Part per trillion nitric oxide measurement by optical feedback cavity-enhanced absorption spectroscopy in the mid-infrared

International audienc

Towards high-precision isotopic analysis of CO2 from ice-core gas bubbles using quantum cascade laser spectroscopy

International audienceThe paleo-climate archive provided by gas stored in bubbles in the ice provides a powerful means to study the ~40% increase in the atmospheric CO2 concentration between glacial and interglacial climates, in combination with numerical modeling studies, to elucidate the underlying physical mechanisms. Of particular interest is, considering the strong correlation between the carbon cycle and climate, and in light of the post-industrial revolution anthropogenic increase of the CO2 concentration. The source of the CO2 released into the atmosphere during previous deglaciations can be constrained from 13CO2 isotopic measurements on CO2 gas stored in bubbles in the ice-cores by the fact that the different CO2 reservoirs (terrestrial biosphere, oceans) and associated mechanisms (biological or physical) have different isotopic signatures. Unfortunately, conventional IRMS measurements on the small quantity of gas available are difficult, tedious, and time-consuming. We report here on the design of an alternative method based on Optical Feedback Cavity Enhanced Absorption Spectrometry (OF-CEAS) using a quantum cascade laser operating near 4.36 μm. The aim of this instrument design is to achieve the measurement of the 13C/12C isotopic ratio (δ13C) with a precision better than 0.05 ‰ on small quantities of the trapped atmospheric CO2. We describe the instrument and show preliminary results

QCL - Optical-Feedback Cavity Enhanced Absorption Spectroscopy For The Analysis Of Atmospheric 13CO2/12CO2 In Ice-Core Gas Bubbles

International audienceIn the context of a globally warming climate it is crucial to study the climate variability in the past and to understand the underlying mechanisms. The composition of gas stored in bubbles in polar ice presents a paleo-climate archive that provides a powerful means to study the exact mechanisms involved in the ~40% increase in the atmospheric CO2 concentration between glacial and interglacial climates. It is particularly important to understand such natural coupling between climate and the carbon cycle, as it will partly determine what natural feedback can be expected on the atmospheric CO2 concentration in a future warmer world. The source of the CO2 released into the atmosphere during previous deglaciations can be constrained from isotopic measurements by the fact that the different CO2 reservoirs (terrestrial biosphere, oceans) and associated mechanisms (biological or physical) have different isotopic signatures. Unfortunately, such isotope studies have been seriously hampered by the experimental difficulty of extracting the CO2 without contamination or fractionation, and measuring the isotope signal off-line on an isotope ratio mass spectrometer (IRMS). Here we present an alternative method that leverages the extreme sensitivity afforded by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) in the Mid-Infrared [1]. This region of the spectrum is accessed by a custom-developed Quantum Cascade Laser operating near 4.35 µm. The feedback to the laser of light that has been spectrally filtered by a high-finesse, V-shaped enhancement cavity has the effect of spectrally narrowing the laser emission and to auto-lock the laser frequency to one of the cavity's longitudinal modes, with clear advantages in terms of acquisition time and signal-to-noise ratio of the measurement. The line strengths in this region are about 5 orders of magnitude higher than in the more easily accessible NIR region near 1.6 µm and about 1000 times higher than at 2 µm. The instrument is temperature stabilization at the mK-level. Together with a small cavity volume of ~20 mL, this enables the analysis of nmol-sized samples with high precision (< 0.05‰) in a fraction of the time required by the conventional IRMS-based technique. We will show preliminary results obtained on synthetic samples. [1] Maisons G., Gorrotxategi Carbajo P., Carras M., Romanini D.: Optical-feedback cavity-enhanced absorption spectroscopy with a quantum cascade laser, Opt. Lett., 35, 3607, 2010. [2] Morville J., Kassi S., Chenevier M., and Romanini D.: Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking, Appl. Phys., B 80, 1027-1038, 2005