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Characterization of a laser spectrometer for Nitric Oxide trace detection in Antarctica

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Nitric oxide (NO) is a molecule of high interest at very low concentration (trace level) in a large range of applications. It is the most abundant of the nitrogen oxides (NOx) that are involved in the atmospheric ozone cycle. Hence, despite its low concentration level in the atmosphere (ranging from a few ppb -part per billion- down to several ppt -part per trillion- in very remote and unpolluted areas), NO plays a major role in environmental monitoring, atmospheric chemistry, and in the study of air-snowpack interaction processes. The LAME group at LIPhy is well known in France and abroad for being at the fore-front in the development of ultrasensitive spectroscopic techniques for the selective and quantitative measurements of molecules present in a gas at trace level. Very recently we have developed an instrument optimized for in-situ NO detection that is at the state of the art with a detection limit below 1 ppt [1]. This sensitivity is adapted to the most stringent application. In particular it will allow the monitoring of NO in remote regions such as Antarctica where concentration drops below 10 ppt [2]. In collaboration with the IGE laboratory (Institute for Geosciences and Environment), we plan to deploy this instrument for direct and in-situ measurements in Antarctica during summer 2020.

Laser spectroscopy technique :
By means of the coupling of a laser to a high finesse optical cavity, the effective absorption length is enhanced up to tens of kilometers, while the actual cavity length is only about 50 cm allowing for a compact set-up for in-situ measurements. Our group at LIPhy has developed an original technique called Optical Feedback-Cavity Enhanced Absorption Spectroscopy (OF-CEAS) that relies on the sensitivity of a semi-conductor laser to optical feedback [1]. OF-CEAS, enables the design of a compact instrument with very high detection sensitivity and very high molecule specificity. Additional advantages are that the method does not require routine calibration with certified gas mixtures and that the resulting robust instruments can be operated by non-specialists.

NO analyzer in the mid infrared region (MIR) :
The NO analyzer operates with an Interband Cascade Laser (ICL) that became recently available in the mid-infrared region (MIR) where strong absorption lines can be addressed, allowing to obtain a sub-ppb sensitivity. Nonetheless, addressing large intensity absorption lines, optical saturation of the molecular transitions has to be accounted for.

Internship objective :
According to the duration of the internship, the student will be in charge of different studies required prior to the Antarctica campaign. She/he will perform an in depth study of optical saturation effects in OF-CEAS analyzers that operate in the MIR. A model will be developed and different analyzers will be available for data acquisition. Additionally, the student will characterize the absolute accuracy of the instrument at very low concentration (subppb) by comparison with another NO detector (chemiluminescence based instrument) and with a calibrated dilution gas line.

Expected skills :
The candidate will have knowledge of optics, lasers and spectroscopy and an interest in modeling and instrumentation.

Supervisor :
Irène VENTRILLARD irene.ventrillard@univ-grenoble-alpes.fr
http://www-liphy.ujf-grenoble.fr/-LAME

[1] L. Richard, D. Romanini, I. Ventrillard, Sensors, 18, 1997 (2018).
[2] R. Grilli, M. Legrand,, A Kukui, G. Méjean, S. Preunkert, D. Romanini, Geophys. Res. Lett., 40, 791 (2013) -