ABSTRACT:The study describes significant outcomes of the 'Metrology for Meteorology' project, MeteoMet, which is an attempt to bridge the meteorological and metrological communities. The concept of traceability, an idea used in both fields but with a subtle difference in meaning, is at the heart of the project. For meteorology, a traceable measurement is the one that can be traced back to a particular instrument, time and location. From a metrological perspective, traceability further implies that the measurement can be traced back to a primary realization of the quantity being measured in terms of the base units of the International System of Units, the SI. These two perspectives reflect long-standing differences in culture and practice and this project -and this study -represents only the first step towards better communication between the two communities. The 3 year MeteoMet project was funded by the European Metrology Research Program (EMRP) and involved 18 European National Metrological Institutes, 3 universities and 35 collaborating stakeholders including national meteorology organizations, research institutes, universities, associations and instrument companies. The project brought a metrological perspective to several long-standing measurement problems in meteorology and climatology, varying from conventional ground-based measurements to those made in the upper atmosphere. It included development and testing of novel instrumentation as well as improved calibration procedures and facilities, instrument intercomparison under realistic conditions and best practice dissemination. Additionally, the validation of historical temperature data series with respect to measurement uncertainties and a methodology for recalculation of the values were included.

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We report on an interband cascade laser (ICL)-absorption spectrometer for absolute, calibration-free, atmospheric CO amount fraction measurements, addressing direct traceability of the results. The system combines first-principles direct tunable diode laser absorption spectroscopy (dTDLAS) with a metrological validation. Using a multipath cell with 76 m path length, our detection limit is 0.5 nmol/mol at Δt=14  s. The system is highly linear (slope: 0.999±0.008) in the amount fraction range of 0.1-1000 μmol/mol and thus is interesting for industrial as well as environmental applications. The sensor repeatability at 300 nmol/mol is 0.06 nmol/mol (with Δt=10  min). The sensor's absolute response is in excellent agreement with the gravimetric values of a set of primary gas standards used to test the sensor accuracy. The relative expanded uncertainty (k=2) of the measured CO amount fraction is 2.8%. Due to this performance and the calibration-free approach, the spectrometer may be used as an optical transfer standard (OTS) if gas standards are for whatever reason not available or applicable, e.g., for airborne instruments. Our dTDLAS approach has shown excellent stability and accuracy in H₂O detection [Appl. Phys. B116, 883 (2014)APPCDL0721-726910.1007/s00340-014-5775-4] even when compared to primary standards. We therefore deduce that the ICL spectrometer (after its adaptation to field conditions, similar to our H₂O spectrometers) has good potential to meet the 2 nmol/mol compatibility goal stated by the World Meteorological Organization for atmospheric CO measurements, and serve as an OTS which does not need frequent calibrations using reference gases.

View the article online for updates and enhancements. Related contentMetrological quantification of CO in biogas using laser absorption spectroscopy and gas chromatography Javis A Nwaboh, Stefan Persijn, Karine Arrhenius et al. Abstract Employing direct absorption spectroscopy and using a spectrometer comprising a single-pass and a multipass white cell, we probed the R(12) line of carbon dioxide (CO 2 ) in the combination band around 2 μm. Gravimetric gas standards containing CO 2 , between 300 and 60 000 μmol mol −1 (0.03% to 6%), in N 2 were quantified by means of the TILSAM method. The spectrometric results were compared with the gravimetric reference values. We describe our implementation of the 'Guide to the Expression of Uncertainty in Measurements' to infrared laser-spectrometric gas analysis. Data quality objectives are addressed by uncertainty and traceability flags. Uncertainty budgets are presented to show the quality of the results and to demonstrate software-assisted uncertainty assessment. The relative standard uncertainties of the spectrometrically measured CO 2 amount fractions at, e.g., ambient levels of 360 μmol mol −1 and at exhaled breath gas levels of 50 mmol mol −1 were 1.4% and 0.7%, respectively. Our detection limit was 2.2 μmol mol −1. The reproducibility of individual results was in the ±1% range. Furthermore, we measured collisional broadening coefficients of the R(12) line of CO 2 at 4987.31 cm −1 . The relative standard uncertainties of the measured self-, nitrogen-, oxygen-and air-broadening coefficients were in the ±1.7% range.

We report a new direct tunable diode laser absorption spectroscopy (dTDLAS) sensor for absolute measurements of HO in methane, ethane, propane, and low CO natural gas. The sensor is operated with a 2.7 µm DFB laser, equipped with a high pressure single pass gas cell, and used to measure HO amount of substance fractions in the range of 0.31-25 000 µmol/mol. Operating total gas pressures are up to 5000 hPa. The sensor has been characterized, addressing the traceability of the spectrometric results to the SI and the evaluation of the combined uncertainty, following the guide to the expression of uncertainty in measurement (GUM). The relative reproducibility of HO amount of substance fraction measurements at 87 µmol/mol is 0.26% (0.23 µmol/mol). The maximum precision of the sensor was determined using a HO in methane mixture, and found to be 40 nmol/mol for a time resolution of 100 s. This corresponds to a normalized detection limit of 330 nmol mol·m Hz. The relative combined uncertainty of HO amount fraction measurements delivered by the sensor is 1.2%.

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We employed a comparison method to determine the optical path length of gas cells which can be used in spectroscopic setup based on laser absorption spectroscopy or FTIR. The method is based on absorption spectroscopy itself. A reference gas cell, whose length is a priori known and desirably traceable to the international system of units (SI), and a gas mixture are used to calibrate the path length of a cell under test. By comparing spectra derived from pressure-dependent measurements on the two cells, the path length of the gas cell under test is determined. The method relies neither on the knowledge of the gas concentration nor on the line strength parameter of the probed transition which is very rarely traceable to the SI and of which the uncertainty is often relatively large. The method is flexible such that any infrared light source and infrared active molecule with isolated lines can be used. We elaborate on the method, substantiate the method by reporting results of this calibration procedure applied to multipass and single pass gas cells of lengths from 0.38 m to 21 m, and compare this to other methods. The relative combined uncertainty of the path length results determined using the comparison method was found to be in the ±0.4% range.

Biogas has a vital role in the future market for renewable energy. When upgraded to biomethane, it can be injected into natural gas grids if the level of certain impurities complies with the specifications in EN16723. For some of these impurities, suitable measurement methods are lacking which hamper the quality control of biomethane to be injected into natural gas networks. Here, we report on the evaluation of three detection methods suitable for carbon monoxide (CO) in biogas and biomethane applications for which EN16723 specifies an upper limit of 0.1% (1000 µmol ⋅ mol −1 ). Two of these methods are based on laser absorption spectroscopy (LAS) and one on gas chromatography (GC). Both LAS spectrometers employ direct absorption spectroscopy and operate at 4.6 µm, probing a single CO absorption line in the fundamental CO band. One of them, direct tunable diode laser absorption spectroscopy (dTDLAS), is based on a new interband cascade laser specially designed for biogas and biomethane applications, while the other is based on quantum cascade laser absorption spectroscopy (QCLAS). The GC is equipped with two packed columns (HayeSep Q and molecular sieve 5A) and a thermal conductivity detector. CO amount fraction results in biogas matrices derived using these three measurement methods and are compared to amount fraction values of different, gravimetrically prepared reference gas standards of CO in biogas. These were used to validate the measurement capabilities. The measured CO amount fraction results from LAS and GC covered 10-30 000 µmol ⋅ mol −1 (system measurement ranges, LAS: 3-1000 µmol ⋅ mol −1 , GC: 500-30 000 µmol ⋅ mol −1 ) and were in excellent agreement with the gravimetric values of the gas standards. At 400 µmol ⋅ mol −1 , the guide to the expression of uncertainty in measurement compliant relative standard uncertainties of our calibration-free dTDLAS and the gas-calibrated QCLAS systems are estimated to be 1.4% versus 0.5%, respectively. The relative standard uncertainty of the GC CO measurements at 5075 µmol ⋅ mol −1 is 1.3%. This work demonstrates that, by means of GC and LAS, relative standard uncertainties of 1.4% and below can be reached for CO measurements in biogas and that cost-optimized calibration-free approaches not requiring frequent use of gas standards have become available.