Incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS)-based strategy for direct measurement of aerosol extinction in a lidar blind zone

In this Letter, the development of a custom-designed incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) and its application to in situ measurement of aerosol extinction near the ground surface are described in an effort to address the issue of missing data in the light detection and ranging (lidar) blind zone in the first hundreds of meters of the observation range. Combined measurements of aerosol extinction at the same location using lidar remote sensing at 355 nm and in situ IBBCEAS operating in the UV spectral region around 370 nm showed results with a good correlation (${{\rm R}^2} = {0.90}$R²=0.90) between the two measurement techniques. This Letter highlights a new strategy for near-end lidar calibration, using a ground-based compact and robust IBBCEAS located at the lidar measurement site to determine the vertical profile of the aerosol extinction coefficient with a higher accuracy.

Analysis of the Stable Isotope Ratios (18O/16O, 17O/16O, and D/H) in Glacier Water by Laser Spectrometry

A compact isotope ratio sensor based on laser absorption spectroscopy at 2.7 μm was developed for high precision and simultaneous measurements of the D/H, ¹⁸O/¹⁶O and ¹⁷O/¹⁶O isotope ratios in glacier water. Measurements of the oxygen and hydrogen isotope ratios in glacier water demonstrate a 1σ precision of 0.3‰ for δ¹⁸O, 0.2‰ for δ¹⁷O, and 0.5‰ for δ²H, respectively. The δ values of the working standard glacier water obtained by the calibrated sensor system is basically identical to the IRMS measurement results with a very high calibration accuracy from 0.17‰ to 0.75‰. Preliminary results on the reproducibility measurements display a standard deviation of 0.13‰ for δ¹⁸O, 0.13‰ for δ¹⁷O, and 0.64‰ for δ²H, respectively.

High-accuracy and high-sensitivity spectroscopic measurement of dinitrogen pentoxide (N2O5) in an atmospheric simulation chamber using a quantum cascade laser

A spectroscopic instrument based on a mid-infrared external cavity quantum cascade laser (EC-QCL) was developed for high-accuracy measurements of dinitrogen pentoxide (N₂O₅) at the ppbv-level. A specific concentration retrieval algorithm was developed to remove, from the broadband absorption spectrum of N₂O₅, both etalon fringes resulting from the EC-QCL intrinsic structure and spectral interference lines of H₂O vapour absorption, which led to a significant improvement in measurement accuracy and detection sensitivity (by a factor of 10), compared to using a traditional algorithm for gas concentration retrieval. The developed EC-QCL-based N₂O₅ sensing platform was evaluated by real-time tracking N₂O₅ concentration in its most important nocturnal tropospheric chemical reaction of NO₃ + NO₂ ↔ N₂O₅ in an atmospheric simulation chamber. Based on an optical absorption path-length of L_eff = 70 m, a minimum detection limit of 15 ppbv was achieved with a 25 s integration time and it was down to 3 ppbv in 400 s. The equilibrium rate constant K_eq involved in the above chemical reaction was determined with direct concentration measurements using the developed EC-QCL sensing platform, which was in good agreement with the theoretical value deduced from a referenced empirical formula under well controlled experimental conditions. The present work demonstrates the potential and the unique advantage of the use of a modern external cavity quantum cascade laser for applications in direct quantitative measurement of broadband absorption of key molecular species involved in chemical kinetic and climate-change related tropospheric chemistry.

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.

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.

Kalman filtering real-time measurements of H2O isotopologue ratios by laser absorption spectroscopy at 2.73 microm

Kalman adaptive filtering was applied for the first time, to our knowledge, to the real-time simultaneous determination of water isotopic ratios using laser absorption spectroscopy at 2.73 microm. Measurements of the oxygen and hydrogen isotopologue ratios delta(18)O, delta(17)O, and delta(2)H in water showed a 1-sigma precision of 0.72 per thousand for delta(18)O, 0.48 per thousand for delta(17)O, and 0.84 per thousand for delta(2)H, while sampling the output of the tuned Kalman filter at 1 s time intervals. Using a standard running average technique, averaging over approximately 30 s is required to obtain the same level of precision.

Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy

Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide (HCN), carbon monoxide (CO), formaldehyde (H2CO), and water (H2O) have been observed from 500 to 2400 GHz. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of HCN and CO. The quantification of the measurement was validated by the means of a calibration gas containing CO. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of HCN equal to 9 parts in 10(6).

Photomixing at 1.55 microm in ion-irradiated In(0.53)Ga(0.47)As on InP

We report the first demonstration of a terahertz photomixer made of ion-irradiated In(0.53)Ga(0.47)As lattice-matched to InP and fiber-optic coupled with the drive lasers. A continuous-wave radiation is generated at frequencies up to 0.8 THz by photomixing two continuous-wave laser diodes around 1.55 microm. The measured 3dB-down bandwidth of 300 GHz yields a carrier lifetime of 0.53 ps, in agreement with the value of 0.41 ps measured in pump probe experiments. The detected signal is at the most 15 dB lower than the one obtained from similar photomixers fabricated from low-temperature-grown GaAs.

Refractive-Index Changes of Standard Telecommunication Fiber through Exposure to Femtosecond Laser Pulses at 810 cm

We report on permanent refractive-index changes as great as 6 x 10(-3) in standard fibers on irradiation with tightly focused femtosecond (120-fs) near-IR (lambda = 800-nm) pulses. The refractive-index increase was measured through changes in the transmission spectrum of an UV photowritten Bragg-grating-based Fabry-Perot interferometer. We depict the dependence of the index variation on exposure time. The changes in refractive index can be optimized after a few seconds of irradiation. The measured induced excess loss increases during near-IR exposure.