”