Tunable Diode Laser Absorption Spectrometer Research Articles

Offsets in fiber-coupled diode laser hygrometers caused by parasitic absorption effects and their prevention

Large systematic errors in absorption spectrometers (AS) can be caused by ‘parasitic’ optical absorption (parA) outside the measurement region. In particular, calibration-free direct tunable diode laser AS (dTDLAS) can take advantage of an effective parA-compensation to provide correct absolute values. However, parA also negatively affects calibrated AS in calibration frequency and stability. A common strategy to suppress parA in TDLAS systems is to fiber-couple the light source and even the detector. However, this can be a critical approach if the TDL spectrometer is validated/calibrated under laboratory conditions in ambient humidity and used afterwards in much drier and variable conditions, for example in aircrafts. This paper shows that, e.g., ‘hermetically sealed’ butterfly packages, despite fiber coupling, can possess fixed as well as variable parA sections. Two new methods for absolute parA-quantification in dTDLAS were developed, including a novel, fiber-coupled, parA-free I0-detector for permanent parA-monitoring. Their dependences on ambient humidity/pressure and temporal behavior were studied. For the example of a 1.4 µm dTDLAS hygrometer SEALDH-II with a commercial DFB-laser module and an extractive 1.5 m path cell, we quantified the parA-induced signal offsets and their dependence on cell pressure. The conversion of parA-uncertainty into H2O signal uncertainty was studied and an updated uncertainty budget including parA-uncertainty was derived. The studies showed that parA in commercial laser modules can cause substantial, systematic concentration offsets of ≈25 ppmv fixed and ≈100 ppmv variable offsets for one meter absorption path. Applying our parA-quantification techniques these offsets could be compensated by a factor of 20 to an overall offset uncertainty of 4.5 ppmv m−1. Finally, we developed an innovative, integrated, µ-pumped closed-loop air drying unit for the parA minimization and temporal stabilization in airborne laser hygrometers. This compact and light weight dryer eliminates the variable parA by ambient humidity in less than 120 min and is well suited for airborne applications as it fulfils all airborne operation and safety restrictions.

The detectability of nitrous oxide mitigation efficacy in intensively grazed pastures using a multiple-plot micrometeorological technique

Abstract. Methodologies are required to verify agricultural greenhouse gas mitigation at scales relevant to farm management. Micrometeorological techniques provide a viable approach for comparing fluxes between fields receiving mitigation treatments and control fields. However, they have rarely been applied to spatially verifying treatments aimed at mitigating nitrous oxide emission from intensively grazed pastoral systems. We deployed a micrometeorological system to compare N2O flux among several ~1.5 ha plots in intensively grazed dairy pasture. The sample collection and measurement system is referred to as the Field-Scale Nitrous Oxide Mitigation Assessment System (FS-NOMAS) and used a tuneable diode laser absorption spectrometer to measure N2O gradients to high precision at four locations along a 300 m transect. The utility of the FS-NOMAS to assess mitigation efficacy depends largely on its ability to resolve very small vertical N2O gradients. The performance of the FS-NOMAS was assessed in this respect in laboratory and field-based studies. The FS-NOMAS could reliably resolve gradients of 0.039 ppb between a height of 0.5 and 1.0 m. The gradient resolution achieved corresponded to the ability to detect an inter-plot N2O flux difference of 26 μg N2O–N m−2 h−1 under the most commonly encountered conditions of atmospheric mixing (quantified here by a turbulent transfer coefficient), but this ranged from 11 to 59 μg N2O–N m−2 h−1 as the transfer coefficient ranged between its 5th and 95th percentile. Assuming a likely value of 100 μg N2O–N m−2 h−1 for post-grazing N2O fluxes from intensively grazed New Zealand dairy pasture, the system described here would be capable of detecting a mitigation efficacy of 26% for a single (40 min) comparison. We demonstrate that the system has considerably greater sensitivity to treatment effects by measuring cumulative fluxes over extended periods.

A new method for continuously measuring the δ13C of soil CO2 concentrations at different depths by laser spectrometry

SummarySoil carbon dioxide (CO2) efflux is an important component of the carbon (C) cycle but the biological and physical processes involved in soil CO2 production and transport are not fully understood. To improve our knowledge, we present a new approach to measure simultaneously soil CO2 concentrations and efflux, and their respective isotopic signatures (δ13C‐CO2). To quantify soil air 13CO2 and 12CO2 concentrations, we adapted a method based on CO2 diffusion from soil pores into tubes with a highly gas‐permeable membrane wall. These tubes were placed horizontally at different depths in the soil. Air was sampled automatically from the tubes and injected through a diluting system into a tuneable diode laser absorption spectrometer. The CO2 and δ13C‐CO2 vertical profiles were thus obtained at hourly intervals. Our tests demonstrated the absence of fractionation in the membrane tubes for δ13C‐CO2. Subsequently, we set up field experiments for two forest soils, which showed that natural soil CO2 concentrations and δ13C‐CO2 were not affected significantly by the measurement system. While δ13C‐CO2 in air‐filled pores below 5 cm was constant over 3 days, we observed large diurnal variations in δ13C‐CO2 efflux. However, the average difference between the two measurements was close to −4.4‰, which supports steady‐state diffusion over this 3‐day period. This new method seems to be a very effective way to measure the δ13C‐CO2 profile of the soil atmosphere, and demonstrates that the fractionation that occurs during diffusion is the main transport process that affects the δ13C‐CO2 of the soil CO2 efflux on a daily timescale while advection may account for within‐day variations.

Pressure-broadening and narrowing coefficients and temperature dependence measurements of CO2 at 2.68 μm by laser diode absorption spectroscopy for atmospheric applications

By using a tunable diode laser absorption spectrometer in conjunction with a cryogenically cooled multipath cell, we have revisited the air-induced pressure-broadening coefficients and the narrowing coefficients related to the Dicke effect, as well as the temperature dependences, for the R(18) and R(20) lines of the (10°1)I←(00°0) vibrational band at 2.68μm of carbon dioxide. The selected transitions are used to probe in situ CO2 in the troposphere and the lower stratosphere by using balloon-borne laser sensors. The achieved measurements are thoroughly compared to existing former determinations. The impact of processing the in situ atmospheric CO2 spectra with this new set of molecular data is reported.

Absolute, spatially resolved, in situ CO profiles in atmospheric laminar counter-flow diffusion flames using 2.3 μm TDLAS

We developed a new, spatially traversing, direct tunable diode laser absorption spectrometer (TDLAS) for quantitative, calibration-free, and spatially resolved in situ measurements of CO profiles in atmospheric, laminar, non-premixed CH4/air model flames stabilized at a Tsuji counter-flow burner. The spectrometer employed a carefully characterized, room temperature distributed feedback diode laser to detect the R20 line of CO near 2,313 nm (4,324.4 cm−1), which allows to minimize spectral CH4 interference and detect CO even in very fuel-rich zones of the flame. The burner head was traversed through the 0.5 mm diameter laser beam in order to derive spatially resolved CO profiles in the only 60-mm wide CH4/air flame. Our multiple Voigt line Levenberg–Marquardt fitting algorithm and the use of highly efficient optical disturbance correction algorithms for treating transmission and background emission fluctuations as well as careful fringe interference suppression permitted to achieve a fractional optical resolution of up to 2.4 × 10−4 OD (1σ) in the flame (T up to 1,965 K). Highly accurate, spatially resolved, absolute gas temperature profiles, needed to compute mole fraction and correct for spectroscopic temperature dependencies, were determined with a spatial resolution of 65 μm using ro-vibrational N2-CARS (Coherent anti-Stokes Raman spectroscopy). With this setup we achieved temperature-dependent CO detection limits at the R20 line of 250–2,000 ppmv at peak CO concentrations of up to 4 vol.%. This permitted local CO detection with signal to noise ratios of more than 77. The CO TDLAS spectrometer was then used to determine absolute, spatially resolved in situ CO concentrations in the Tsuji flame, investigate the strain dependence of the CO Profiles and favorably compare the results to a new flame-chemistry model.

French instruments for in-situ missions: Past, present and future

Two in-situ missions were launched at the end of 2011, soon after the 62nd IAC conference, one to Mars and the other to one of its moons, Phobos. The first – Mars Science Laboratory – has been developed by NASA and will take the largest ever rover, Curiosity, to the surface of Mars. The second mission, Phobos-Grunt under the responsibility of Roscosmos, will carry out orbital and in-situ experiments before sending Phobos samples to Earth. As of June 2012, MSL is on its way to Mars: landing is planned on August 6th, 2012 at 07h 31 AM (French time). On the other hand, unfortunately, Phobos-Grunt fell back to Earth in an uncontrolled re-entry on January15th , 2012, after rocket burns intended to set the craft on a course for Mars had failed 2 months earlier, shortly after launch.CNES is privileged, together with French scientific laboratories from CNRS, to have contributed to both missions. More specifically, we participate in two MSL instruments:–The mast part of ChemCam (Chemistry Camera) which will analyse by spectrometry the plasma light emitted by Martian rocks after a laser shot. ChemCam—Mast Unit encompasses a laser, a telescope, a camera and the associated electronics.–The Gas Chromatograph (SAM-GC), one of the three SAM (Sample Analysis at Mars) instruments. SAM detects a wide range of organic components from the atmosphere and the ground. It will also search for carbon isotopes, as well as noble gas isotopes.French contributions to Phobos Grunt involve:–The Gas-Chromatograph (GC) and the Tunable Diode Laser Absorption Spectrometer (TDLAS) of the Gas Analytic Package (GAP) which will characterise the molecular soil composition.–The supply of two panoramic cameras (PANCAM), of a pair of stereoscopic cameras (STEREO PAIR), and of a visible microscope (MicrOmega VIS).–The IR spectral microscope (MicrOmega IR), a new instrument that will perform the first in-situ characterisation by microscopic spectral imaging of the mineralogical and molecular composition of a probably nondifferentiated body. This characterisation will be decisive in determining the origin of Phobos, and as a reference for the sample analyses.Before describing extensively these contributions and their objectives, the paper will put them into perspective by presenting previous French involvement in surface missions, in particular on Cassini—Huygens, and on Rosetta—Philae. We will then elaborate on future missions, by presenting our participation in the in-situ segment of the ESA–NASA ExoMars mission, in the DLR–CNES Mascot asteroid lander to be carried by JAXA's Hayabusa 2 spacecraft, and other candidate missions such as GEMS (Geophysical Monitoring Station, in the final list for NASA's Discovery programme to be definitely selected for realisation in 2012), and Selene 2. We will conclude by highlighting the synergy between these missions for the various instrument families.

A tunable diode laser absorption spectrometer for formaldehyde atmospheric measurements validated by simulation chamber instrumentation

Abstract A tunable diode laser absorption spectrometer (TDLAS) for formaldehyde atmospheric measurements has been set up and validated through comparison experiments with a Fourier transform infrared spectrometer (FT-IR) in a simulation chamber. Formaldehyde was generated in situ in the chamber from reaction of ethene with ozone. Three HCHO ro-vibrational line intensities (at 2909.71, 2912.09 and 2914.46 cm −1) possibly used by TDLAS were calibrated by FT-IR spectra simultaneously recorded in the 1600-3200 cm −1 domain during ethene ozonolysis, enabling the on-line deduction of the varying concentration for HCHO in formation. The experimental line intensities values inferred confirmed the calculated ones from the updated HITRAN database. In addition, the feasibility of stratospheric in situ HCHO measurements using the 2912.09 cm −1 line was demonstrated. The TDLAS performances were also assessed, leading to a 2s detection limit of 88 ppt in volume mixing ratio with a response time of 60 sec at 30 Torr and 294 K for 112 m optical path. As part of this work, the room-temperature rate constant of this reaction and the HCHO formation yield were found to be in excellent agreement with the compiled literature data.

Is mesophyll conductance to CO2 in leaves of three Eucalyptus species sensitive to short-term changes of irradiance under ambient as well as low O2?

Mesophyll conductance to CO2 (g m) limits the diffusion of CO2 to the sites of carboxylation, and may respond rapidly (within minutes) to abiotic factors. Using three Eucalyptus species, we tested the rapid response of g m to irradiance under 21% and 1% O2. We used simultaneous measurements of leaf gas exchange and discrimination against 13CO2 with a tuneable diode laser absorption spectrometer. Measurements under 1% O2 were used to limit uncertainties due to 13C-12C fractionation occurring during photorespiration. Switching irradiance from 600 to 200µmolm-2s-1 led to a ≈60% decrease of g m within minutes in all species under both 21% O2 and 1% O2. The g m response to irradiance is unlikely to be a computation artefact since using different values for the parameters of the discrimination model changed the absolute values of g m but did not affect the relative response to irradiance. Simulations showed that possible rapid changes of any parameter were unable to explain the observed variations of g m with irradiance, except for13C-12C fractionation during carboxylation (b), which, in turn, is dependent on the fraction of leaf C assimilated by phospho-enol pyruvate carboxylase (PEPc) (β). g m apparently increased by ≈30% when O2 was switched from 21% to 1% O2. Again, possible changes of β with O2 could explain this apparent g m response to O2. Nevertheless, large irradiance or O2-induced changes in β would be required to fully explain the observed changes in g m, reinforcing the hypothesis that g m is responsive to irradiance and possibly also to O2.

Self-induced pressure shift and temperature dependence measurements of CO 2 at 2.05 μm with a tunable diode laser spectrometer

By using a high resolution tunable diode laser absorption spectrometer combined with a cryogenically cooled optical multi-pass cell, we have measured the self-induced pressure shift coefficients for 8 transitions in the R branch of the (20 01) III ← (00 00) I band of carbon dioxide around 2.05 μm. This spectral region is of particular interest for the monitoring of atmospheric CO 2 with Differential Absorption Lidars (DiAL). The measurement of these shift coefficients was realized at five different temperatures ranging from 218 to 292 K in order to determine their temperature dependence. The results are thoroughly compared to previous values reported in the literature for the (20 01) III ← (00 00) I band of CO 2. The temperature dependence of the self-induced pressure shifts are reported experimentally for the first time for this specific CO 2 band.

Mesophyll conductance to CO₂, assessed from online TDL-AS records of ¹³CO₂ discrimination, displays small but significant short-term responses to CO₂ and irradiance in Eucalyptus seedlings.

Mesophyll conductance (gm) is now recognized as an important limiting process for photosynthesis, as it results in a significant decrease of CO2 diffusion from substomatal cavities where water evaporation occurs, to chloroplast stroma. Over the past decade, an increasing number of studies proposed that gm can vary in the short term (e.g. minutes), but these variations are still controversial, especially those potentially induced by changing CO2 and irradiance. In this study, gm data estimated with online 13C discrimination recorded with a tunable diode laser absorption spectrometer (TDL-AS) during leaf gas exchange measurements, and based on the single point method, are presented. The data were obtained with three Eucalyptus species. A 50% decrease in gm was observed when the CO2 mole fraction was increased from 300 μmol mol−1 to 900 μmol mol−1, and a 60% increase when irradiance was increased from 200 μmol mol−1 to 1100 μmol mol−1 photosynthetic photon flux density (PPFD). The relative contribution of respiration and photorespiration to overall 13C discrimination was also estimated. Not taking this contribution into account may lead to a 50% underestimation of gm but had little effect on the CO2- and irradiance-induced changes. In conclusion, (i) the observed responses of gm to CO2 and irradiance were not artefactual; (ii) the respiratory term is important to assess absolute values of gm but has no impact on the responses to CO2 and PPFD; and (iii) increasing irradiance and reducing the CO2 mole fraction results in rapid increases in gm in Eucalyptus seedlings.

Tunable diode laser measurement of pressure-induced shift coefficients of CO 2 around 2.05 μm for Lidar application

Atmospheric carbon dioxide (CO 2) is one of the main contributors to the greenhouse effect. A global monitoring of CO 2 from space is foreseen as a key issue to quantify its sources and sinks at a regional scale and to better predict future levels of CO 2 and their effect on climate change. Differential Absorption Lidar (DiAL) is a promising and novel spectroscopic technique for remote sensing CO 2 spatial and temporal concentration distribution with a high level of accuracy. However, a precise knowledge of spectroscopic parameters of CO 2 molecular transitions and their dependence with temperature and pressure is required for reducing the uncertainty on DiAl measurements. Hence, to support remote sensing of carbon dioxide in the troposphere, we report on the accurate determination of air pressure-induced shift coefficients for eight absorption lines belonging to the R branch of (20 01) III←(00 00) I band of CO 2 at 2.05 μm. Purposely, a high-resolution tunable diode laser absorption spectrometer (TDLAS) coupled to a cryogenically cooled optical cell was implemented. From these measurements, we have further determined the temperature-dependencies of the air pressure-induced shift coefficients.

A lightweight near-infrared spectrometer for the detection of trace atmospheric species

This paper describes the development and deployment of a lightweight in situ near-infrared tunable diode laser absorption spectrometer (TDLAS) for balloon-borne measurements of trace species such as methane in the upper troposphere and stratosphere. The key feature of the instrument design is its ability to provide high sensitivity measurements with better than 1 part in 10(6) Hz(-1/2) optical sensitivity in a lightweight package weighing as little as 6 kg, and maintaining this level of performance over the wide range of conditions experienced during field measurements. The absolute accuracy for methane measurements is approximately 10% limited by uncertainties in determining the gas temperature in the measurement volume. The high sensitivity and high temporal resolution (2.3 s measurement period) enables details of the fine-scale structure in the atmosphere to be measured. The TDLAS instrument has been used on a number of major international measurement campaigns. Intercomparison with other instruments during these campaigns have confirmed the comparability of the results from this instrument with measurements made by a range of other techniques, and demonstrated the instruments suitability for studies of atmospheric dynamics, transport, and mixing processes.

Photosynthetic carbon isotope discrimination and its relationship to the carbon isotope signals of stem, soil and ecosystem respiration

• Photosynthetic carbon (C) isotope discrimination (Δ(Α)) labels photosynthates (δ(A) ) and atmospheric CO(2) (δ(a)) with variable C isotope compositions during fluctuating environmental conditions. In this context, the C isotope composition of respired CO(2) within ecosystems is often hypothesized to vary temporally with Δ(Α). • We investigated the relationship between Δ(Α) and the C isotope signals from stem (δ(W)), soil (δ(S)) and ecosystem (δ(E)) respired CO(2) to environmental fluctuations, using novel tuneable diode laser absorption spectrometer instrumentation in a mature maritime pine forest. • Broad seasonal changes in Δ(Α) were reflected in δ(W,) δ(S) and δ(E). However, respired CO(2) signals had smaller short-term variations than Δ(A) and were offset and delayed by 2-10 d, indicating fractionation and isotopic mixing in a large C pool. Variations in δ(S) did not follow Δ(A) at all times, especially during rainy periods and when there is a strong demand for C allocation above ground. • It is likely that future isotope-enabled vegetation models will need to develop transfer functions that can account for these phenomena in order to interpret and predict the isotopic impact of biosphere gas exchange on the C isotope composition of atmospheric CO(2).

A Compact Tunable Diode Laser Absorption Spectrometer to Monitor CO2 at 2.7 µm Wavelength in Hypersonic Flows

Since the beginning of the Mars planet exploration, the characterization of carbon dioxide hypersonic flows to simulate a spaceship’s Mars atmosphere entry conditions has been an important issue. We have developed a Tunable Diode Laser Absorption Spectrometer with a new room-temperature operating antimony-based distributed feedback laser (DFB) diode laser to characterize the velocity, the temperature and the density of such flows. This instrument has been tested during two measurement campaigns in a free piston tunnel cold hypersonic facility and in a high enthalpy arc jet wind tunnel. These tests also demonstrate the feasibility of mid-infrared fiber optics coupling of the spectrometer to a wind tunnel for integrated or local flow characterization with an optical probe placed in the flow.

Absolute diode laser-based in situ detection of HCl in gasification processes

The release of HCl is an important parameter for industrial combustion and gasification processes, which must be determined in the ppm range for active process control and optimization. Based on a low power vertical-cavity surface-emitting laser (VCSEL) at 1.74 μm, we developed a new tuneable diode laser absorption spectrometer for calibration-free, absolute in situ HCl detection using the H35Cl (2 ← 0) R(3) absorption line with minimized cross-sensitivity to CO2 and H2O. The spectrometer was applied to in situ measurements in a gasification process (T = 1,130°C, P = 1 atm, L = 28 cm) and yielded an optical resolution of 2.3·10−4, i.e. a HCl sensitivity of 45 ppm (13 ppm·m).

Characterizing a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of atmospheric ammonia

Abstract. A compact, fast-response Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of ammonia (NH3) has been evaluated under both laboratory and field conditions. Absorption of radiation from a pulsed, thermoelectrically cooled QC laser occurs at reduced pressure in a 0.5 L multiple pass absorption cell with an effective path length of 76 m. Detection is achieved using a thermoelectrically-cooled Mercury Cadmium Telluride (HgCdTe) infrared detector. A novel sampling inlet was used, consisting of a short, heated, quartz tube with a hydrophobic coating to minimize the adsorption of NH3 to surfaces. The inlet contains a critical orifice that reduces the pressure, a virtual impactor for separation of particles, and additional ports for delivering NH3-free background air and calibration gas standards. The level of noise in this instrument has been found to be 0.23 ppb at 1 Hz. The sampling technique has been compared to the results of a conventional lead salt Tunable Diode Laser Absorption Spectrometer (TDLAS) during a laboratory intercomparison. The effect of humidity and heat on the surface interaction of NH3 with sample tubing was investigated at mixing ratios ranging from 30–1000 ppb. Humidity was seen to worsen the NH3 time response and considerable improvement was observed when using a heated sampling line. A field intercomparison of the QC-TILDAS with a modified Thermo 42CTL chemiluminescence-based analyzer was also performed at Environment Canada's Centre for Atmospheric Research Experiments (CARE) in the rural town of Egbert, ON between May–July 2008. Background tests and calibrations using two different permeation tube sources and an NH3 gas cylinder were regularly carried out throughout the study. Results indicate a very good correlation at 1 min time resolution (R2 = 0.93) between the two instruments at the beginning of the study, when regular background subtraction was applied to the QC-TILDAS. An overall good correlation of R2 = 0.85 was obtained over the entire two month data set, where the majority of the spread can be attributed to differences in inlet design and background subtraction methods.

An injection method for measuring the carbon isotope content of soil carbon dioxide and soil respiration with a tunable diode laser absorption spectrometer

We present a novel technique in which the carbon isotope ratio (delta(13)C) of soil CO(2) is measured from small gas samples (<5 mL) injected into a stream of CO(2)-free air flowing into a tunable diode laser absorption spectrometer (TDL). This new method extends the dynamic range of the TDL to measure CO(2) mole fractions ranging from ambient to pure CO(2), reduces the volume of sample required to a few mL, and does not require field deployment of the instrument. The measurement precision of samples stored for up to 60 days was 0.23 per thousand. The new TDL method was applied with a simple gas well sampling technique to obtain and measure gas samples from shallow soil depth increments for CO(2) mole fraction and delta(13)C analysis, and subsequent determination of the delta(13)C of soil-respired CO(2). The method was tested using an artificial soil system containing a controlled CO(2) source and compared with an independent method using the TDL and an open soil chamber. The profile and chamber estimates of delta(13)C of an artificially produced CO(2) flux were consistent and converged to the delta(13)C of the CO(2) source at steady state, indicating the accuracy of both methods under controlled conditions. The new TDL method, in which a small pulse of sample is measured on a carrier gas stream, is analogous for the TDL technique to the development of continuous-flow configurations for isotope ratio mass spectrometry. While the applications presented here are focused on soil CO(2), this new TDL method could be applied in a number of situations requiring measurement of delta(13)C of CO(2) in small gas samples with ambient to high CO(2) mole fractions.

More evidence for very short-lived substance contribution to stratospheric chlorine inferred from HCl balloon-borne in situ measurements in the tropics

Abstract. Volume mixing ratio (vmr) vertical profiles of hydrogen chloride (HCl) are retrieved from in situ measurements performed by a balloon-borne infrared tunable diode laser absorption spectrometer (SPIRALE) during two balloon flights in the tropics (Teresina, Brazil, 5.1° S–42.9° W) in June 2005 and June 2008. HCl vertical profiles obtained from 15 to 31 km are presented and analysed to estimate the contribution of very short-lived substances (VSLS) to total stratospheric chlorine. Both retrieved vertical profiles of HCl from these flights agree very well with each other, with estimated overall uncertainties of 6% on vmr between 23 and 31 km. Upper limits of HCl vmr as low as 20 pptv in June 2008 and 30 pptv in June 2005 are inferred in the upper part of the tropical tropopause layer (TTL). Backward trajectory calculations and such low amounts suggest that the air masses sampled correspond to typical background conditions, i.e. neither influenced by recent tropospheric nor stratospheric air. Taking into account the recently reported VSL source gas measurements obtained in similar conditions (Laube et al., 2008) and the main intermediate degradation product gas COCl2 (Fu et al., 2007), a total VSLS contribution of 85±40 pptv to stratospheric chlorine is inferred. This refines the WMO (2007) estimation of 50 to 100 pptv, which was not taking into account any HCl contribution. In addition, comparisons of HCl measurements between SPIRALE and the Aura MLS satellite instrument in the tropical lower and middle stratosphere lead to a very good agreement. The previous agreement between MLS-deduced upper stratospheric total chlorine content and modelled values including 100 pptv of VSLS (Froidevaux et al., 2006) is thus supported by our present result about the VSLS contribution.

Tracing of recently assimilated carbon in respiration at high temporal resolution in the field with a tuneable diode laser absorption spectrometer after in situ 13CO2 pulse labelling of 20-year-old beech trees

The study of the fate of assimilated carbon in respiratory fluxes in the field is needed to resolve the residence and transfer times of carbon in the atmosphere-plant-soil system in forest ecosystems, but it requires high frequency measurements of the isotopic composition of evolved CO2. We developed a closed transparent chamber to label the whole crown of a tree and a labelling system capable of delivering a 3-h pulse of 99% 13CO2 in the field. The isotopic compositions of trunk and soil CO2 effluxes were recorded continuously on two labelled and one control trees by a tuneable diode laser absorption spectrometer during a 2-month chase period following the late summer labelling. The lag times for trunk CO2 effluxes are consistent with a phloem sap velocity of about 1 m h(-1). The isotopic composition (delta13C) of CO2 efflux from the trunk was maximal 2-3 days after labelling and declined thereafter following two exponential decays with a half-life of 2-8 days for the first and a half-life of 15-16 days for the second. The isotopic composition of the soil CO2 efflux was maximal 3-4 days after labelling and the decline was also well fitted with a sum of two exponential functions with a half-life of 3-5 days for the first exponential and a half-life of 16-18 days for the second. The amount of label recovered in CO2 efflux was around 10-15% of the assimilated 13CO2 for soil and 5-13% for trunks. As labelling occurred late in the growing season, substantial allocation to storage is expected.

Introduction to a Virtual Special Issue: probing the carbon cycle with 13C

Introduction to a <i>Virtual Special Issue</i>: probing the carbon cycle with ¹³C