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We used the recently developed commercially available Delta Ray isotope ratio\udinfrared spectrometer (IRIS) to continuously measure the CO2 concentration\udc and its isotopic composition δ13C and δ18O in a\udmanaged beech forest in central Germany. Our objectives are (a) to\udcharacterize the Delta Ray IRIS and evaluate its internal calibration\udprocedure and (b) to quantify the seasonal variability of c,\udδ13C, δ18O and the isotopic composition of nighttime net\udecosystem CO2 exchange (respiration) Reco13C and\udReco18O derived from Keeling plot intercepts. The analyzer's\udminimal Allan deviation (as a measure of precision) was below 0.01 ppm for\udthe CO2 concentration and below 0.03 ‰ for both δ values.\udThe potential accuracy (defined as the 1σ deviation from the\udrespective linear regression that was used for calibration) was approximately\ud0.45 ppm for c, 0.24 ‰ for 13C and 0.3 ‰ for\ud18O. For repeated measurements of a target gas in the field, the\udlong-term standard deviation from the mean was 0.3 ppm for c and below\ud0.3 ‰ for both δ values. We used measurements of nine\uddifferent inlet heights to evaluate the isotopic compositions of nighttime\udnet ecosystem CO2 exchange Reco13C and\udReco18O in a 3-month measurement campaign in a beech\udforest in autumn 2015. During this period, an early snow and frost event\udoccurred, coinciding with a change in the observed characteristics of both\udReco13C and Reco18O. Before the first snow,\udReco13C correlated significantly (p  <  10−4) with\udtime-lagged net radiation Rn, a driver of photosynthesis and\udphotosynthetic discrimination against 13C. This correlation became\udinsignificant (p  >  0.1) for the period after the first snow, indicating a\uddecoupling of δ13C of respiration from recent assimilates. For\ud18O, we measured a decrease of 30 ‰ within 10 days in\udReco18O after the snow event, potentially reflecting the\udinfluence of 18O depleted snow on soil moisture. This decrease was\ud10 times larger than the corresponding decrease in δ18O in ambient\udCO2 (below 3 ‰) and took 3 times longer to recover (3 weeks\udvs. 1 week). In summary, we conclude that (1) the new Delta Ray IRIS with\udits internal calibration procedure provides an opportunity to precisely and\udaccurately measure c, δ13C and δ18O at field sites and\ud(2) even short snow or frost events might have strong effects on the isotopic\udcomposition (in particular 18O) of CO2 exchange on an ecosystem scale

atmospheric measurement techniques

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