Andrea Pogány

Development and Metrological Characterization of a Tunable Diode Laser Absorption Spectroscopy (TDLAS) Spectrometer for Simultaneous Absolute Measurement of Carbon Dioxide and Water Vapor

Simultaneous detection of two analytes, carbon dioxide (CO2)and water vapor (H2O), has been realized using tunable diode laser absorption spectroscopy (TDLAS) with a single distributed feedback diode laser at 2.7 μm. The dynamic range of the spectrometer is extended from the low parts per million to the percentage range using two gas cells, a single-pass cell with 0.77 m, and a Herriott-type multipass cell with 76 m path length. Absolute measurements were carried out, i.e., amount fractions of the analytes were calculated based on previously determined spectral line parameters, without the need for an instrument calibration using gas standards. A thorough metrological characterization of the spectrometer is presented. We discuss traceability of all parameters used for amount fraction determination and provide a comprehensive uncertainty assessment. Relative expanded uncertainties (k = 2, 95% confidence level) of the measured amount fractions are shown to be in the 2-3% range for both analytes. Minimum detectable amount fractions are 0.16 μmol/mol for CO2 and 1.1 μmol/mol for H20 for 76 m path length and 5 s averaging time. This corresponds to normalized detection limits of 27 μmol/mol m Hz_1/2 for CO2 and 221 μmol/mol m Hz−1/2 for H2O. Precision of the spectrometer, determined using Allan variance analysis, is 3.3 nmol/mol for CO2 and 21 nmol/mol for H2O. The spectrometer has been validated using reference gas mixtures with known CO2 and H2O amount fractions. An application example of the absolute TDLAS spectrometer as a reference instrument to validate other sensors is also presented.

Optical Path Length Calibration: A Standard Approach for Use in Absorption Cell-Based IR-Spectrometric Gas Analysis

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 ap rioriknown 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.

A metrological approach to improve accuracy and reliability of ammonia measurements in ambient air

The environmental impacts of ammonia (NH3) in ambient air have become more evident in the recent decades, leading to intensifying research in this field. A number of novel analytical techniques and monitoring instruments have been developed, and the quality and availability of reference gas mixtures used for the calibration of measuring instruments has also increased significantly. However, recent inter-comparison measurements show significant discrepancies, indicating that the majority of the newly developed devices and reference materials require further thorough validation. There is a clear need for more intensive metrological research focusing on quality assurance, intercomparability and validations. MetNH3 (Metrology for ammonia in ambient air) is a three-year project within the framework of the European Metrology Research Programme (EMRP), which aims to bring metrological traceability to ambient ammonia measurements in the 0.5–500 nmol mol−1 amount fraction range. This is addressed by working in three areas: (1) improving accuracy and stability of static and dynamic reference gas mixtures, (2) developing an optical transfer standard and (3) establishing the link between high-accuracy metrological standards and field measurements. In this article we describe the concept, aims and first results of the project.

Cantilever-enhanced Photoacoustic Spectroscopy of Ammonia with Accelerated Gas Exchange

We present a modified cantilever-enhanced photoacoustic spectrometer for trace ammonia detection with a detection limit of 20 ppb using a new accelerated gas exchange (AGE) procedure to attenuate negative effects due to ammonia adsorption.

High-Accuracy Ammonia Line Intensity Measurements at 1.5 µm

We present ammonia line intensities measured by tunable diode laser absorption spectroscopy, with up to 10 times lower uncertainty than data in the HITRAN2012 database. We validated our results in independent spectroscopic measurements.

Spectral reference data of molecules relevant to Earth's atmosphere: impact of European metrology research on atmospheric remote sensing

European metrology research has seen a tremendous change of focus concerning research impacting specific fields of applications. The European Metrology Research Programme (EMRP)1 in its different calls on environmental and energy subjects has revealed many new metrology projects devoted to problems, applications, and stakeholder needs in atmospheric sensing, pollution management, air quality assessments, and new energy technologies. We present the current status of development of a European infrastructure for traceable spectral reference data to be used, e.g., in remote sensing or for new developments of field-employable spectrometric transfer standards. This is demonstrated by means of standardized measurement approaches we are developing and by new measurement results regarding H2O, CO2, and N2O molecular line parameters, in this paper pressure broadening coefficients. Molecular line data are required to process raw spectra in order to extract column concentrations or local emission rates of specific analytes. Without molecular line data, all instruments were to be calibrated frequently by means of certified reference gas mixtures which were to keep available onboard throughout the instrument’s life time. At present, many instruments use line data from managed line collections like HITRAN and GEISA. These comprise paramount information on many thousands of lines for many different molecular species, but, modern remote sensing applications, like CO2 emission monitoring by satellites, tend to significantly tighten data quality objectives and thus require improved data quality that go quite frequently beyond that of the present database entries. In this presentation, we will show how metrology attempts to benefit this aim.