Trace water is one of the most critical matrix contaminants in ultra-high purity (UHP) process gases, like argon (Ar) and nitrogen (N₂), and many others. Even trace amounts can severely degrade the quality of many products reliant on these gases. Despite its importance to advanced technology sectors, notably semiconductor manufacturing, it has proven quite difficult to realize preparative or analytical trace water metrology over the full amount fraction range needed or in the broad spectrum of industrially relevant matrix gases. Within the EU-funded PROMETH2O project consortium, this challenge has been addressed through the development or significant improvement of traceable measurement methods and standards spanning 5 nmol mol⁻¹ to 5 µmol mol⁻¹, tailored for use in UHP process gas production, such as Ar, N2 and hydrogen (H2). The measurement ranges were extended and the uncertainties were improved, while being consistent with current best practice at primary humidity standards laboratories. These capabilities were validated in applications relevant to process instrumentation and the gas industry. A distributed metrological infrastructure at various European National Metrology Institutes and partner sites now provides SI-traceable trace water measurements in various UHP, strongly supporting and extending the calibration capabilities for the gas and semiconductor industries and the associated stakeholders.

We describe thorough metrological characterization of a cavity ring-down spectrometer, combined with proprietary spectral evaluation algorithm. Our measurements show good linearity, good accuracy and short response time of the instrument in the 50-400 nmol/mol amount fraction range. Our spectral evaluation algorithm enables decreasing the offset to 0.12 nmol/mol from the values around 0.6-0.9 nmol/mol reported using the built-in data evaluation of the commercial spectrometer. We have successfully eliminated cross-sensitivity by water vapor via an advanced spectral fitting approach using line parameters published in the HITRAN2020 database. Our spectrometer has been compared to a traceable reference gas standard and the results show that it is a good candidate to become an optical gas standard for ammonia amount fraction measurements under laboratory conditions.

Main text The report summarises the results of the CCT-K8 key comparison in the field of humidity. The main target of this comparison was to obtain the degree of equivalence (DoE) between realizations of local scales of dew point temperature of humid gas (air), in the range from 30 °C to 95 °C, among the participating national metrology institutes. The report gives details of the scheme, procedure and analysis of the comparison. The overall results display a very good agreement of the nine participating laboratories. The results show excellent agreement between the realized scales with no detected outliers. The performance of the travelling standards was excellent and can be attributed to the intensive preconditioning and selection of the platinum resistance thermometers, prior to their integration into the measuring head assembly, that led to insignificant drift. In the conclusion, some lessons learned are reported that could be beneficial for future key comparisons. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA). To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/ . The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Methane (CH4) is a greenhouse gas (GHG) with both anthropogenic and natural sources. It also contributes to air quality problems through its role in tropospheric ozone formation. Key source categories of anthropogenic CH4 emissions in Europe are the agricultural sector (~50 %), waste (~22 %), and energy (~15 %), which makes them the focus of intense research for developing mitigation actions. Stable isotope ratio measurement in CH4 provide the information needed to verify emissions by source type. To provide comparable and accurate atmospheric CH4 isotope ratios, there is an increasing need to develop metrological harmonized measurements protocols and procedures. In addition, there is a lack of a metrological infrastructure for source signature information needed to interpret atmospheric isotope ratio measurements, as well as an assessment of uncertainties in atmospheric transport models and inverse estimates of Europe's CH4 emissions.Here, we present the isoMET project that aims to (a) develop a harmonised in situ CH4 isotope dataset of ambient air in Europe to resolve compatibility issues of measurements of δ13C or δ2H in CH4 across multiple laboratories, b) develop a sustainable metrological infrastructure for a dataset for δ13C(CH4) and δ2H(CH4)-emissions source measurements in Europe and to evaluate the potential for source apportionment through clumped isotopes, c) use atmospheric chemistry transport modelling to inform the work in (a) and (b), creating estimates of the minimum measurement requirements for deployed instruments. References[1] isoMET project available at: https://www.npl.co.uk/21grd04-isomet[2] J. A. Nwaboh, J. Mohn, M. Fatima, T. Arnold, V. Ebert, Metrology for European emissions verification on methane isotopes (isoMET), CCQM GAWG-IRWG Workshop on Carbon Dioxide and Methane Stable Isotope Ratio Measurements, LATU (Uruguay), 2023Acknowledgements: The project 21GRD04 isoMET project has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. Empa has received funding from the Swiss State Secretaritat for Education, Research and Innovation (SERI).

Accurate measurements of amount fractions and isotopic compositions of greenhouse gas such as carbon dioxide (CO2) and methane (CH4) provide valuable insights on their atmospheric composition and origin. Commonly used field deployable commercial laser spectrometers that measure amount fractions and isotopic ratios are often calibrated with reference gases with certified amount fractions and/or isotopic composition. Reference gases, also known as calibration reference materials (CRMs), can be for example synthetic mixtures of e.g. CO2 in N2, where the gas matrix N2 does not match that of the sample (e.g. ambient air) to be measured. A mismatch in the composition of the gas matrix of a CRM and sample can lead to a considerable bias in the amount fraction or isotopic ratio results of the sample due to changes in the measured spectra which e.g. are not perfectly captured by the analysers’ fitting routine.In this work, we demonstrate the quantification of matrix effects for two commercial CRDS analysers measuring CO2 and CH4 amount fractions and isotope ratios. In our experiments with synthetic air gas matrix where the O2 concentration was varied, we measured (for a 1 % change in the O2 concentration in the gas matrix) a relative change of 0.15 % for the amount fractions of two major CO2 isotopologues and 0.07 % for the amount fractions of two major CH4 isotopologues. Similarly, in terms of isotopic δ13C values, we found matrix effects larger than 0.2 for both CO2 and CH4. We present options for correcting the gas matrix effects and discuss the underlying assumptions made during the analysis. Amount fraction results for CO2 and CH4 are reported including δ13C isotope ratio results. Our work concludes that a matrix mismatch when using a commercial laser spectrometer can lead to considerable biases in amount fraction and isotope ratio results, and appropriate correction approaches have to be applied in order to achieve accurate and reliable results.Acknowledgements: This work has received partial funding from the EMPIR programme (19ENV05 STELLAR project) co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. Part of this work has also received funding from the European Partnership on Metrology (21GRD04 isoMET project), co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States.

Mid-infrared laser absorption spectroscopy enables rapid and nondestructive analysis of methane clumped isotopes. However, current analytical methods require a sample size of 20 mL STP (0.82 mmol) of pure CH4 gas, which significantly limits its application to natural samples. To enhance the performance of spectroscopic measurement of methane clumped isotopes, we established a laser spectroscopic platform with newly selected spectral windows for clumped isotope analysis: 1076.97 cm-1 for 12CH2D2 and 1163.47 cm-1 for 13CH3D, and a custom-built gas inlet system. These spectral windows were identified through an extensive spectral survey on newly recorded high-resolution Fourier transform infrared (FTIR) spectra across the wavelength range of 870-3220 cm-1, thereby addressing gaps for 12CH2D2 in existing spectral databases. In addition, we implemented several key technological advances, which result in superior control and performance of sample injection and analysis. We demonstrate that for small samples ranging from 3 to 10 mL (0.12-0.41 mmol) of CH4 gas, a measurement precision comparable to high-resolution isotope ratio mass spectrometry for Δ12CH2D2 (∼1.5‰) can be achieved through 3 to 8 repetitive measurements using a recycle-refilling system within a few hours. Samples larger than 10 mL can be quantified in under 20 min. At the same time, for Δ13CH3D analysis a repeatability of 0.05‰, superior to mass spectrometry, was realized. These advancements in reducing sample size and shortening analysis time significantly improve the practicality of the spectroscopic technique for determining the clumped isotope signatures of natural methane samples, particularly for applications involving low CH4 concentrations or requiring consecutive analyses, which are feasible in conjunction with an automated preconcentration system.

Depositional ice growth is an important process for cirrus cloud evolution, but the physics of ice growth in atmospheric conditions is still poorly understood. One major challenge in constraining depositional ice growth models against observations is that the early growth rates of ice crystals cannot be directly observed, and proposed models require assumptions about the functional dependence of physical processes that are still highly uncertain. Neural ordinary differential equations (NODE's) are a recently developed machine learning method that can be used to learn the derivative of an unknown function. Here we use NODE's to learn the functional dependence of unknown physics in the depositional ice growth model by optimizing against experimental measurements of ice crystal mass. We find a functional form for the depositional ice growth model that best fits 290 mass time series of ice crystals grown in a levitation diffusion chamber. We use symbolic regression to derive an equation for the function learned by the NODE model, which includes additional terms proportional to ice crystal mass in the capacitance growth model. We evaluate this functional form against experimental data sets from the AIDA Aerosol and Cloud Chamber, finding that our new proposed model for depositional ice growth accurately reproduces experimental results in the early stages of ice crystal growth.

Hydrogen chloride (HCl) is a colourless, toxic, and highly corrosive gas that poses risks to both human health and the environment. It is emitted from various sources, including fossil fuel combustion and municipal waste incineration. Consequently, most governments have established stack emission limits for HCl and monitor the emission levels accordingly. To ensure reliable measurement data from chimneys, periodic calibration of HCl monitors is essential using standard gas mixtures. However, due to its highly corrosive nature and strong tendency to adsorb onto the inner surface of cylinders and gas tubes, developing HCl standard gas mixtures in high-pressure cylinders remains a challenge. This comparison aims to evaluate calibration and measurement capabilities to ensure international equivalence for HCl in nitrogen at an amount fraction of 30 µmol mol -1 . This comparison result showed that four participants (NPL, VSL, PTB, and KRISS) agreed with their KCRV, but two participants (NIST and VNIIM) were discrepant. The consensus value underlined the showed agreement. To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/ . The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The quickly developing drone technology can be used efficiently in the field of pipeline leak detection. The aim of this article is to provide drone mission concepts for detecting releases from pipelines. It provides an overview of the current applications of natural gas pipeline surveys, it considers environmental conditions by plume modelling, it discusses suitable commercially available sensors, and develops concepts for routine monitoring of pipelines and short term missions for localising and identifying a known leakage. Suitable platforms depend on the particular mission and requirements concerning sensors and legislation. As an illustration, a feasibility study during a release experiment is introduced. The main challenge of this study was the variability of wind direction on a time scale of minutes, which produces considerable differences to the plume simulations. Nevertheless, the leakage rates derived from the observations are in the same order of magnitude as the emission rates. Finally the results from the modeling, the release experiment and possible drone scenarios are combined and requirements for future application derived.

We have obtained low-uncertainty absorption cross sections of sulfur hexafluoride (SF6) in mixtures with synthetic air at 296 K in the mid-infrared region. An explicit uncertainty calculation for every wavenumber has been performed. The integrated intensities in the region of the two IR-active fundamental bands ν3 and ν4 were evaluated with expanded uncertainties below 1.2 %. Several combination bands, involving ν3, ν4 and ν5 fundamentals were also targeted. The integrated intensities higher than 10–19 cm/molec (transitions ν2+ν4, ν1+ν3, ν2+ν3 and ν5+ν6) were determined with uncertainties below 2.8 %. Data files are available online, doi: 10.7795/720.20231027.

Accurate measurements of amount fractions and isotopic compositions of greenhouse gas such as carbon dioxide (CO2) and methane (CH4) provide valuable insights on their atmospheric composition and origin. Commonly used field deployable commercial laser spectrometers that measure amount fractions and isotopic ratios are often calibrated with reference gases with certified amount fractions and/or isotopic composition. Reference gases, also known as calibration reference materials (CRMs), can be for example synthetic mixtures of e.g. CO2 in N2, where the gas matrix N2 does not match that of the sample (e.g. ambient air) to be measured. A mismatch in the composition of the gas matrix of a CRM and sample can lead to a considerable bias in the amount fraction or isotopic ratio results of the sample due to changes in the measured spectra which e.g. are not perfectly captured by the analysers’ fitting routine. In this work, we demonstrate the quantification of matrix effects for two commercial CRDS analysers measuring CO2 and CH4 amount fractions and isotope ratios. In our experiments with synthetic air gas matrix where the O2 concentration was varied, we measured (for a 1 % change in the O2 concentration in the gas matrix) a relative change of 0.15 % for the amount fractions of two major CO2 isotopologues and 0.07 % for the amount fractions of two major CH4 isotopologues. Similarly, in terms of isotopic δ13C values, we found matrix effects larger than 0.2 for both CO2 and CH4. We present options for correcting the gas matrix effects and discuss the underlying assumptions made during the analysis. Amount fraction results for CO2 and CH4 are reported including δ13C isotope ratio results. Our work concludes that a matrix mismatch when using a commercial laser spectrometer can lead to considerable biases in amount fraction and isotope ratio results, and appropriate correction approaches have to be applied in order to achieve accurate and reliable results. Acknowledgements: This work has received partial funding from the EMPIR programme (19ENV05 STELLAR project) co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. Part of this work has also received funding from the European Partnership on Metrology (21GRD04 isoMET project), co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States.

PTB is developing a new thermodynamic, primary standard for gas humidity based on the principle of a 2-pressure 2-temperature humidity generator [Deschermeier, R., Bubser, F., Ebert, E. "Conception and Design of a Novel Primary Humidity Generator", In: Posterprasentation TEMPMEKO 2019, Chengdu, China]. The new generator will cover a frost-/dewpoint range from -40 degrees C up to +95 degrees C temperature, while allowing a significantly higher maximum flow rate of 20 L/min and a maximum pressure within the generator block of 1.5 MPa. Depending on the target dew-/frostpoint temperature the gas phase saturation will be over liquid or solid water. PTB thereby significantly extends the available working range of its primary realization for calibration and research projects. The new, highly modular concept - in terms of mechanical, electronic and software - ensures a maximized flexibility for further adaptations of the system to upcoming requirements from industry or environmental sciences.

We have obtained low-uncertainty absorption cross sections, together with explicit uncertainties for every wavenumber, for mixtures of tetrafluoromethane (CF4) with air at 296 K. The integrated intensities in the region of the two fundamental bands v3 and v4 as well as four combination bands, namely v1 + v4, v2 + v4, v3 + v4, and v2 + v3, were also evaluated. The expanded uncertainties are below 1.3 % for the fundamental and below 3 % for the combination bands. An intensity estimation for the forbidden v1 band is also obtained. Spectral data is available online: doi: 10.7795/720.20230920.

Abstract. Ice growth from vapor deposition is an important process for the evolution of cirrus clouds, but the physics of depositional ice growth at the low temperatures (<235 K) characteristic of the upper troposphere and lower stratosphere is not well understood. Surface attachment kinetics, generally parameterized as a deposition coefficient αD, control ice crystal habit and also may limit growth rates in certain cases, but significant discrepancies between experimental measurements have not been satisfactorily explained. Experiments on single ice crystals have previously indicated the deposition coefficient is a function of temperature and supersaturation, consistent with growth mechanisms controlled by the crystal's surface characteristics. Here we use observations from cloud chamber experiments in the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) aerosol and cloud chamber to evaluate surface kinetic models in realistic cirrus conditions. These experiments have rapidly changing temperature, pressure, and ice supersaturation such that depositional ice growth may evolve from diffusion limited to surface kinetics limited over the course of a single experiment. In Part 1, we describe the adaptation of a Lagrangian parcel model with the Diffusion Surface Kinetics Ice Crystal Evolution (DiSKICE) model (Zhang and Harrington, 2014) to the AIDA chamber experiments. We compare the observed ice water content and saturation ratios to that derived under varying assumptions for ice surface growth mechanisms for experiments simulating ice clouds between 180 and 235 K and pressures between 150 and 300 hPa. We found that both heterogeneous and homogeneous nucleation experiments at higher temperatures (>205 K) could generally be modeled consistently with either a constant deposition coefficient or the DiSKICE model assuming growth on isometric crystals via abundant surface dislocations. Lower-temperature experiments showed more significant deviations from any depositional growth model, with different ice growth rates for heterogeneous and homogeneous nucleation experiments.

Main text The pilot study CCQM-P204 was aimed at evaluating the level of compatibility of laboratories' measurement capabilities to value assign isotope ratios in samples of pure CO 2 gas, expressed as isotope delta values relative to the relevant international scale: δ 13 C VPDB and δ 18 O VPDB-CO2 . Pure CO 2 gas samples were prepared by the BIPM in batches of 10 samples of the same gas and circulated to participants for measurement. Each participant received four samples of CO 2 with a different nominal δ 13 C VPDB value: −1 ‰; −9 ‰; −35 ‰; and −42 ‰. The BIPM was also responsible for evaluating the homogeneity and stability of the samples. The co-coordinator IAEA received one sample per batch to confirm the batch-to-batch homogeneity. Within-batch and between-batch inhomogeneity was assessed and found to be negligible in comparison to the spread of results reported by participants. Participants used the analytical technique of their choice to measure the isotope delta values. They were requested to report the measurement results together with detailed information on their traceability, measurement methods and data treatment. Results of the comparison were to be compiled by the BIPM and evaluated jointly by the BIPM and the IAEA. The majority of participants reported results using DI-IRMS, and those that reported results based on laser spectroscopy techniques showed a very similar dispersion of results as for DI-IRMS, although generally with greater uncertainty. A total of nineteen participants reported their measurements, with two of them reporting results with different reference materials to provide more insight into the traceability of the measurements. The results were reported with traceability to three different VPDB scale realizations, notably VPDB, VPDB-LSVEC and VPDB2020, with 8, 7 and 6 results reported respectively for each of these. Participants agreed that results based on VPDB and VPDB2020 scale realizations should, in principle, lead to consistent results, whereas those based on VPDB-LSVEC should show a bias that increased as samples became more depleted in 13 C, with the bias approaching 0.2 ‰ for the most depleted sample. This bias was demonstrated by the participant reporting the most precise measurements based on the VPDB-LSVEC realizations, whereas for 2 participants using VPDB-LSVEC scale realizations other issues dominated the consistency of their results. The 3 laboratories using the NIST (8562,8563, 8564) reference materials (reported as on the VPDB-LSVEC scale), were highly consistent with each other, but the reported bias for the VPDB-LSVEC realization was not evident, with the historical method used for value assignment of the NIST RMs, and their relatively large uncertainty, being identified as possible causes for this. In general, for all results the dispersion was greater than expected based on the measurement uncertainties reported by participants. This dispersion increased as the samples became more depleted in 13 C, so that results that were traceable to realizations of the VPDB scale that could be considered equivalent (VPDB and VPDB2020) did not lead to ensembles that were fully consistent within their stated uncertainties. Either the reduced chi-squared or Birge Ratio provide easily calculated quantities to characterise lack of consistency in a data set, where consistent data would lead to values of unity for either of these, and discrepant data leading to increased values. This is most readily demonstrated by considering results based on DI-IRMS with traceability to the VPDB scale through either VPDB and VPDB 2020 realizations, where the standard deviation of 16 results was 0.043 ‰ and a Birge Ratio of 2.7 calculated for nominally −1‰ for δ 13 C, and the standard deviation was 0.12 ‰ and a Birge Ratio of 2.9 calculated at nominally −9 ‰ for δ 18 O. For the samples where the nominal δ 13 C value was −42 ‰, the standard deviation of 17 results was 0.085 ‰ and a Birge Ratio of 4.5 calculated for δ 13 C, and the standard deviation was 0.24 ‰ and a Birge Ratio of 3.4 calculated for δ 18 O at nominally −36 ‰. The observed magnitude of the standard deviation of results can also be compared to the standard uncertainty of the IAEA-603 materials certified values (0.01 ‰ for δ 13 C and 0.04 ‰ δ 18 O) and the smallest standard uncertainties reported by a participant (0.005 ‰ for δ 13 C, and 0.01 ‰ for δ 18 O). These results indicate an underestimation of uncertainty for reported results, especially for those with the smallest uncertainties. Components of uncertainty that should be reviewed before future comparisons include: the uncertainty contribution from reference materials; the uncertainty associated with the phosphoric acid reaction with carbonate reference materials; corrections and uncertainties related to cross-contamination effects in the IRMS; appropriate methods for combining uncertainties of multiple reference materials and accounting for their correlations. A retreatment of results, which normalizes results to the −1 ‰ and −42 ‰ δ 13 C samples, leads to improvement in the consistency of results as demonstrated for measurements on the nominally −35 ‰ δ 13 C, −30 ‰ δ18O samples for which the standard deviation is reduced to 0.034 ‰ and 0.057 ‰ for δ 13 C and δ 18 O respectively (from 0.072 ‰ and 0.198 ‰ without normalization). The results of the comparison indicate that once issues of non-ideal methods and use of LSVEC are removed, discrepancies in results arise from the challenges in accurately transferring delta values from carbonate reference materials to CO 2 gaseous samples, and that these issues can be reduced by having appropriate gaseous reference standards for calibration when measuring CO 2 gaseous samples. This is consistent with the identical treatment principle that is preferred in the isotope ratio measurement community. An analysis of results is presented in this report, with further consideration of the impact of the measurement method, the scale, and the reference materials. Uncertainties reported by participants are detailed and analysed, highlighting important differences in the uncertainty sources considered. Although CCQM-P204 was a comparison organised within the CCQM/GAWG and IRWG, no reference value was calculated, mainly because not all results appeared to be strictly on the same scale. Instead, a list of recommendations is provided to encourage more harmonised measurement practices and reach better consistency in future comparisons on similar materials. To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/ . The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Data sets used in the publication Re-evaluating cloud chamber constraints on depositional ice growth in cirrus clouds-- Part 1: Model description and sensitivity tests K.D. Lamb, J. Harrington, B.W. Clouser, E.J. Moyer, L. Sarkozy, V. Ebert, O. Möhler, H. Saathoff. Atmospheric Chemistry and Physics (2023)

Simultaneous measurements of the five most abundant methane isotopologues is demonstrated using a quantum cascade laser spectrometer. The instrument enables rapid and high-precision analysis of Δ13CH3D and Δ12CH2D2 in the natural methane samples.

Non-impact effects in the absorption spectra of HCl in various collision-partners are investigated both experimentally and theoretically. Fourier transform spectra of HCl broadened by CO2, air, and He have been recorded in the 2-0 band region at room temperature and for a wide pressure range, from 1 to up to 11.5 bars. Comparisons between measurements and calculations using Voigt profiles show strong super-Lorentzian absorptions in the troughs between successive lines in the P and R branches for HCl in CO2. A weaker effect is observed for HCl in air, while for HCl in He, Lorentzian wings are in very good agreement with measurements. In addition, the line intensities retrieved by fitting the Voigt profile on the measured spectra decrease with the density of the perturber. This perturber-density dependence decreases with the rotational quantum number. For HCl in CO2, the decrease in the retrieved line intensity can reach 2.5% per amagat for the first rotational quantum numbers. This number is about 0.8% per amagat for HCl in air, while for HCl in He, no density dependence of the retrieved line intensity is observed. Requantized classical molecular dynamics simulations have been performed for HCl-CO2 and HCl-He in order to simulate the absorption spectra for various perturber-density conditions. The density dependence of the intensities retrieved from the simulated spectra and the predicted super-Lorentzian behavior in the troughs between lines are in good agreement with experimental determinations for both HCl-CO2 and HCl-He. Our analysis shows that these effects are due to incomplete or ongoing collisions, which govern the dipole auto-correlation function at very short times. The effects of these ongoing collisions strongly depend on the details of the intermolecular potential: they are negligible for HCl-He but significant for HCl-CO2 for which a line-shape model beyond the impact approximation will be needed to correctly model the absorption spectra from the center to the far wings.

A novel flow switching setup for the dynamic generation and metrological detection of fast, isolated H2O concentration changes is presented and characterized. Based on this flow setup, very accurate, static H2O concentrations as well as highly dynamic H2O step changes of several 1000 ppmv (mu mol/mol) can be generated and repeated. First experiments show that temporal water vapor concentration gradients of up to 10000 ppmv/s can be generated and measured. Based on this setup, a dynamic hygrometer characterization method was developed and demonstrated using a polymer-based hygrometer as device under test (DUT). The polymer hygrometer (with about 180 ppmv/s) proved to be more than 15 times slower than the optical reference hygrometer (with 3000 ppmv/s) and could be modeled and described well with a first order lowpass. To estimate the dynamics of the spatial and temporal H2O-profile, a fast, traversable, local sampling probe was developed and used in combination with a fast, extractive laser hygrometer (called SEALDH-II). The modeling of the H2O distribution in the dynamically operated flow channel enables the calculation of the dynamic concentration at the position of the DUT based on the data of the spatially averaging open-path laser reference hygrometer. In the future, this calculation will be used to determine a transfer function between the optical, open path reference hygrometer and the position of the DUT in order to extract the ideal step response of the DUT from the measured data of the flow channel.

The use of optical circular multipass absorption cells (CMPAC) in an open-path configuration enables the sampling free analysis of cylindrical gas flows with high temporal resolution and only minimal disturbances to the sample gas in the pipe. Combined with their robust unibody design, CMPACs are a good option for many applications in atmospheric research and industrial process monitoring. When deployed in an open-path configuration, the effects of inhomogeneities in the gas temperature and composition have to be evaluated to ensure that the resulting measurement error is acceptable for a given application. Such an evaluation needs to consider the deviations caused by spectroscopic effects, e.g., nonlinear effects of temperature variations on the intensity of the spectral line, as well as the interaction of the temperature and concentration field with the characteristic laser beam pattern of the CMPAC. In this work we demonstrate this novel combined evaluation approach for the CMPAC used as part of the tunable diode laser absorption spectroscopy (TDLAS) reference hygrometer in PTB’s dynH2O setup for the characterization of the dynamic response behavior of hygrometers. For this, we measured spatially resolved, 2D temperature and H2O concentration distributions, and combined them with spatially resolved simulated spectra to evaluate the inhomogeneity effects on the line area of the used H2O spectral line at 7299.43 cm−1. Our results indicate that for dynH2O, the deviations caused by the interaction between large concentration heterogeneities and the characteristic sampling of the beam pattern of the CMPAC are three orders of magnitude larger than deviations caused by small temperature heterogeneity induced spectroscopic effects. We also deduce that the assumption that the “path-integrated” H2O concentration derived with the open-path CMPAC setup represents an accurate H2O area average in the flow section covered by the CMPAC in fact shows significant differences of up to 16% and hence does not hold true when large H2O concentration gradients are present.