Thomas Decarpenterie

Photoacoustic spectroscopy for trace gas detection with cryogenic and room-temperature continuous-wave quantum cascade lasers

The main characteristics that a sensor must possess for trace gas detection and pollution monitoring are high sensitivity, high selectivity and the capability to perform in situ measurements. The photacoustic Helmholtz sensor developed in Reims, used in conjunction with powerful Quantum Cascade Lasers (QCLs), fulfils all these requirements. The best cell response is # 1200 V W−1 cm and the corresponding ultimate sensitivity is j 3.3 × 10−10 W cm−11 Hz−11/2. This efficient sensor is used with mid-infrared QCLs from Alpes Lasers to reach the strong fundamental absorption bands of some atmospheric gases. A first cryogenic QCL emitting at 7.9 μm demonstrates the detection of methane in air with a detection limit of 3 ppb. A detection limit of 20 ppb of NO in air is demonstrated using another cryogenic QCL emitting in the 5.4 μm region. Real in-situ measurements can be achieved only with room-temperature QCLs. A room-temperature QCL emitting in the 7.9 μm region demonstrates the simultaneous detection of methane and nitrous oxide in air (17 and 7 ppb detection limit, respectively). All these reliable measurements allow the estimated detection limit for various atmospheric gases using quantum cascade lasers to be obtained. Each gas absorbing in the infrared may be detected at a detection limit in the ppb or low-ppb range.

Continuous-wave quantum cascade lasers absorption spectrometers for trace gas detection in the atmosphere

Mid infra-red absorption spectrometry based on continuous-wave distributed feedback (DFB) quantum cascade laser (QCL) is more and more widely used for trace gas detection and pollution monitoring. The main advantages of this technique are high sensitivity, high selectivity and a potential for extreme compactness. Various examples of trace gas detection for atmospheric detection will be presented in this paper. Commercial QCLs available on the shelves were first implemented. A cryogenic QCL emitting at 6.7 μm was used to demonstrate the detection of water vapor and its isotopes. A room-temperature QCL was then used to simultaneously detect methane and nitrous oxide at 7.9 μm. Recently, we have developed a room-temperature top grating DFB QCL designed around 4.5 μm for the demonstration of N2O detection in the ppb range. Atmospheric applications of these spectrometers will be presented. The improvements of QCL performances make it now possible to develop instruments that are more and more compact and therefore compatible with in situ applications.

Wavelet Denoising for Infrared Laser Spectroscopy and Gas Detection

After a brief introduction to wavelet theory, this paper discusses the critical parameters to be considered in wavelet denoising for infrared laser spectroscopy. In particular, it is shown that measurement dispersion as well as sensibility can be dramatically improved when using wavelet denoising for gas detection by infrared laser absorption spectroscopy.

Atmospheric Measurements by Ultra-Light SpEctrometer (AMULSE) Dedicated to Vertical Profile in Situ Measurements of Carbon Dioxide (CO2) Under Weather Balloons: Instrumental Development and Field Application

The concentration of greenhouse gases in the atmosphere plays an important role in the radiative effects in the Earth’s climate system. Therefore, it is crucial to increase the number of atmospheric observations in order to quantify the natural sinks and emission sources. We report in this paper the development of a new compact lightweight spectrometer (1.8 kg) called AMULSE based on near infrared laser technology at 2.04 µm coupled to a 6-m open-path multipass cell. The measurements were made using the Wavelength Modulation Spectroscopy (WMS) technique and the spectrometer is hence dedicated to in situ measuring the vertical profiles of the CO2 at high precision levels (σAllan = 0.96 ppm in 1 s integration time (1σ)) and with high temporal/spatial resolution (1 Hz/5 m) using meteorological balloons. The instrument is compact, robust, cost-effective, fully autonomous, has low-power consumption, a non-intrusive probe and is plug & play. It was first calibrated and validated in the laboratory and then used for 17 successful flights up to 10 km altitude in the region Champagne—Ardenne, France in 2014. A rate of 100% of instrument recovery was validated due to the pre-localization prediction of the Météo—France based on the flight simulation software.

Des ballons pour le climat

Depuis trois ans, le Centre national d’études spatiales (Cnes) collabore avec l’Agence spatiale canadienne (CSA-ASC) pour réaliser des campagnes de lâchers de ballons stratosphériques ouverts depuis la base de Timmins en Ontario (Canada). Les ballons emportent dans la stratosphère, jusqu’à environ 40 km d’altitude, plusieurs centaines de kilogrammes d’instrumentation scientifique afin de réaliser des observations dans les domaines de la physico-chimie de l’atmosphère ou de l’astronomie. La campagne StratoScience 2015 organisée du 12 août au 30 septembre a débuté par le vol Climat (figure 1).