Interactive comment on “ First measurements of continuous δ 18 OCO 2 with a Fourier Transform InfraRed spectrometer in Heidelberg , Germany ”

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vital.The interaction between atmosphere, biosphere and ocean must be studied in all its complexity.The stable isotopes in CO 2 can provide information about the fluxes between the different carbon reservoirs. 13CO 2 measurements can be used to distinguish between terrestrial biosphere and marine fluxes (Keeling et al., 1989;Ciais et al., 1995), and are also used as tracer for anthropogenic emissions, as most fossil fuel CO 2 emissions are depleted in 13 C relative to those of the biosphere (Tans, 1981).
The interpretation of atmospheric δ 18 O-CO 2 is more complex since the 18 O-CO 2 cycle is strongly coupled to the water cycle (e.g.Francey and Tans, 1987;Farquhar et al., 1993;Cuntz et al., 2003a, b).During CO 2 exchange with soil and leaves, the 18 O isotopes of CO 2 are exchanged with those of H 2 O (Hesterberg and Siegenthaler, 1991).
The equilibration with leaf water is facilitated by carbonic anhydrase (Gillon and Yakir, 2001).The isotopic composition of soil water is determined by the isotopic composition of precipitation, which itself has strong spatial variations (IAEA/WMO GNIP database available at http://isohis.iaea.org).Since precipitation at higher latitudes is depleted in 18 O, the CO 2 from root respiration and heterotrophic respiration is depleted in 18 O too (Farquhar et al., 1993), but due to 18 O enrichment during evapotranspiration, the plant leaf water is enriched in 18 O relative to the soil water (Farquhar et al., 1993).During photosynthesis, CO 2 equilibrates with enriched leaf water and about 2/3 of the CO 2 retro-diffuses into the atmosphere without being assimilated (Tans, 1998) and therewith enriches the atmospheric δ 18 O-CO 2 value.Also diffusion into and out of the soil may change the atmospheric oxygen isotopic composition (Tans, 1998).Therefore, in order to understand and use the δ 18 O-CO 2 measurements, information about the regional isotopic composition of precipitation, as well as environmental parameters such as temperature and water vapour deficit are important (Ciais et al., 1997;Cuntz et al., 2003a).Only then, it will become possible to understand the 18 O-CO 2 fluxes i.e. from and to the terrestrial biosphere (Yakir and Wang, 1996;Langendörfer et al., 2002).
In order to achieve continuous estimates of the two-way gross biogenic fluxes, continuous isotopic data are necessary.However, respective measurements via mass spectrometry are elaborate and time-consuming, which is why not many continuous records Introduction

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Full of 18 O in CO 2 exist so far (Flanagan et al., 1997;Bowling et al., 2003;Pataki et al., 2003).Quantum cascade laser-based absorption spectrometer measure 12 C 16 O 18 O with a high temporal resolution and provided first continuous records (Tuszon et al., 2011;Sturm et al., 2013).In principle, an in-situ Fourier Transform Infrared (FTIR) spectrometer could also allow to obtain a continuous data set of 12 C 16 O 18 O.The FTIR spectrometer uses molecular absorption in the infrared to analyze the amount of trace gases in an air sample (Griffith et al., 2000).Since the 12 C 16 O 18 O isotopologue absorbs in the infrared as well, it is in principle possible to analyse also the 18 O/ 16 O ratio in CO 2 .
However, in their description of the instrument Esler et al. (2000) have remarked that the degree of precision is too poor for a useful determination in natural abundances using a 1 cm −1 resolution spectrometer.The precision of δ 18 O measurements actually achieved has, however, not been stated in Esler et al. (2000).The reason for the difficulty of resolving  1).Careful analysis is required to distinguish between both of them unambiguously with sufficient accuracy and precision.
Here we provide a first assessment of the δ 18 O-CO 2 (and δ 13 C-CO 2 ) measurements obtained with the Heidelberg FTIR instrument in terms of precision and accuracy.Continuous atmospheric data are compared to conventional mass spectrometric observations to investigate the uncertainty of these measurements.Finally first applications of the highly resolved δ 18 O-CO 2 record (along with the continuous CO 2 , CO and δ 13 C-CO 2 records) are discussed and the question is investigated which insight into regional scale carbon exchange processes may be gained through these isotopologue records.

FTIR measurement principle and calibration procedure
The in-situ FTIR used in Heidelberg has been developed at the University of Wollongong, Australia (Griffith et al., 2012).Details concerning the Heidelberg set-up of the spectrometer can be found in Hammer et al. (2013).Basically, the FTIR obtains Introduction

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Full a transmittance spectrum by taking the ratio of the infrared spectra measured with and without a sample in the optical cell.The transmittance spectrum is then evaluated with a fitting algorithm called MALT (Multi-Layer Absorption Transmittance), which generates a theoretical transmittance spectrum for gas samples with different gas compositions.This is described in more detail by Griffith (1996) and Griffith et al. (2012).
The generated theoretical spectrum is compared to the measured spectrum, and the residual of this comparison is minimised by a non-linear least squares fitting algorithm, varying the trace gas composition and instrument parameters in the theoretical spectrum.The raw trace gas concentration can then be derived.

Data evaluation and calibration
In the following, the data evaluation and calibration procedure is described for the isotopologue ratio δ 18 O-CO 2 , but it is done in an analogous way for δ 13 C-CO 2 .
Step 1: calculate the raw δ 18 O value from FTIR measurements The FTIR computes the raw δ 18 O value as the ratio of the raw value of the rare isotopologue and the raw value of the common isotopologue: with R HITRAN = 0.00401126.Note that we here use the expression "Molec-δ 18 O" since commonly the δ-notation is defined with isotope ratios rather than isotopologue ratios.The FTIR implicitly uses the HITRAN scale (Rothman et al., 2005), which is referred to VPDB for δ 13 C and to VSMOW for δ 18 O; but during the calibration (step 3) the final reference scale of the calibrated data can be changed to any other scale.Full Step 2: cross-sensitivity and inter-species interference corrections Even though the software MALT takes into account influences from pressure, temperature and inter-species overlapping absorption bands, residual sensitivity corrections of certain parameters and species need to be performed (see Hammer et al., 2013).A cross-sensitivity correction for sample temperature and pressure, H 2 O abundance and flow rate, as well as an interspecies-sensitivity correction for CO 2 mole fraction is done for every measurement.The sensitivity of Molec-δ 18 O-CO 2 and Molec-δ 13 C-CO 2 to CO 2 are shown in Fig. 2. The set-up of the experiment to determine this sensitivity is described in detail by Hammer et al. (2013).A cubic fit is used to describe the CO 2 interspecies correction.The residual sum of squares between the measurement and the cubic fit divided by the number of measurements is ±0.001 ‰ for δ 13 C and ±0.041 ‰ for δ 18 O with residuals showing no further concentration dependence (Fig. 2b and d).
Step 3: calibration The cross-sensitivity corrected data is calibrated on the VPDB gas scale using a linear instrument response function (typically linear to the degree of R = 0.9999).The calibration (response function) is derived from three reference tanks with known values for CO 2 , δ 13 C-CO 2 and δ 18 O-CO 2 .This response function is determined weekly.Our reference standards span ranges from about 370 µmol mol −1 to 470 µmol mol −1 for CO 2 mole fraction, a δ 13 C VPDB range from −8.7 ‰ to −12.8 ‰ and a δ 18 O VPDB range from −1.9 ‰ to −5.0 ‰ (as determined by the Heidelberg mass spectrometer (MS)).For details on the MS, see Neubert (1998).
Step 4: smoothed working standard correction We have found that the measurement of different cylinder gases on the FTIR show very similar sub-weekly variations of δ 18 O-CO 2 .One can thus use a smoothed working standard correction in order to account for these small instrumental variations Introduction

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Full on a sub-weekly time scale.For this purpose, daily working gas measurements are smoothed and the smoothed residual variations from the long-term mean value of this gas vs. the reference standards are subtracted from all measurements.By performing this correction, the standard deviation of a weekly measured target or surveillance gas reaches about 0.2 ‰.
Step 4 is not obligatory, but could further increase the precision of the measurement.In the data presented in Sect. 4 we have applied a smoothed working standard correction.Figure 3 illustrates the application of the calibration procedure.For δ 13 C, Fig. 3 shows raw (a), corrected (b) and calibrated (c) FTIR measurements against MS reference values of δ 13 C respectively, and Fig. 3d shows the difference between calibrated FTIR measurements and MS values (FTIR-MS) against CO 2 mole fractions.Figure 3e-h show corresponding data for δ 18 O.The cross-sensitivity correction forces Molec-δ 13 C and Molec-δ 18 O on a linear regression line (Fig. 3b and f), so that a linear calibration can then be applied.

Using CO 2total instead of 12 C 16 O 2 in Eq. (1)
Total raw CO 2 mole fraction is retrieved from absorption bands near 3600 cm −1 . 12CO 2 , 13 CO 2 as well as 12 C 16 O 18 O absorb in this region but the minor isotopologue absorptions are weak and are hardly distinguishable.In the stronger absorption band near 2300 cm −1 (see Fig. 1) the CO 2 isotopologues have distinguishable absorption regions and structures and can therefore be obtained individually.The precision of total CO 2 in the 3600 cm −1 range is, however, about 50 % higher than that of 12 CO 2 in the region of 2300 cm −1 , due to an optimum absorption and a lower temperature sensitivity.
Therefore, the raw Molec-δ 18 O and Molec-δ 13 C values are calculated using total CO 2 , instead of 12 CO 2 .As long as the isotopic composition of the sample is close to the isotopic composition of the reference standards, the bias in δ 18 O and δ 13 C is negligible Introduction

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Full (< 0.03 ‰ for δ 13 C and < 0.05 ‰ for δ 18 O) after calibration (step 3).For much depleted cylinder gases, as may be the case for synthetic gas mixtures, the biases may, however, become as large as 0.2 ‰.

Absolute calibration
The described method of calibration (step 3) is an empirical one (Griffith et al., 2012).
In principle, it is equally valid to use the "absolute calibration method".In this case, the isotopologues  (Allison et al., 1995), which takes into account that CO 2 contains two oxygen atoms.In Sect.5, we will briefly discuss possible advantages or disadvantages of both methods.

Direct cylinder comparison to mass spectrometric values
In order to check the calibration as well as the compatibility of the

Reproducibility
The reproducibility can be estimated and monitored by measuring standard gases every day or week under reproducible conditions.The averaging time for cylinder measurements is 9 min.The standard deviation of the measured gases is a good measure of estimating the reproducibility of our measurements.For δ 18 O-CO 2 , the daily working cylinder reproducibility was found to be 0.27 ‰ for the period from December 2012 to October 2013.The reproducibility for δ 13 C-CO 2 was 0.04 ‰ (see Fig. 5a and b).Thus, the reproducibility is dominated by the repeatability, suggesting a very good long-term stability of the instrumental setup.The reproducibility of the δ 18 O-CO 2 measurements is much (ca.1/5th) smaller than the diurnal ambient air variability, at least for the measurement site in Heidelberg (see Fig. 7).Note that in our calibration procedure we now use the daily measured cylinder (working gas) in a final correction step (step 4) to account for sub-weekly variations of the Introduction

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Full instrument response.Since we only recognized the need to correct for this variability well after commence of the measurements, we do not yet have a long-term record for a real target.Therefore, Fig. 5 displays the working standard measurements without any sub-weekly smoothing applied, which is an upper estimate of the reproducibility of real measurements where step 4 of our calibration procedure is applied in addition.

Compatibility of ambient air measurements
We have shown in Sect.2.3 that the FTIR measures cylinders with high compatibility to the Heidelberg mass spectrometer.In order to show that there is no bias introduced when measuring ambient air, a so-called diurnal cycle "event" was evaluated.An automated flask sampler (Neubert et al., 2004)  In this section, we will give a short prospect on how to potentially use a highly resolved δ 18 O-CO 2 record in an area such as Heidelberg in order to disentangle regional scale carbon exchange processes.For this purpose, we look at two very different periods in which the FTIR measured δ 18 O along with δ 13 C in CO 2 , total CO 2 and CO in Heidelberg (see Fig. 7).
In order to interpret the atmospheric δ 18 O-CO 2 variation, the isotopic signature of the processes influencing the isotopic content must be known.In the Heidelberg catchment area, the most important CO 2 fluxes are associated with plant photosynthesis, leaf and soil respiration as well as fossil fuel burning.Discrimination during photosynthesis tends to enrich atmospheric CO 2 with respect to 18 O and 13 C.Typical mean δ 13 C fractionation relative to the atmosphere during photosynthesis is about -(2-8) ‰ for C4 plants and about -(12-20) ‰ for C3 plants (Mook, 1994).During respiration, at first approximation, the 13 CO 2 / 12 CO 2 ratio captured during photosynthesis is also released, which leads to a depletion of the atmospheric 13 CO 2 / 12 CO 2 ratio.In addition, 18 O discrimination during respiration tends to deplete the atmosphere in its  Farquhar et al., 1993;Cuntz et al., 2003b).
Further, the δ 18 O source signature will be influenced also by the invasion flux (Tans, 1998;Miller et al., 1999), which occurs when CO 2 diffuses into the soil, partially equilibrates with soil water and retro-diffuses into the atmosphere.Seibt et al. (2006) have Introduction

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Full shown that invasion fluxes can lead to more depleted apparent δ 18 O soil respiration source signatures in presence of carbonic anhydrase.However Wingate et al. (2008) found more enriched apparent δ 18 O soil respiration source signatures in presence of carbonic anhydrase.Miller et al. (1999) found that in most settings and especially in dry ground and for short residence times (corresponding to high boundary layer mixing heights), the effect will be rather small (< 5 ‰).The magnitude of this effect for our catchment area cannot be quantified without intensive sampling and isotopic soil flux modelling and thus, it will only be considered in a sense that a larger uncertainty must be attributed to the soil respiration flux.(Widory and Javoy, 2003;Kaul, 2007).With these landmarks of isotopic signatures, we can now look at the Heidelberg atmospheric CO 2 records that show values around δ 18 O VPDB of −2 ‰ to −4 ‰ in winter and 0 ‰ to −2 ‰ in summer while δ 13 C-CO 2 varies between −8 ‰ and −12 ‰ (Fig. 7).
Since all CO 2 sources with a negative isotopic signature compared to atmospheric CO 2 lead to δ 13 C-CO 2 or δ 18 O-CO 2 depletion, a differentiation between different depleting sources is difficult.Therefore we used the following approach: we first constructed an artificial δ 13 C-CO 2 and δ 18 O-CO 2 record using the slope and offset of so called "Keeling plots" (Keeling, 1958) Note that in the nighttime reference periods, for which the reference slope and offsets were calculated, we can neglect any photosynthetic sinks.Therefore, the δ 13 C source signature of the reference period can be interpreted as the flux-weighted average of all sources (Miller and Tans, 2003).We then applied the parameters from the reference period to the entire CO 2 record to calculate artificially constructed δ 13 C constr and δ 18 O constr values over time (t).
The constructed δ 13 C and δ 18 O records are shown in burgundy and light blue in Fig. 7a, b, g and h.During the reference period, in which the Keeling plot slopes and offsets were derived, the Keeling plot had a high correlation (r 2 > 0.85) and showed an isotopic 13 C and 18 O signature, which was typical for the respective season (δ 13 C-CO 2 ≈ −25 ‰ in the winter and −27 ‰ in the summer period, δ 18 O-CO 2 ≈ −28 ‰ in the winter and −12 ‰ in the summer period).To identify enriching or depleting sources and sinks relative to the reference period, we then calculated the difference between the real measured and the artificially constructed (Eq.4) δ 13 C and δ 18 O record (Fig. 7e, k, f and l).
Negative ∆δ values occur in periods when the apparent sources are more depleted than in the reference period and positive values occur when apparent sources are more enriched than in the reference period.During photosynthetic CO 2 uptake the equilibration of back-diffusing CO 2 with enriched leaf water leads to an enrichment of atmospheric δ 18 O and thus to positive ∆δ 18 O values.We, thus, now have a tool at hand, which allows differentiating between more and less depleted fluxes relative to the reference period.

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Full In the wintertime, relative fossil fuel contributions in the Heidelberg catchment area are higher than in the summer time (Levin et al., 2003).Fossil fuel CO 2 emissions lead to high concentration of CO 2 (Fig. 7d) and deplete atmospheric CO 2 in its heavy isotopes 13 C and 18 O (original measurements: dark blue and red in Fig. 7a and b).During incomplete combustion of fossil fuels, CO (Fig. 7c) is often emitted as well.A typical example of a pollution event can be seen in Fig. 7 (left panel) on 21 December 2012.The difference between the measured and artificially constructed δ 13 C (Fig. 7e) decreases rapidly on the 21 December, indicating a source (mix) which is more depleted in 13 C than during the reference period (δ 13 C ref = −25 ‰).The strong influence of a more 13 C depleted source mix points towards a high contribution from fossil fuel sources, including domestic heating (natural gas).At the same time the isotopic signature of δ 18 O is very close to the isotopic signature during the reference period (−28 ‰) and increases during the pollution event.The different behavior of δ 13 C and δ 18 O in CO 2 points towards a larger influence from traffic or natural gas combustion, as both sources are slightly more enriched in 18 O, but less enriched in 13 C with respect to coal-fired combustion (Schumacher et al., 2011).One can see that the fact that different fossil fuel types influence both stable isotopes 13 C and 18 O in CO 2 in a different way can be used to differentiate between different emission groups in situations when biogenic fluxes are low (i.e. in winter).However, for a quantitative analysis the exact isotopic signatures of all fluxes in the area of influence must be known.
In the summer time, we expect biosphere fluxes to be much larger than during winter and at the same time fossil fuel (especially residential heating) emissions to be smaller than in winter.In fact, we do not find large deviations in δ 13 CO 2 from those determined in the reference period (−27 ‰), pointing towards a relatively constant mixture of biogenic and fossil fuel emissions.On the other hand, the measured δ 18 O decreases rapidly on the 3 July compared to the reference period with a source isotopic signature of ≈ −12 ‰.This decrease is not accompanied with changes of any other tracer, such as CO, δ 13 CO 2 or CO 2 .A possible explanation for the decrease is a change in the hydrological conditions.After four dry days, a sudden heavy rain occurred in Heidelberg

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Full on the 3 July (see dashed bar in Fig. 7, right panel).The rainfall replenished the water reservoirs and a new equilibrium value between the soil and leaf water reservoirs and CO 2 most probably caused the atmospheric δ 18 O-CO 2 to become depleted relative to the reference period.This example shows how closely coupled δ 18 O in the water and carbon cycle are.In order to quantitatively use the δ 18 O-CO 2 records it is thus crucial to study also the hydrological conditions, such as precipitation and its isotopic signature.

Instrumental challenges
In Sect. 2 we have described two possible evaluation and calibration methods; the absolute (Sect.2.2.2) and the empirical method (Sect.2.1).Even though in principle both methods should lead to the same results, they differ slightly by about 0.11±0.03‰ for δ 13 C (mean ± standard deviation for a two-monthly period in 2014) and by 0.08 ± 0.15 ‰ for δ 18 O.The discrepancy between both calibration methods is most likely due to small inaccuracies in interspecies interference corrections.The empirical calibration requires a large CO 2 -cross-sensitivity correction over a large CO 2 range (see Fig. 2c).
Only if the CO 2 interspecies-sensitivity correction is applied with high accuracy, reliable δ 18 O can be obtained.For the absolute calibration, no CO 2 correction is required, but a very accurate determination of all CO 2 isotopologue calibration equations is vital.Small errors in the inter-species interference corrections of the CO 2 isotopologues can lead to large errors in the δ 13 C and δ 18 O values.The decision on which method to use should thus be based on which correction can be performed with higher accuracy.In this work we have found for the Heidelberg spectrometer that the empirical calibration method better fits the Heidelberg mass spectrometer values.Introduction

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Full For δ 13 C, the different carbon sources and sinks are relatively well understood, but for δ 18 O, high temporal and spatial variability of the respiratory and photosynthetic fluxes (due to a strong variation of environmental parameters such as temperature, humidity and geographical position) makes it difficult to separate the different CO 2 fluxes.For our qualitative study, we could use observations from Neubert (1998) in the catchment area of Heidelberg, as well as globally resolved model data for assimilation isofluxes from Cuntz et al. (2003b).However, for a quantitative apportionment of the CO 2 fluxes at a high temporal resolution, sampling of the isotopic content of precipitation, soil respiration and foliage gas exchange in the catchment area will be necessary on similarly high temporal resolution (Stern et al., 1999).Further, invasion flux models are required to quantify the effect of this process at the measurement site.All these unknowns largely limit current applicability of our new continuous isotope measurements.Only future sophisticated regional models of the water and the carbon cycle could fully exploit the wealth of new information now available.

Summary and conclusion
The accurate analysis of δ 18 O in CO 2 using FTIR spectrometry is novel.The CO 18 O measurements of the FTIR in Heidelberg were evaluated with regard to long-term stability, reproducibility and accuracy.We found that the reproducibility of daily working standard gas measurements, averaged over nine minutes, is δ targets (WMO, 2009), a number of interesting scientific applications seem possible using a FTIR.In particular, investigation of processes that govern the δ 18 O variability of atmospheric CO 2 on the regional scale seem very promising.However, quantitative analysis requires comprehensive knowledge on the isotopic signature of different CO 2 sources and sinks as well as of the influencing water reservoirs.Introduction

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Full

First
order to study the impact and fate of increasing anthropogenic CO 2 emissions to the atmosphere, quantitative understanding of the processes in the carbon cycle is Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | FTIR and the Heidelberg mass spectrometer (MS), different test cylinders were measured in March and April 2014 on both instruments and values were compared.The Heidelberg mass spectrometric values are linked to the VPDB scale via three pure CO 2 reference gases (RM8562, RM8563, RM8564).The FTIR reference cylinders were calibrated by the Heidelberg mass spectrometer (MS) and thus the FTIR and the MS are on the same scale.For all cylinder measurements with the mass spectrometer, cylinder air was filled into evacuated flasks, which were analysed like regular flask samples, since pressure regulator effects often disturb the MS analyses.Discussion Paper | Discussion Paper | Discussion Paper | 3 Characterization of δ 18 O-CO 2 and δ 13 C-CO 2 measurements with the Heidelberg FTIR 3.1 Allan deviation An Allan deviation test was performed on the FTIR system on the 5/6 August 2013 with a reference gas cylinder with a δ 13 C value of −9.96 ± 0.07 ‰ and a δ 18 O value of −1.24 ± 0.25 ‰.The Allan deviation will be used as a measure for the repeatability.It can be seen in Fig. 4a and b that the Allan deviations after a three minute measurement are about δ 13 C = ±0.05‰ and δ 18 O = ±0.45‰.After 10 min, they decrease to δ 13 C = ±0.025‰ and δ 18 O = ±0.25 ‰ and after 30 min, the Allan deviation reaches δ 13 C = ±0.015‰ and δ 18 O = ±0.15‰.In Heidelberg, a typical diurnal variation of δ 18 O-CO 2 is in the order of 1 ‰; after about 30 min integration time the repeatability is thus typically 15 % of this diurnal signal and should therefore be good enough to significantly distinguish ambient δ 18 O-CO 2 signals (see Sect. 4 and Fig. 7).
Discussion Paper | Discussion Paper | Discussion Paper | collected and dried (dew point −40 • C) ambient air from the same intake line as the FTIR into 2 L glass flasks.Every flask was flushed for two hours and then pressurized to 2 bar absolute pressure and closed.Then the next flask was opened, flushed and filled to 2 bar.Pressurizing the flasks takes about 5 min.With this procedure, a diurnal isotopic profile with a two-hourly resolution could be captured in the flasks and analyzed by mass spectrometry.These values were then compared to the nine-minutely averaged values from the FTIR spectrometer (see Fig.6).The averaging was made to account for atmospheric variability and a possible time asynchrony between the event sampler and the FTIR, but also to reduce the noise on the FTIR measurement.δ 18O agrees well within its measurement uncertainty.The mean residual and standard error for δ 18 O is 0.08±0.14‰, which shows that the FTIR succeeds in measuring 18 O in CO 2 with good accuracy.Thus, a high compatibility of the 18 O isotope measurements of better than 0.1 ‰ can be confirmed.The larger discrepancy (and variability) in δ 18 O than in direct cylinder gas comparisons (Sect.2.3) reflects the fact that there are more components contributing to the difference between the FTIR and the MS value, such as the flasks itself, which could be slightly wet and then change the δ 18 O value of the CO 2 , or some other possible interference of the automated flask sampler.For δ 13 C, the mean residual is 0.01 ± 0.02 ‰.
Discussion Paper | Discussion Paper | Discussion Paper | determined from measured values in an exemplary and short night time reference period (ref) (gray bars in Fig. 7) according to: Discussion Paper | Discussion Paper | Discussion Paper | δ meas (ref) = slope(ref) × 1/CO 2 (ref) + offset(ref) Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 5.2 Quantitative interpretation of continuous δ 13 C and δ 18 O record 18 O = ±0.27‰.The Allan deviation of three-minutely δ 18 O-measurements was determined to about ±0.45 ‰ and it decreases to ±0.15 ‰ for hourly averages.Depending on the magnitude of atmospheric variability and on the field of application, it could be worthwhile to aggregate three-minutely δ 18 O measurements to half-or one-hourly averages.Even then, a much higher temporal resolution than provided by mass-spectrometric analyses can be achieved.Even though the FTIR precision does not reach the WMO ILC Discussion Paper | Discussion Paper | Discussion Paper |

Figure 3 .Figure 4 .
Figure 3. (a) Raw, (b) cross-and interspecies corrected (but still un-calibrated) and (c) calibrated δ 13 C measurements and (e) raw, (f) cross-and interspecies corrected (but still uncalibrated) (g) calibrated δ 18 O measurements of different target cylinders against the massspectrometric measurement of the same cylinders.Lowest panels: (d) Calibrated FTIR δ 13 C value minus reference value measured by the Heidelberg mass spectrometer, (h) same as (d) for δ 18 O, both plotted vs. the CO 2 mole fraction of the samples.The red lines in the lowest panels give the mean difference between the FTIR-measured values and the mass spectrometer and the grey areas show the standard deviation of the differences.

Figure 6 .Figure 7 .
Figure 6.Diurnal cycle event sampled on the 3/4 March 2014 at the Institut für Umweltphysik in Heidelberg.Red: GC concentration (in case of CO 2 ) or mass spectrometric isotopologue value (in case of isotopologues) of flasks samples, blue: 9 minly averaged values from FTIR, black: continuous 3 min values from the FTIR, (a) CO 2 mole fraction, (b) δ 13 C-CO 2 value (c) residual of 9-minuetly averaged FTIR δ 13 C and mass spectrometric measurement (FTIR -mass spectrometer), (d) δ 18 O-CO 2 value, (e) residual of 9-minuetly averaged FTIR δ 18 O and mass spectrometric measurement (FTIR -mass spectrometer).All error bars on the (blue) averaged FTIR data are the standard deviation during the 9 min of averaging time.The error bars on the (red) mass spectrometric values show the typical reproducibility of our mass spectrometric measurements.The residual (FTIR-MS) has an error bar which combines the mass spectrometric and the FTIR uncertainties.In general, the FTIR reproduces the mass spectrometric values very well.The uncertainty given here combines the variability of atmospheric signal during the flask filling time as well as the reproducibility of the mass spectrometer and the FTIR measurements.

Figure 7 .
Figure 7. Trace gas records in winter (left panel) and summer (right panel) in Heidelberg.(a) and (g) show the measured (dark blue) and artificially constructed (light blue) δ 13 C value, (b) and (h) the measured (red) and artificially constructed (burgundy) δ 18 O value, (c) and (i) the measured CO value, (d) and (j) the measured CO 2 value.Panels (e) and (k) give the difference between the measured and constructed δ 13 C value with a mean isotopic signature of δ 13 C ≈ −25 ‰ in the wintertime and δ 13 C ≈ −27 ‰ in the summer time.(f) and (l) give the difference between the measured and constructed δ 18 O value with a mean isotopic signature of δ 18 O VPDB ≈ −28 ‰ in the wintertime and δ 18 O VPDB ≈ −12 ‰ in the summer time.Gray vertical bars indicate the "reference periods", in which the isotopic signature for artificially constructed δ 13 C and δ 18 O was determined.The dashed vertical bar in the right panel shows a period of high precipitation.Gray horizontal bars in (f) and (l) mark the 1σ-uncertainty of the isotope measurements.