Eric Fertein

Short-lived species detection of nitrous acid by external-cavity quantum cascade laser based quartz-enhanced photoacoustic absorption spectroscopy

Spectroscopic detection of short-lived gaseous nitrous acid (HONO) at 1254.85 cm−1 was realized by off-beam coupled quartz-enhanced photoacoustic spectroscopy (QEPAS) in conjunction with an external cavity quantum cascade lasers (EC-QCL). High sensitivity monitoring of HONO was performed within a very small gas-sample volume (of ∼40 mm3) allowing a significant reduction (of about 4 orders of magnitude) of air sampling residence time which is highly desired for accurate quantification of chemically reactive short-lived species. Calibration of the developed QEPAS-based HONO sensor was carried out by means of lab-generated HONO samples whose concentrations were determined by direct absorption spectroscopy involving a ∼109.5 m multipass cell and a distributed feedback QCL. A minimum detection limit (MDL) of 66 ppbv (1 σ) HONO was achieved at 70 mbar using a laser output power of 50 mW and 1 s integration time, which corresponded to a normalized noise equivalent absorption coefficient of 3.6 × 10−8 cm−1 W/Hz1/...

Sensitive and selective detection of OH radicals using Faraday rotation spectroscopy at 2.8 µm.

We report on the development of a Faraday rotation spectroscopy (FRS) instrument using a DFB diode laser operating at 2.8 µm for the hydroxyl (OH) free radical detection. The highest absorption line intensity and the largest gJ value make the Q (1.5) double lines of the 2Π3/2 state (υ = 1 ← 0) at 2.8 µm clearly the best choice for sensitive detection in the infrared region by FRS. The prototype instrument shows shot-noise dominated performance and, with an active optical pathlength of only 25 cm and a lock-in time constant of 100 ms, achieves a 1σ detection limit of 8.2 × 10(8) OH radicals/cm3.

A Quantum Cascade Laser-Based Optical Sensor for Continuous Monitoring of Environmental Methane in Dunkirk (France)

A room-temperature continuous-wave (CW) quantum cascade laser (QCL)-based methane (CH4) sensor operating in the mid-infrared near 8 μm was developed for continuous measurement of CH4 concentrations in ambient air. The well-isolated absorption line (7F2,4 ← 8F1,2) of the ν4 fundamental band of CH4 located at 1255.0004 cm−1 was used for optical measurement of CH4 concentration by direct absorption in a White-type multipass cell with an effective path-length of 175 m. A 1σ (SNR = 1) detection limit of 33.3 ppb in 218 s was achieved with a measurement precision of 1.13%. The developed sensor was deployed in a campaign of measurements of time series CH4 concentration on a site near a suburban traffic road in Dunkirk (France) from 9 to 22 January 2013. An episode of high CH4 concentration of up to ~3 ppm has been observed and analyzed with the help of meteorological parameters combined with back trajectory calculation using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model of NOAA.

Sensing atmospheric reactive species using light emitting diode by incoherent broadband cavity enhanced absorption spectroscopy

We overview our recent progress in the developments and applications of light emitting diode-based incoherent broadband cavity enhanced absorption spectroscopy (LED-IBBCEAS) techniques for real-time optical sensing chemically reactive atmospheric species (HONO, NO3, NO2) in intensive campaigns and in atmospheric simulation chamber. New application of optical monitoring of NO3 concentration-time profile for study of the NO3-initiated oxidation process of isoprene in a smog chamber is reported.

Measurement of the D/H, 18O/16O, and 17O/16O Isotope Ratios in Water by Laser Absorption Spectroscopy at 2.73 μm

A compact isotope ratio laser spectrometry (IRLS) instrument was developed for simultaneous measurements of the D/H, 18O/16O and 17O/16O isotope ratios in water by laser absorption spectroscopy at 2.73 μm. Special attention is paid to the spectral data processing and implementation of a Kalman adaptive filtering to improve the measurement precision. Reduction of up to 3-fold in standard deviation in isotope ratio determination was obtained by the use of a Fourier filtering to remove undulation structure from spectrum baseline. Application of Kalman filtering enables isotope ratio measurement at 1 s time intervals with a precision (<1‰) better than that obtained by conventional 30 s averaging, while maintaining a fast system response. The implementation of the filter is described in detail and its effects on the accuracy and the precision of the isotope ratio measurements are investigated.

Circular Regression in a Dual-Phase Lock-In Amplifier for Coherent Detection of Weak Signal

Lock-in amplification (LIA) is an effective approach for recovery of weak signal buried in noise. Determination of the input signal amplitude in a classical dual-phase LIA is based on incoherent detection which leads to a biased estimation at low signal-to-noise ratio. This article presents, for the first time to our knowledge, a new architecture of LIA involving phase estimation with a linear-circular regression for coherent detection. The proposed phase delay estimate, between the input signal and a reference, is defined as the maximum-likelihood of a set of observations distributed according to a von Mises distribution. In our implementation this maximum is obtained with a Newton Raphson algorithm. We show that the proposed LIA architecture provides an unbiased estimate of the input signal amplitude. Theoretical simulations with synthetic data demonstrate that the classical LIA estimates are biased for SNR of the input signal lower than −20 dB, while the proposed LIA is able to accurately recover the weak signal amplitude. The novel approach is applied to an optical sensor for accurate measurement of NO2 concentrations at the sub-ppbv level in the atmosphere. Side-by-side intercomparison measurements with a commercial LIA (SR830, Stanford Research Inc., Sunnyvale, CA, USA ) demonstrate that the proposed LIA has an identical performance in terms of measurement accuracy and precision but with simplified hardware architecture.

Monitoring of nitrous acid (HONO) by external-cavity quantum cascade laser-based off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS)

In this presentation, we report on the high-sensitivity and high-selectivity measurement of HONO by off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) in a very small gas sample volume (of ~ 40 mm3) resulting in a ultrashort residence time of less than 10 ms (compared to ~ 7 min for a conventional 210 m multipass cell or ~ 10-min. integration time for currently used chemical analytical instruments). A minimum detection limit of 66 ppbv (1σ) HONO was achieved at 70 mbar using a laser output power of 50 mW and 1 s integration time. This MDL was down to 7 ppbv at the optimal integration time of 150 s.

Filter-free measurements of black carbon absorption using photoacoustic spectroscopy

A photoacoustic (PA) spectrophone was developed for filter-free measurement of black carbon (BC) mass absorption coefficient (MAC) in the spectral region of ~ 442 nm using a blue diode laser. The PA sensor was characterized and calibrated using NO2 at standard calibrated concentrations, as well as at indoor and outdoor air concentrations via a side-by-side intercomparison with a commercial NOx analyzer. Black carbon (graphite) and volcanic ash samples were measured under laboratory condition. MAC of BC and volcanic ash sample were determined based on the measured particles optical absorption coefficient by the PA spectrophone, in combination with the mass concentration measured using a scanning mobility particle sizer (SMPS). A minimum detectable absorption coefficient (1σ) of ~ 1.2 Mm-1 for BC was achieved with a time solution of 1 second.

Monitoring of nitrous acid (HONO) by off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) using external-cavity quantum cascade laser

Spectroscopic monitoring of short-lived gaseous nitrous acid (HONO) near 8 µm was realized by off-beam coupled quartz-enhanced photoacoustic spectroscopy (QEPAS) using an external cavity quantum cascade lasers with an ultra-short residence time for air sampling.

Real Time Determination of Water Isotope Ratios by Laser Absorption Spectroscopy at 2.73 µm Using Kalman Filter

Kalman filter was applied to measurements of water isotopologue ratios by laser spectroscopy at 2.73 µm. The results obtained in 1-s showed a 1? precision that required ~30-s averaging when using a simple average approach.