Introduction

AMT

Atmospheric Measurement Techniques

AMT

Atmos. Meas. Tech.

1867-8548

Copernicus Publications

Göttingen, Germany

10.5194/amt-9-5607-2016

Stratospheric Air Sub-sampler (SAS) and its application to analysis of Δ17O(CO2) from small air samples collected with an AirCore

Mrozek

Dorota Janina

dorota.mrozek@gmail.com van der Veen

Carina

Hofmann

Magdalena E. G.

Chen

Huilin

https://orcid.org/0000-0002-1573-6673

Kivi

Rigel

https://orcid.org/0000-0001-8828-2759

Heikkinen

Pauli

Röckmann

Thomas

https://orcid.org/0000-0002-6688-8968

1Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht University, Princetonplein 5, 3584CC Utrecht, the Netherlands 2Centre for Isotope Research (CIO), Energy and Sustainability Research Institute Groningen, University of Groningen, the Netherlands 3Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA 4Finnish Meteorological Institute (FMI), Arctic Research, Sodankyä, Finland

Dorota Janina Mrozek (dorota.mrozek@gmail.com)

25November2016

9 11 56075620 9April2016 15July2016 8October2016 18October2016

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We present the set-up and a scientific application of the Stratospheric Air Sub-sampler (SAS), a device to collect and to store the vertical profile of air collected with an AirCore in numerous sub-samples for later analysis in the laboratory. The SAS described here is a 20 m long 1/4 inch stainless steel tubing that is separated by eleven valves to divide the tubing into 10 identical segments, but it can be easily adapted to collect smaller or larger samples. In the collection phase the SAS is directly connected to the outlet of an optical analyzer that measures the mole fractions of CO2, CH4 and CO from an AirCore sampler. The stratospheric part (or if desired any part of the AirCore air) is then directed through the SAS. When the SAS is filled with the selected air, the valves are closed and the vertical profile is maintained in the different segments of the SAS. The segments can later be analysed to retrieve vertical profiles of other trace gas signatures that require slower instrumentation. As an application, we describe the coupling of the SAS to an analytical system to determine the 17O excess of CO2, which is a tracer for photochemical processing of stratospheric air. For this purpose the analytical system described by was adapted for analysis of air directly from the SAS. The performance of the coupled system is demonstrated for a set of air samples from an AirCore flight in November 2014 near Sodankylä, Finland. The standard error for a 25 mL air sample at stratospheric CO2 mole fraction is 0.56 ‰ (1σ) for δ17O and 0.03 ‰ (1σ) for both δ18O and δ13C. Measured Δ17O(CO2) values show a clear correlation with N2O in agreement with already published data.

Introduction

Monitoring and studying the distribution of greenhouse gases throughout the atmospheric column is an important constituent of understanding contemporary climate change. Carbon dioxide (CO2) is the most important molecule of the atmospheric carbon cycle and the increase of its abundance in the atmosphere is the primary factor of recent radiative forcing . In the stratosphere the 17O excess of CO2 (expressed as Δ17O(CO2)) is a valuable long-lived tracer for stratospheric chemistry and atmospheric circulation patterns. Measurement of the mole fraction and oxygen isotopic composition of CO2 provides information on both transport times and photochemical lifetimes of CO2 . Over the past decades several sampling campaigns have been carried out to observe and to understand the oxygen isotope enrichments of stratospheric CO2 . Unfortunately, measurement campaigns with rockets, balloons and aircraft platforms are expensive and therefore it is in general difficult to obtain stratospheric samples.

The simple and lightweight sampling system AirCore provides new opportunities to sampling the high altitude atmosphere at relatively low cost. The AirCore device consists of a long (usually 100 m of longer) piece of coiled stainless steel tubing that is lifted to the stratosphere on a balloon with one end open and the other end closed. During ascent, the AirCore empties because of the decrease in pressure; during descent, ambient air successively fills the AirCore coil again as pressure increases. The atmospheric profile information in the coil is preserved because of limited gas diffusion inside the long tube. This means that the altitude profiles of various trace gases such as CO2, CH4 and CO can be determined by processing the content of the AirCore through a fast analytical system quickly after recovery of the sampler.

Previously, the air from an AirCore was vented after analysis with a real time gas analyzer and not used for other, more sophisticated and slower, analyses. We developed a Stratospheric Air Sub-sampler (SAS) that collects (the stratospheric fraction of) air from an AirCore directly after online analysis and stores it in different segments of the SAS so that the profile is preserved. The SAS can be easily transported and processed, for example for the isotopic composition of trace gases or for halocarbon analysis. The limitation of the method is that the analytical system must be capable of analysing very small air samples, since the total stratospheric fraction of the AirCore profile is only of the order of 250 mL at ambient temperature and pressure, which is split into multiple segments in the SAS.

For the work presented here we apply the SAS concept to measurement of the 17O excess of CO2 in the stratospheric sub-samples. The analytical system described in was modified to allow air from the SAS to be flushed directly by the reference air into a new sample introduction unit at ambient pressures. The change of the oxidation reagent from CeO2 powder to CuO wires reduced peak broadening and allowed detection of the equilibrated CO2 peak without focusing on liquid nitrogen trap. The improved system is fully automated; only the change of the individual SAS segments is made manually. The successful coupling of the SAS and the 17O analysis system was demonstrated by CO2 stable isotope measurements on stratospheric air obtained from an AirCore flight near Sodankylä, Finland, in November 2014.

A schematic diagram showing the overall procedure described in this work, from AirCore sampling on the site to IRMS analysis in laboratory.

Schematic diagram of the analytical system for trace gas analyses with a Picarro instrument and for transferring the air from the AirCore coil into the Stratospheric Air Sub-Sampler (SAS). The three-way valves in the SAS are in “open to the right” position when the AirCore air is transferred into the SAS and closed after sub-sampling. The crossed circles are conventional valves. The open circles at the inlet of the fill-gas and the calibration-gas cylinder represent cylinder valves and pressure regulators. The black circle represents the shut-off valve at the end of the AirCore coil. The drying tube is filled with magnesium perchlorate (Mg(ClO4)2) from Sigma-Aldrich.

We use the common delta notation to quantify isotopic composition, iδ = iRSA/iRST-1, where iR represents the heavy-to-light isotope ratio 17R = (17O / 16O) or 18R = (18O / 16O) of a sample (index SA) or international standard (index ST). The δ values are expressed in ‰. The international reference material for oxygen isotopes is Vienna Standard Mean Ocean Water (VSMOW) with 17RVSMOW = 382.7-2.1+1.7 × 10-6 and 18RVSMOW = 2005.2 ± 0.45 × 10-6 . For quantifying the 17O excess of CO2 we use the exponential definition Δ17O = [1 + δ17O] / [1 + δ18O]λ-1 with λ of 0.528, but note that also other definitions are in use .

Method

The stratospheric air samples are collected in a two-step procedure. First, air samples from the surface up to the ceiling altitude of a stratospheric balloon flight (typically 30 km) are obtained with an AirCore system. Second, the stratospheric part of the collected air samples is recovered into an SAS after online analysis of trace gas concentrations. The details of the first step are here briefly described in Sect. . Section and provide the information relevant to the stratospheric air samples: the sub-sampling method and the description of the analytical system used for CO2 isotope measurement. We include a schematic illustration for describing all the instruments and procedure in Fig. .

Stratospheric air sampling with AirCore

The AirCore device is made of stainless steel tubing coated with SilcoNert^®1000 and has a total length of 100 m (40 m at 1/4 inch and 60 m at 1/8 inch, with a wall thickness of 0.01 inch). The payload that includes the AirCore and a radiosonde (Vaisala, type RS92-SGPL) weighs about 3.6 kg. Before each flight the AirCore is filled with a standard dry fill-gas with known CO2, CH4 and CO mole fractions (CO2 = 386.10 ± 0.09 ppm; CH4 = 1880 ± 2 ppb; CO = 7972 ± 5 ppb) and closed with a shut-off valve at the inlet (Swagelok, part number SS-1GS4). A stratospheric balloon (Totex, type Tx3000) is used to launch the payload high into the stratosphere near Sodankylä, Finland. Just before launching the AirCore, the shut-off valve is opened, so that during the ascent the fill-gas leaves the AirCore coil due to the drop in pressure. After reaching an altitude of approximately 30 km, the balloon bursts, and the descent of the payload on a parachute begins. Ambient air flows into the AirCore and the air from higher altitudes is continuously compressed and pushed towards the closed end of the AirCore by air from lower altitudes. The shut-off valve is closed automatically about 10 s after the landing, and the AirCore is quickly recovered and transported to the Finnish Meteorological Institute (FMI) laboratory for analysis.

Sub-sampling into SAS

The vertical profiles of CO2, CH4 and CO mole fractions are measured within 2–3 h after landing of the AirCore at FMI using a gas analyzer (Picarro, model G2401), as presented in Fig. . The originally closed end of the AirCore coil is connected to the analyzer, while the end with the shut-off valve is connected to the fill-gas cylinder. By opening both ends of the AirCore, the air in the AirCore is flushed into the analyzer by the fill-gas. The flow rate is controlled at the outlet of Picarro instrument and set to 38.2 mL min-1. Calibration air can be analysed before and after measurement of the AirCore air. After the trace gas analysis, the top (i.e. stratospheric) part of the air collected with the AirCore (less than 20 % of the total collected air) is transferred into an SAS that is sent to Utrecht University for CO2 isotope analysis.

The timing of the sub-sampling process is based on the flow rate of the air through the analyzer and the pump. The timing of the sub-sampling procedure has to be precisely established to ensure that the desired fraction of gas is collected with the SAS. To establish the timing for the sub-sampling procedure, we injected a spike of highly CO2-enriched gas into the inlet of the Picarro analyzer and then connected the outlet of the pump to the inlet of the Picarro analyzer. Thus a closed loop without the SAS was established, and the CO2 spike was measured multiple times as it circulated through the analyzer and the pump. This experiment allowed us to determine the timing of air travelling from the inlet of the analyzer to the inlet of the SAS. The flow rate was measured with a flow meter, and the timing for the sub-sampling procedure was established. Also, the membrane pump (Picarro Inc.) that is used for the sub-sampling was carefully tested for leaks to avoid contamination of stratospheric air with ambient laboratory air during the sub-sampling process. The outlet of the pump was connected to the gas analyzer (Picarro Inc., model G2401-m) to form a closed loop, which was filled with air with high mole fractions of CO2, CO and CH4. The flow rate was set to 35 mL min-1 and the air enclosed in this closed system was circulated nine times while measuring the CO2, CO, CH4 and H2O mole fractions. The CO2 mole fractions stayed between 940.4 and 941.0 ppm and a small initial fluctuation quenched after nine cycles. The rate of change of CO2 mole fraction based on these measurements with a very high CO2 mole fraction difference between the sample and ambient air was 0.1 ppm of CO2 per minute. The effect of such a small contamination on the isotopic composition of CO2 is therefore negligible (< 0.001 ‰ given the maximum Δ17O(CO2) of 5 ‰; see below). We note, however, that CO gets contaminated in the sub-sampling process at a rate of 17 ppb min-1 for CO (from a starting value of 500 ppb), so CO measurements would be compromised.

The SAS used for the CO2 isotope measurements in Utrecht is made of 10 2 m long pieces of 1/4 inch diameter stainless steel tubing, which are connected by 11 Swagelok valves (part number SS-3CXS4) to form the 20 m long SAS. The tubes are bent to form identical rings to facilitate easy handling and transport. In the following, we refer to the rings as “SAS segments” and the valves as “three-way valves”. The three-way valves are open and connect the segments when the stratospheric part of the AirCore air fills the SAS and are kept closed when the sub-sampling process is finished. Each SAS segment contains about 25 mL of the AirCore air. Segment 1 corresponds to the highest altitude of the AirCore flight, and segment 10 contains air from the lower-stratosphere.

The idea of the SAS that is described here in detail has already been successfully implemented to enable the radiocarbon analysis of stratospheric CO2 at the Centre for Isotope Research in Groningen, the Netherlands .

Continuous flow system to measure the isotopic composition of CO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>

The analytical system to measure the isotopic composition (δ13C, δ18O and Δ17O) of CO2 in each SAS segment is based on the principle presented in . The isotopic composition is measured after gas chromatographic separation of the CO2 from other air constituents. The 17O content cannot be determined directly from the measurement at m/z = 45, because of the isobaric interferences of 13C and 17O. Therefore, the 17O content is obtained from isotope measurements on CO2 before (PreCO2) and after oxygen isotope exchange (PostCO2) with a large reservoir of oxygen. Instead of using cerium (IV) oxide (CeO2) as exchange material , we use in our new system copper oxide (CuO) . A single measurement requires two independent injections of 1 mL of air: one for direct measurement (PreCO2) and one for measurement after oxygen isotope exchange (PostCO2), plus 0.7 mL for flushing the injection lines (all at 1 bar pressure). To reduce the measurement uncertainty statistically we perform multiple measurements on one air sample.

In addition to the requirement of overpressure in the injection unit, another disadvantage of the method presented in was the severe peak broadening that was introduced by the strong flow resistance of the CeO2 powder in the oxygen exchange unit, which required re-focusing of the CO2 after equilibration. In the new system the powdered CeO2 in the oxygen exchange unit was replaced with CuO wires. As a result the peak broadening was dramatically reduced by a factor of 7.5 (from 450 to 60 s) and the equilibrated CO2 could be analysed without re-focusing. In addition, we developed a custom-made sample injection unit for samples provided by the SAS, where both sample and reference gas are injected via the SAS.

The improved continuous flow isotope ratio mass spectrometry analytical system (CF-IRMS) analytical system consists of: a sample injection unit to attach the SAS segments, a gas chromatographic column to separate the CO2 from a 1 mL air aliquot, an oxygen isotope exchange unit for CO2 equilibration with CuO and open split interface and an IRMS for isotope measurement (Fig. ). Similar to the method described by Mrozek et al. (2015), three 6-port, 2-position Valco valves (VICI, C6UWM) direct the gas flows through the analytical system. The four main components are described in the following subsections.

Schematic diagram of the CF-IRMS analytical system for complete isotope analysis (including Δ17O) of stratospheric CO2. The system is divided into four units: (a) sample injection, (b) gas chromatographic column (GC) to separate CO2 from air and N2O, (c) optional oxygen isotope exchange unit and (d) isotope detection unit including open split interface and isotope ratio mass spectrometer (IRMS). MFC is mass flow controller, V1–V3 are Valco valves, RefCO2 is Utrecht working reference CO2, the crossed circles are conventional valves and the blue T's are the three-way valves. The SAS three-way valves are normally closed but here they are shown in the sample admission position.

Sample injection

The 10 individual segments of the SAS are measured separately (see Sect. ). After connecting a segment, the volume between the mass flow controller (MFC) and the segment is evacuated with a low vacuum (LV) pump and filled with reference air. All connections in the injection sub-unit are made of 1/8 inch o.d. stainless steel tubing. When the valves of a connected SAS segment are opened, as demonstrated in Fig. , the reference air flushes the stratospheric air sample from the SAS into the analytical system. The flow rate of the MFC is set to 1 mL min-1 during sample gas injection. We use the reference air itself as the carrier for the sample air.

A single Δ17O(CO2) measurement requires two independent injections of the sample gas. The first injection is used for direct isotope measurement of CO2 (PreCO2), the second injection is for measuring the isotopically equilibrated CO2(PostCO2). We inject the gas (reference air or sample air) into the GC column via a 1 mL sample loop in V1. The sample loop is always filled to ambient pressure because it is open to the outside air via the vent. We use extended flushing during sample loop loading (the 1 mL sample loop is flushed with the sample gas at a flow rate of 1 mL min-1 for 80 s) to avoid interference from outside air that may diffuse through the vent tubing into the sample loop.

It takes less than 1 h to measure an individual SAS segment, however, in the present set-up reference air is measured for several hours between the different segments (see Sect. ). Usually two SAS segments are measured in one day. The CF-IRMS system operates fully automated and the only manual step required is the connection of the segments.

CO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula> separation from air

This sub-unit consist of a gas chromatography (GC) capillary column (ParaPLOT Q 25 m × 0.53 mm, Varian). As a carrier we use helium at a flow rate of 4 mL min-1, supplied to the GC through one of the ports in valve V1. The GC is kept inside a heated stainless steel box at 40 ∘C to ensure uniform conditions for gas separation. On the GC column the different air constituents are separated. The air peak (mainly O2 and N2) elutes at 120 s after sample injection, CO2 at 160 s and N2O at 190 s. The air peak leaves the analytical system through the vent in valve V3. Both CO2 and N2O are directed either to IRMS or to the isotope exchange sub-unit. Nitrous oxide in our system does not interfere with CO2 during the isotope ratio measurement because it is fully separated from CO2 and gets destroyed in the CuO oven. This is discussed in detail in Sect. .

Oxygen isotope exchange with CuO

This sub-unit consists of Valco valve 2 (V2) and an optional oxygen equilibration oven, the same as . The Valco valve V2 directs the CO2 aliquot either directly to the IRMS or first into the equilibration oven before entering IRMS. The oven is an assembly of a quartz glass reaction tube (1.4 mm i.d., 3.0 mm o.d., 300 mm length), a tube furnace and a temperature controller. Inside the tube there are oxygenated Cu wires and a Ni catalyst. We refer to this assembly as CuO oven. At a temperature of 900 ∘C, the wires act as a fast and highly efficient oxygen equilibration medium. This is similar to , who for the same purpose used twisted wires of CuO and Pt catalyst, and different from and , who used CeO2 powder. The important advantage of oxygenated Cu / Ni wires over CeO2 powder in our system is the much smaller flow resistance. Therefore, the new exchange unit induces a much smaller peak broadening, and the equilibrated CO2 does not require focusing anymore. As a result, the single analysis time shortened by 250 s in comparison to the method described in (650 s instead of 900 s) and no liquid nitrogen is required.

Before first use, and then on a weekly basis, the CuO oven is conditioned with O2 (Ultra High Purity (UHP), Air Products) at a flow rate of 20 mL min-1 and a temperature of 600 ∘C, following the procedure of . Under these conditions, the copper metal forms a coating of copper (II) oxide on the surface of the Cu wires according to Cu+12O2→CuO.

After oxygenation, the IRMS needs at least 50 measurements for the signal to stabilize. During routine measurements we monitor the δ18O(CO2) RefAir vs. VSMOW signal before and after equilibration to make sure that the equilibration reaction is quantitative.

IRMS interface and mass spectrometric analysis

This sub-unit of the analytical system (Fig. d) consists of a Valco valve 3 (V3), a Nafion^™ dryer, a custom-made open split system and an IRMS (Thermo Fisher Scientific Delta V Advantage). The loop in valve V3 is used to insert an additional volume of approximatively 1 mL volume (1/4 inch o.d. tube connected with Swagelok fittings) into the flow path, which smooths and ensures a compact shape of the PostCO2 peak in the IRMS. The Nafion dryer removes traces of water before the gas stream enters the IRMS via the open split system, which is also used to inject the pure CO2 working gas. The IRMS measures ion current ratios 45/44 and 46/44 that originate from CO2 isotopologues with masses 44, 45 and 46.

Reference air and reference CO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>

Our reference air cylinder (referred to as RefAir in the following) was filled with tropospheric air collected at an altitude of 20 m from the sixth floor of the Buys Ballot building on the Utrecht University campus in July 2014. The isotopic composition of the CO2 in the reference air cylinder was calibrated against air cylinders provided by an intercomparison program of the World Meteorological Organization (WMO) and assigned the following isotope values: δ13CRefAir/VPDB =  -8.09 ± 0.10 ‰ and δ18ORefAir/VSMOW = 41.05 ± 0.20 ‰. Following a mass dependent fractionation relation of tropospheric CO2 , we assign δ17ORefAir/VSMOW = 21.5 ‰, so that Δ17ORefAir = 0.0 ‰. The isotopic composition of the reference air is measured continuously in between the samples to monitor the stability and to correct for long-term trends of the CF-IRMS system.

The δ values for the working reference CO2 that is injected via the open split system are δ13CRefCO2/VPDB = -36.16 ± 0.01 ‰ and δ18ORefCO2/VSMOW = 4.69 ± 0.01 ‰.

Measurement procedure Connecting the sub-sampler to the continuous flow isotope analysis system

We start the CO2 isotope analysis always from the SAS segment with the highest segment number, here segment number 10. The common port of valve no. 11 (the end of segment 10) is connected to the injection line, which leads to V1, and the free port of valve no. 10 (the beginning of segment 10) to the MFC delivering reference air (see Fig. ). Note that the beginning of the segment 10 is also the end of segment 9.

The injection lines are open to the laboratory air when the SAS segments are being exchanged. To avoid mixing of the precious sample air with laboratory air we evacuate the volume between the MFC and the SAS segment with the LV pump and flush this volume with reference air (LV pump exchanged to vent). It is important to depressurize the volume behind the MFC before opening the SAS segment. Overpressure in the injection lines must be avoided so that the small air sample is not pushed out of the inlet system.

After connecting an SAS segment as described above, the three-way valve no. 10 is opened, so that reference air starts slowly flowing through segment 10. Next, the three-way valve no. 11 is opened, so that air is flushed to the sample loop of the injection system (see Fig. ). The sample admission procedure is described in detail in the next section.

Example of a typical IRMS chromatogram of a single Δ17O(CO2) analysis with the CF-IRMS system described here. The eight square peaks are the working reference CO2 (RefCO2) peaks injected directly via the open split interface. The non-equilibrated CO2 (PreCO2) peak and the equilibrated CO2 (PostCO2) peak are the two sample gas peaks.

As the sample air is flushed into the system by the reference air, we simply continue measuring the reference air that is then flowing through the SAS segment. A disadvantage is that the last injections of the sample air are actually a mixture of sample and reference air (see below). The measurements of reference air after the sample measurement are later used for referencing. After the reference air measurements are completed, we close the three-way valves and disconnect segment 10. As the common port of valve no. 10 has to be connected to the injection line when measuring segment 9, segment 10 has to be physically disconnected from the SAS. Segment 9 is then treated the same as segment 10 before. We continue measuring and exchanging SAS segments one by one.

Isotope analysis procedure

Each Valco valve has two possible positions: LOAD and INJECT. The initial Valco valve configuration is shown in Fig. : V1 and V3 are in position LOAD and V2 is in position INJECT. After an SAS segment has been connected and the connecting lines flushed as described above, sample admission starts. The MFC is set to a flow rate of 1 mL min-1 15 s before opening the SAS valve in order to provide a small and reproducible overpressure at the entrance of the SAS segment. We then open the three-way valve towards the SAS segment to admit the reference air from the MFC, and 1 s later the three-way valve before V1. Now, the flow of reference air pushes the AirCore air towards the sample loop in V1. It takes 10 s for the AirCore air to travel from the SAS segment to the sample loop and another 80 s to fill it. At 80 s, the flow rate at the MFC is stopped to save sample and the air is allowed to further expand into and fill the injection loop. At 90 s the loop is fully filled and Valco valve V1 is switched for 40 s to position INJECT to transfer the first aliquot of the sample air into the analytical system. The 1 mL aliquot of air from the SAS segment is transferred to the GC column in a He carrier gas (4 mL min-1). In the GC column, the CO2 is separated from other atmospheric gases. All compounds are directed via Valco valve V2 (INJECT) and Valco valve V3 (LOAD) directly towards the isotope detection unit. The CO2 is injected into the ion source of the IRMS (PreCO2), all other gases are discarded via the open split.

As soon as the CO2 peak (PreCO2) appears on the chromatogram, the second aliquot of the AirCore air is introduced into the system. Similar to the first injection, the sample loop is flushed with the AirCore air for 80 s (MFC is set to a flow rate of 1 mL min-1 between 290 and 370 s). At 380 s valve V1 is switched for 40 s from LOAD to INJECT, and the second aliquot of the AirCore air is injected into the GC column. Non-CO2 gases leave the analytical system through the open split capillary between 500 and 520 s (V2 in position INJECT and V3 in position LOAD). At 545 s, V2 switches to position LOAD, and the CO2 is directed to the isotope exchange unit. After the isotope exchange reaction, the equilibrated CO2 (PostCO2) is flushed further to V3. An additional 1 mL stainless steel volume in front of Valco valve V3 has been added to smoothen the peak shape of the equilibrated CO2 (PostCO2), leading to improved precision. The equilibrated CO2 is detected on the chromatogram between 560 and 640 s.

Figure presents an example of an IRMS chromatogram. A single measurement including two injections takes 650 s. The CO2 peak from the first injection (PreCO2) is detected between 250 and 280 s, and the equilibrated CO2 peak from the second injection (PostCO2) between 560 and 640 s. The eight working reference CO2 peaks (RefCO2) are injected to the IRMS directly via the open split interface: four before detection of the PreCO2 (0–200 s) and four before detection of the PreCO2 (300–500 s). The two sample peaks (PreCO2 and PostCO2) have a different shape because the second one has passed the exchange unit and the additional volume.

Raw-δ45CO2 data (‰ vs RefCO2) from a complete measurement sequence of an SASsegment. SA is the air sample stored in the SAS segment. RefAir before SA refers to the reference air passing through an empty SAS segment and defines IRMS stability before the new SA is connected. After exchanging the SAS segments we obtain five “pure” sample air measurements. Subsequent to the SA measurement, the reference air first mixes into the sample air; it takes about 10 measurements until pure reference air is processed. RefAir after SA refers to the reference air passing through the SAS segment after the SA has been completely flushed out. For the sample air presented here the “before equilibration' points overlap with the “after equilibration” points and cannot be distinguished.

Statistical improvement of Δ17O analytical error by combining multiple (n) measurements of the same gas into packages and taking the standard error of these packages.

Test group Number of packages Number of measurements Random error SE theoretical: SE experimental: name in the group in the package: (n) factor: 1/(n) SD/(n) SD over Δ17O mean/(n) A 1 270 0.061 0.074 B 2 135 0.086 0.105 0.205 C 3 90 0.105 0.129 0.256 D 5 54 0.136 0.166 0.170 E 6 45 0.149 0.182 0.245 F 9 30 0.183 0.223 0.251 G 10 27 0.192 0.235 0.302 H 15 18 0.236 0.288 0.316 I 18 15 0.258 0.315 0.345 J 27 10 0.316 0.386 0.405 K 30 9 0.333 0.407 0.472 L 45 6 0.408 0.498 0.531 M 54 5 0.447 0.546 0.570 N 270 1 1.000 1.220 1.220

The bold values describe the uncertainty for a single measurement and the error expected for the five repeated measurements on one air sample stored in the SAS.SD: standard deviation.

For each SAS segment we run a sequence of 35 individual measurements (of two injections each). The results show that 5 of these measurements represent pure sample gas (SA), 10 of them contain sample-reference air mixtures, and 20 are pure reference air (RefAir) measurements (see Fig. ). Theoretically, we should be able to analyse nine SA aliquots from each SAS segment (25/2.7 mL), but as described above the reference air is used as carrier gas and mixes with the sample air. Since we know the isotopic composition of the reference air, the method can potentially be improved by extracting SA information also from the “mixed” SA/RefAir peaks, which may decrease measurement uncertainty statistically (see Table ).

IRMS chromatogram demonstrating the separation of N2O from CO2 on the GC column and the absence of an N2O peak after the hot CuO / Ni isotope exchange unit. The artificially prepared mixture of N2O and CO2 was injected to the analytical system following the procedure described in Sect. . In order to observe the N2O peak the fifth working RefCO2 peak was omitted in the chromatogram and the measurement time was extended to 750 s.

Performance of the continuous flow isotope analysis system Separation and destruction of N<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>O

Nitrous oxide (N2O) interferes with the mass spectrometric analysis of CO2 because the isotopologues of both CO2 and N2O fall on the mass 44, 45 and 46 collectors and cannot be separated with conventional IRMS instruments . These mass interferences can lead to significant analytical biases for δ13C(CO2), δ17O(CO2) and δ18O(CO2) values and, as result, decrease Δ17O of stratospheric air by as much as 3 ‰ . Note that N2O and CO2 can also not be separated cryogenically .

To measure CO2 isotopes in an air sample without interference from N2O we completely separate N2O from CO2 before the isotope ratio analysis on the GC column . In addition, for the measurement of PostCO2, N2O is quantitatively destroyed in the hot isotope exchange unit .

To demonstrate successful GC separation and N2O decomposition inside the CuO oven at 900 ∘C we connected a dilution of 400 ppm N2O in synthetic air to the injection sub-unit of our analytical system. The N2O enriched gas was placed at the position of a flask, as shown in Fig. . The flask was opened and the N2O enriched gas filled the injection lines and the 4 mL inner volume of MFC. Next, we changed the position of the three-way valve in front of the MFC so that the reference air was flowing towards the MFC. As a consequence, the reference air mixed with the N2O-rich gas and both N2O and CO2 were observed on the chromatogram. We monitored the N2O peak before and after the isotope exchange reaction: for the non-heated aliquot, the GC column separated completely the N2O from the CO2; for the heated aliquot, the N2O was destroyed completely in the CuO oven (see Fig. ). For the experiment shown in Fig. , the chromatogram was extended to 750 s instead of the normal length of 650 s because N2O elutes after CO2. Additionally, the fifth working RefCO2 peak was omitted in order to observe the N2O peak. The test shows that N2O can be effectively removed on CuO / Ni wires at 900 ∘C and confirms the results of . This removal method can potentially be applied to other trace gas measurement techniques.

Efficiency of oxygen isotope equilibration in the CuO oven. The δ18O(CO2) values of RefAir and a mixture of RefCO2 in synthetic air are shown before and after isotope equilibration. The last point of the RefAir measurement sequence after exchange is missing because the peak of the last run was accidentally not registered.

CuO–CO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula> equilibration efficiency

To quantify the efficiency of oxygen isotope equilibration in the CuO oven we analysed two samples containing CO2 with very different isotopic composition. The first one was our RefAir and the second one was a synthetic mixture of RefCO2 diluted to 400 ppm CO2 in synthetic air. The samples were injected via a stainless tube that is similar to a segment of the SAS, but longer (4 m length 1/4 inch o.d.). First, the reference air was injected multiple times through this tube. Next, we filled the injection tube with the RefCO2 dilution and continued the measurements. Figure presents the results. The isotopic difference between the two CO2 samples was about 36 ‰ before isotope exchange. After the isotopic exchange reaction both gases were equilibrated to δ18O = (19.03 ± 0.18) ‰. From the difference in the oxygen isotopic composition between RefAir and RefCO2 dilution before and after oxygen isotope exchange, the oxygen exchange efficiency in CuO oven was calculated to be > 99.5 %. We conclude that the oxygen exchange reaction with CuO/Ni wires at 900 ∘C is complete.

Error analysis

In order to investigate the long-term stability of our CF-IRMS system and the measurement uncertainty, 540 aliquots of reference air were injected continuously to the CF-IRMS system for 50 h, comprising 270 individual measurements of the complete isotopic composition of CO2. The standard deviation of Δ17O(CO2) over all 270 measurements was 1.22 ‰. This is the error that we assign to a single measurement with the new analytical system. In our previously published method , the Δ17O(CO2) standard deviation in such a long-term stability test was 1.68 ‰. The improvement compared to the system described in is due to the replacement of the isotope exchange medium from powdered CeO2 to CuO wires and through abandonment of a liquid nitrogen trap for re-focusing of the isotopically equilibrated CO2. For δ18O(CO2) and δ13C(CO2) the standard deviation over all 270 measurements was 0.06 and 0.07 ‰, respectively, vs. 0.16 and 0.09 ‰ in .

To investigate how the measurement error is reduced statistically with multiple measurements on an air sample, we divided the 270 measurements from the stability test into different packages of n numbers of measurements (two packages of 135 measurements each, three packages of 90 measurements each, etc.). The experimental standard error (SE) for each set of packages was then compared to the theoretically expected error. Calculations for Δ17O(CO2) are shown in Table ; the results for Δ17O(CO2), δ18O(CO2) and δ13C(CO2) are displayed in Fig. . Since the SE is defined as σ/n0.5 the expected slope on this double logarithmic plot is -0.50. The error reduction follows the theoretically expected relation quite well. When the two groups with only two and three members each are neglected, the linear fit to the data has a slope of -0.46 ± 0.02 for Δ17O, -0.42 ± 0.03 for δ18O and -0.45 ± 0.03 for δ13C. Within the error, this is close to the theoretical slope of -0.50.

Correlation of ln(SE) vs. ln(n) for Δ17O(CO2) (based on data from Table ), δ18O(CO2) and δ13C(CO2). SE is the standard error of the mean, and n is the number of measurements in a package. The experimental SE of Δ17O, δ18O and δ13C are shown as circles, triangles and black diamonds respectively. The dashed/dotted lines are the theoretically expected slopes for the linear correlation between ln(SE) and ln(n). For a packages of n≤ 54, the experimentally derived slopes are -0.46 ± 0.02 for Δ17O, -0.42 ± 0.03 for δ18O and -0.45 ± 0.03 for δ13C. The values are close to the theoretical slope of -0.50 (dotted/dashed lines). Based on this experiment we expect an uncertainty of ±0.57 ‰ for Δ17O of an AirCore air sample stored in an individual SAS segment.

We conclude that repeated measurements on one air sample (up to 54 repetition that corresponds to 2.6 µmol CO2) can reduce the uncertainty in Δ17O(CO2) to 0.2 ‰. More than 54 repetitions on one air sample improves the Δ17O(CO2) uncertainty only marginally. This uncertainty is in the same range as previously reported techniques for large samples, most of which did not allow the measurement of very small samples. reported an uncertainty of 0.1 ‰ using a BrF5-based technique, obtained 0.2 ‰ with a two-step fluorination method, reported 0.33 ‰ with a CeO2 exchange method and improved this method to an uncertainty of 0.12 ‰. The method is the only technique that also targeted very small sample sizes (like our system) and they reported an uncertainty of 0.35 ‰. For five repeated measurements on one stratospheric air sample stored in the SAS (an example of a CF-IRMS measurement sequence is given in Fig. ) we therefore expect an uncertainty of 0.57 ‰ for Δ17O(CO2) and 0.03 ‰ for both δ18O(CO2) and δ13C(CO2). This will be compared to the reproducibility of the actual SAS measurements in Sect. .

Stratospheric CO2 mole fraction profile for an AirCore descent near Sodankylä, Finland (67∘ N), in November 2014, as a function of altitude (km) and pressure (mbar). The numbers and the various line styles indicate the sub-sampler segments into which the respective part of the air from the AirCore coil was transferred after the trace gas analysis. The general trend of the profile agrees with the already published data . The distribution of the air sample in the SAS segments is proportional to the pressure change during the AirCore descent: here 16 mbar per SAS segment.

AirCore flight on 5 November 2014

The AirCore payload was launched about 100 km upwind from Sodankylä, using a meteorological balloon (67.35∘ N, 26.93∘ E). The AirCore was released from the balloon at 34.5 km altitude (at 4.2 hPa) and the coil filled with air during the balloon descent. Time of launch was 10:04 UTC, while the payload landed at 12:40 UTC; thus the total flight duration was 2 h and 36 min. During the balloon ascent the payload travelled south-east towards Sodankylä due to winds in the stratosphere and troposphere. The payload landed with a parachute 53.8 km east from the analysis laboratory (67.24∘ N, 27.84∘ E). After landing the AirCore was transported to FMI. The analysis of trace gases started 2 h and 15 min after the landing of the payload. In the laboratory the mole fractions of CO2, CH4 and CO were measured and the stratospheric air was transferred into the SAS. The SAS was sent to the laboratory in Utrecht for CO2 isotope analysis.

Assigning the trace gas data to the SAS segments

We assigned the trace gas mixing ratios measured with the Picarro instrument to the individual SAS segments based on the flow rate of the carrier gas and the time required to fill the SAS. As the SAS has 10 segments, the air in each segment represents 38.6 s of trace gas measurements with the Picarro instrument. The second piece of information that is necessary for correct assignment is the starting time for the sub-sampling. This starting time was established by taking into account the transfer time of air from the outlet of the Picarro analyser to the inlet of the SAS and the time required to flush out the initial portion of fill-gas from the AirCore. To ensure that no fill-gas was transferred into the SAS, the top fraction of the stratospheric air (which immediately follows the fill-gas) also had to be discarded. This way we lost the stratospheric air from above 25 km. The timing of sub-sampling has in the meantime been improved to discard less stratospheric air in future samplings.

Comparison of the linear negative correlation between Δ17O(CO2) and N2O from measurements of the AirCore/SAS samples (red rhombuses) presented here with literature data from (blue crosses) and (black triangles). For the AirCore dataset, N2O mole fractions were deduced from CH4 mole fractions as described in the text. The horizontal error bars are a combination of the range of CH4 mole fractions that is combined within a single SAS segment and the error of the CH4–N2O transfer function. The vertical error bars represent Δ17O(CO2) uncertainty of 0.56 ‰ (1σ).

Figure shows which part of the stratospheric AirCore air is stored in which segment of the SAS, both as function of altitude and pressure. Note that the SAS segments are equally spaced in pressure change (here 16 mbar/segment) during the descent rather than altitude. For example, the SAS segment 1 contains air from 24.5 to 21.4 km, while the SAS segment 2 contains air from 21.4 to 19.2 km. An analysis of the trace gas profiles from the AirCore flight will be published separately.

<inline-formula><mml:math display="inline"><mml:mrow><mml:msup><mml:mi mathvariant="normal">Δ</mml:mi><mml:mn>17</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula>O(CO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>) analysis of the AirCore air stored in the SAS

The stratospheric part of the AirCore air from 5 November 2014 flight was measured in Utrecht between 25 November and 1 December 2014. The results showed that segments 10 and 8 were contaminated with CO2 from outside air. The precise origin of this contamination could not be determined, but it could have happened when connecting SAS segments to the injection sub-unit of the CF-IRMS system. Due to an accidental instability in the helium carrier gas flow, the air sample from the SAS segment 6 was lost.

As shown in Fig. , we were able to perform five repeated CF-IRMS measurements on the other seven stratospheric air samples stored in SAS. The average (1σ) standard deviation of the raw molecular mass ratios (45/44 and 46/44) of an air sample was 0.04 and 0.02 ‰ for non-equilibrated CO2 and 0.07 and 0.11 ‰ for equilibrated CO2 respectively. The main contribution to the measurement error of Δ17O is the uncertainty in the isotope ratio 45/44 . Based on the reproducibility of m/z 45 of equilibrated and non-equilibrated CO2 ( (0.04)2+(0.07)2× 15), the uncertainty in δ17O for a single measurement was 1.25 ‰ and for a package of five measurements it was 0.56 ‰ (1.25 ‰/5). The error of 0.56 ‰ determined for the real AirCore air measurements agrees very well with the uncertainty of 0.57 ‰ expected from the long-term stability test (see Sect. ). The standard deviations for all seven successfully measured SAS segments were 0.08 ‰ for δ13C and 0.05 ‰ for δ18O. This results in the standard error of 0.03 ‰ for δ13C and 0.02 ‰ for δ18O for a package of five repeated analyses from an SAS segment. This is again very similar to the expected error from the long-term stability test as presented in Fig. .

The mole fraction of N2O is a good tracer for the photochemical processing of long-lived trace gases in the stratosphere . Nitrous oxide was not measured in the AirCore air, but CH4 was measured and we use the relationship between stratospheric CH4 and N2O mole fractions at high latitudes to translate the CH4 mole fractions to N2O mole fractions. For this purpose we used CH4–N2O data from two cryosampler flights in the Arctic in 2009 and 2011 . The fit function for the CH4–N2O dataset was provided to us by Andreas Engel, Goethe University Frankfurt, Germany. The stratospheric pseudo-N2O profile derived this way was then averaged according to the 10 individual SAS segments. A detailed discussion on obtaining N2O mole fractions from the measured CH4 will be provided together with the scientific interpretation of the data in Mrozek et al. (in prep.).

In Fig. we show the N2O mole fractions deduced from the CH4 profile vs. the Δ17O(CO2) data of the AirCore air samples analysed from the SAS. The linear negative correlation between N2O and Δ17O(CO2) is well known from previous studies and was discussed before . Here, we use this correlation as an additional, independent check on the system calibration. The AirCore air samples agree with the already published data, suggesting that our measurements are well calibrated. The scatter of our data is similar to the variability of the data presented by Kawagucci et al. (2008). The data presented by Wiegel et al. (2013) show smaller variability, but our measurements were obtained from samples of 25 mL of air at ambient pressure only; this is up to 80 times smaller sample size than used in the measurements of Wiegel et al. (2013). The variability of our data around the linear fit is within the analytical uncertainty of 0.56 ‰ (1σ) per SAS segment. The uncertainty of the N2O mole fraction in Fig. reflects the standard deviation of the N2O profile stored in an individual SAS segment. It is up to 20 ppb (1σ) for the high-altitude AirCore air samples where N2O showed a strong trend and 2 ppb (1σ) for lower altitude samples where N2O is more constant. The complete CH4, CO and CO2 mole fraction dataset and in-depth analysis on CO2 isotope measurements from the 5 November 2014 AirCore flight over Sodankylä, Finland, will be published separately.

We conclude that air sampling with an AirCore, followed by sub-sampling of stratospheric air in 10 segments of the SAS and analysis with the analytical system described here provides a relatively cost-effective technique for obtaining vertical profiles of the complete isotopic composition of CO2 in the stratosphere.

Conclusions

This article describes the concept of the SAS, a device constructed to recover and store the stratospheric part of air obtained with an AirCore sampler. The SAS described here is an assembly of 20 m, 1/4 inch o.d., stainless steel tubing divided into 10 segments. Each SAS segment contains 25 mL of air sample at ≈ 980 mbar. During the procedure of sub-sampling air into the SAS, the air from an AirCore is measured for CO2, CH4 and CO mole fractions with a Picarro analyzer. This leads to some mixing of air and partial loss of the vertical information, but once stored in the SAS the stratospheric profile of the AirCore air after online analysis is preserved and the individual sub-samples can be supplied to relatively slow analytical instrumentation. The SAS is easy to construct and simple to use, also in the field.

To illustrate the scientific possibilities, the SAS was coupled to a continuous flow analytical system for measurement of the 17O excess of CO2. The air from the SAS was supplied via a new sample introduction system to allow isotope analysis on very small samples. The sample injection occurs at ambient pressure and we use the reference air as the carrier gas. The determination of Δ17O of CO2 is performed by measuring CO2 before and after complete oxygen isotope exchange with a large oxygen reservoir provided by CuO / Ni. The standard error for a 25 mL air sample at stratospheric CO2 mole fraction is 0.56 ‰ (1σ) for Δ17O and ‰ (1σ) for both δ18O and δ13C. The analytical system operates free of liquid nitrogen because the isotopically equilibrated CO2 does not require a focusing step, unlike the method.

The concept of SAS and its coupling to the analytical system for measurement of Δ17O was validated through measurements on stratospheric air samples obtained during an AirCore flight over Sodankylä, Finland, in November 2014. Δ17O shows the expected negative linear correlation with N2O, which provides an independent check on the system calibration. In the future, we plan routine measurements of stratospheric profiles of Δ17O together with the mole fractions of CO2, CH4 and CO from AirCore flights on a regular basis, which could expedite the use of the isotope signatures for studying stratospheric circulation patterns and reaction mechanisms. The concept of the SAS will further broaden the scientific questions that can be addressed by AirCore soundings (e.g. ).

Data availability

The datasets are available from https://www.projects.science.uu.nl/atmosphereclimate/Data.php (APCG, 2016).

Acknowledgements

This work was funded by the Marie-Skłodowska Curie ITN INTRAMIF (Initial Training Network in Mass Independent Fractionation) as a part of the European Community's Seventh Framework Program (FP7/2007–2013), grant agreement 237890. Research at the FMI has been partly supported by the EU project GAIA-CLIM and the Academy of Finland grant number 140408. We thank Andreas Engel, Goethe University Frankfurt, Germany, for sharing with us the stratospheric N2O–CH4 correlation function. We also appreciate the efforts of the Associate Editor, Murray Hamilton, whose comments helped to shape this paper. Edited by: M. Hamilton Reviewed by: two anonymous referees

References Alexander et al.(2001)Alexander, Vollmer, Jackson, Weiss, and Thiemens

Alexander, B., Vollmer, M. K., Jackson, T., Weiss, R. F., and Thiemens, M. H.: Stratospheric CO2 isotopic anomalies and SF6 and CFC tracer concentrations in the Arctic polar vortex, Geophys. Res. Lett., 28, 4103–4106, 2001.

APCG: IMAU-APCG measurements data, available at: https://www.projects.science.uu.nl/atmosphereclimate/Data.php, last access: 17 November 2016.

Assonov and Brenninkmeijer(2001)

Assonov, S. S. and Brenninkmeijer, C. A. M.: A new method to determine the 17O isotopic abundance in CO2 using oxygen isotope exchange with a solid oxide, Rapid Commun. Mass Sp., 15, 2426–2437, 10.1002/rcm.529, 2001.

Assonov and Brenninkmeijer(2005)

Assonov, S. S. and Brenninkmeijer, C. M.: Reporting small Δ17O values: existing definitions and concepts, Rapid Commun. Mass Sp., 19, 627–636, 10.1002/rcm.1833, 2005.

Assonov and Brenninkmeijer(2006)

Assonov, S. S. and Brenninkmeijer, C. A. M.: On the N2O correction used for mass spectrometric analysis of atmospheric CO2, Rapid Commun. Mass Sp., 20, 1809–1819, 10.1002/rcm.2516, 2006.

Baertschi(1976)

Baertschi, P.: Absolute 18O content of standard mean ocean water, Earth Planet. Sci. Lett., 31, 341–344, 10.1016/0012-821X(76)90115-1, 1976.

Bhattacharya and Thiemens(1989)

Bhattacharya, S. and Thiemens, M.: Oxygen Isotopic Fractionations in Symmetry Dependent Chemical Reactions, in: Lunar and Planetary Science Conference, 20, p. 71, 1989.

Boering et al.(2004)Boering, Jackson, Hoag, Cole, Perri, Thiemens, and Atlas

Boering, K. A., Jackson, T., Hoag, K. J., Cole, A. S., Perri, M. J., Thiemens, M., and Atlas, E.: Observations of the anomalous oxygen isotopic composition of carbon dioxide in the lower stratosphere and the flux of the anomaly to the troposphere, Geophys. Res. Lett., 31, L03109, 10.1029/2003GL018451, 2004.

Brenninkmeijer and Röckmann(1998)

Brenninkmeijer, C. A. M. and Röckmann, T.: A rapid method for the preparation of O2 from CO2 for mass spectrometric measurement of 17O / 16O ratios, Rapid Commun. Mass Sp., 12, 479–483, 1998.

Chen et al.(2016)Chen, Kivi, Heikkinen, de Vreis, Hatakka, Laurila, Sweeney, and Tans

Chen, H., Kivi, R., Heikkinen, P.and Kers, B., de Vreis, M., Hatakka, J., Laurila, T., Sweeney, C., and Tans, P.: High-latitude balloon observations of CO2/CH4/CO using AirCore: evaluation of Sodankylä TCCON retrievals, in prep., 2016.

Engel et al.(2016)Engel, Bönisch, Schwarzenberger, Haase, Grunow, Abalichin, and Sala

Engel, A., Bönisch, H., Schwarzenberger, T., Haase, H.-P., Grunow, K., Abalichin, J., and Sala, S.: Long-term validation of ESA operational retrieval (version 6.0) of MIPAS Envisat vertical profiles of methane, nitrous oxide, CFC11, and CFC12 using balloon-borne observations and trajectory matching, Atmos. Meas. Tech., 9, 1051–1062, 10.5194/amt-9-1051-2016, 2016.

Ferretti et al.(2000)Ferretti, Lowe, Martin, and Brailsford

Ferretti, D. F., Lowe, D. C., Martin, R. J., and Brailsford, G. W.: A new gas chromatograph-isotope ratio mass spectrometry technique for high-precision, N2O-free analysis of δ13C and δ18O in atmospheric CO2 from small air samples, J. Geophys. Res.-Atmos, 105, 6709–6718, 10.1029/1999JD901051, 2000.

Foucher et al.(2011)Foucher, Chédin, Armante, Boone, Crevoisier, and Bernath

Foucher, P. Y., Chédin, A., Armante, R., Boone, C., Crevoisier, C., and Bernath, P.: Carbon dioxide atmospheric vertical profiles retrieved from space observation using ACE-FTS solar occultation instrument, Atmos. Chem. Phys., 11, 2455–2470, 10.5194/acp-11-2455-2011, 2011.

IPCC(2013)Ciais, Sabine, Bala, Bopp, Brovkin, Canadell, Chhabra, DeFries, Galloway, Heimann, Jones, Le Quéré, Myneni, Piao, and Thornton

IPCC 2013: Ciais, P., Sabine, C., Bala, G., Bopp, L., Brovkin, V., Canadell, J., Chhabra, A., DeFries, R., Galloway, J., Heimann, M., Jones, C., Le Quéré, C., Myneni, R., Piao, S., and Thornton, P.: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, edited by: Stocker, T. F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S. K., Boschung, J., Nauels, A., Xia, Y., Bex, V. and Midgley, P. M., Tech. rep., 10.1017/CBO9781107415324.015, 2013.

Kaiser(2009)

Kaiser, J.: Reformulated 17O correction of mass spectrometric stable isotope measurements in carbon dioxide and a critical appraisal of historic “absolute” carbon and oxygen isotope ratios, Geochim. Cosmochim. Ac., 72, 1312–1334, 10.1016/j.gca.2007.12.011, 2009.

Kaiser et al.(2006)Kaiser, Engel, Borchers, and Röckmann

Kaiser, J., Engel, A., Borchers, R., and Röckmann, T.: Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N2O isotope distribution, Atmos. Chem. Phys., 6, 3535–3556, 10.5194/acp-6-3535-2006, 2006.

Karion et al.(2010)Karion, Sweeney, Tans, and Newberger

Karion, A., Sweeney, C., Tans, P., and Newberger, T.: AirCore: An innovative atmospheric sampling system, J. Atmos. Ocean. Techn., 27, 1839–1853, 10.1175/2010JTECHA1448.1, 2010.

Kawagucci et al.(2005)Kawagucci, Tsunogai, Kudo, Nakagawa, Honda, Aoki, Nakazawa, and Gamo

Kawagucci, S., Tsunogai, U., Kudo, S., Nakagawa, F., Honda, H., Aoki, S., Nakazawa, T., and Gamo, T.: An analytical system for determining Δ17O in CO2 using continuous flow-isotope ratio MS, Anal. Chem., 77, 4509–4514, 10.1021/ac050266u, 2005.

Kawagucci et al.(2008)Kawagucci, Tsunogai, Kudo, Nakagawa, Honda, Aoki, Nakazawa, Tsutsumi, and Gamo

Kawagucci, S., Tsunogai, U., Kudo, S., Nakagawa, F., Honda, H., Aoki, S., Nakazawa, T., Tsutsumi, M., and Gamo, T.: Long-term observation of mass-independent oxygen isotope anomaly in stratospheric CO2, Atmos. Chem. Phys., 8, 6189–6197, 10.5194/acp-8-6189-2008, 2008.

Lämmerzahl et al.(2002)Lämmerzahl, Röckmann, Brenninkmeijer, Krankowsky, and Mauersberger

Lämmerzahl, P., Röckmann, T., Brenninkmeijer, C. M., Krankowsky, D., and Mauersberger, K.: Oxygen isotope composition of stratospheric carbon dioxide, Geophys. Res. Lett., 29, 2–5, 2002.

Mahata et al.(2012)Mahata, Bhattacharya, Wang, and Liang

Mahata, S., Bhattacharya, S. K., Wang, C.-H., and Liang, M.-C.: An improved CeO2 method for high-precision measurements of 17O / 16O ratios for atmospheric carbon dioxide., Rapid Commun. Mass Sp., 26, 1909–1922, 10.1002/rcm.6296, 2012.

Mook and Jongsma(1987)

Mook, W. G. and Jongsma, J.: Measurement of the N2O correction for 13C /12C ratios of atmospheric CO2 by removal of N2O, Tellus B, 39B, 96–99, 10.1111/j.1600-0889.1987.tb00274.x, 1987.

Mook and van der Hoek(1983)

Mook, W. G. and van der Hoek, S.: The N2O correction in the carbon and oxygen isotopic analysis of atmospheric CO2, Chem. Geol., 41, 237–242, 10.1016/S0009-2541(83)80021-7, 1983.

Mrozek et al.(2015)Mrozek, van der Veen, Kliphuis, Kaiser, Wiegel, and Röckmann

Mrozek, D. J., van der Veen, C., Kliphuis, M., Kaiser, J., Wiegel, A. A., and Röckmann, T.: Continuous-flow IRMS technique for determining the 17O excess of CO2 using complete oxygen isotope exchange with cerium oxide, Atmos. Meas. Tech., 8, 811–822, 10.5194/amt-8-811-2015, 2015.

Park et al.(2004)Park, Atlas, and Boering

Park, S., Atlas, E. L., and Boering, K. A.: Measurements of N2O isotopologues in the stratosphere: Influence of transport on the apparent enrichment factors and the isotopologue fluxes to the troposphere, J. Geophys. Res.-Atmos., 109, D01305, 10.1029/2003JD003731, 2004.

Paul et al.(2016)Paul, Chen, Been, Kivi, and Meijer

Paul, D., Chen, H., Been, H. A., Kivi, R., and Meijer, H. A. J.: Radiocarbon analysis of stratospheric CO2 retrieved from AirCore sampling, Atmos. Meas. Tech., 9, 4997–5006, 10.5194/amt-9-4997-2016, 2016.

Röckmann et al.(2003)Röckmann, Kaiser, Brenninkmeijer, and Brand

Röckmann, T., Kaiser, J., Brenninkmeijer, C. A. M., and Brand, W. A.: Gas chromatography/isotope-ratio mass spectrometry method for high-precision position-dependent 15N and 18O measurements of atmospheric nitrous oxide., Rapid Commun. Mass Sp., 17, 1897–908, 10.1002/rcm.1132, 2003.

Thiemens et al.(1995)Thiemens, Jackson, Zipf, Erdman, and Egmond

Thiemens, M., Jackson, T., Zipf, E. C., Erdman, P. W., and Egmond, V. C.: Carbon dioxide and oxygen isotope anomalies in the mesosphere and stratosphere, Science, 270, 969–972, 1995.

Wiegel et al.(2013)Wiegel, Cole, Hoag, Atlas, Schauffler, and Boering

Wiegel, A. A., Cole, A. S., Hoag, K. J., Atlas, E. L., Schauffler, S. M., and Boering, K. A.: Unexpected variations in the triple oxygen isotope composition of stratospheric carbon dioxide, P. Natl. Acad. Sci., 110, 17680–17685, 10.1073/pnas.1213082110, 2013.

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