Grating Spectrometry and Spatial Heterodyne Fourier Transform Spectrometry: Comparative Noise Analysis for Raman Measurements

The desire for portable Raman spectrometers is continuously driving the development of novel spectrometer architectures where miniaturisation can be achieved without the penalty of a poorer detection performance. Spatial heterodyne spectrometers are emerging as potential candidates for challenging the dominance of traditional grating spectrometers, thanks to their larger etendue and greater potential for miniaturisation. This paper provides a generic analytical model for estimating and comparing the detection performance of Raman spectrometers based on grating spectrometer and spatial heterodyne spectrometer designs by deriving the analytical expressions for the performance estimator (signal-to-noise ratio, SNR) for both types of spectrometers. The analysis shows that, depending on the spectral characteristics of the Raman light and on the values of some instrument-specific parameters, the ratio of the SNR estimates for the two spectrometers ([Formula: see text]) can vary as much as by two orders of magnitude. Limit cases of these equations are presented for a subset of spectral regimes which are of practical importance in real-life applications of Raman spectroscopy. In particular, under the experimental conditions where the background signal is comparable or larger than the target Raman line and shot noise is the dominant noise contribution, the value of [Formula: see text] is, to a first order of approximation, dependent solely on the relative values of each spectrometer’s etendue and on the number of row pixels in the detector array. For typical values of the key instrument-specific parameters (e.g., etendue, number of pixels, spectral bandwidth), the analysis shows that spatial heterodyne spectrometer-based Raman spectrometers have the potential to compete with compact grating spectrometer designs for delivering in a much smaller footprint (10–30 times) levels of detection performance that are approximately only five to ten times poorer.

Synchrotron radiation and long path cryogenic cells: New tools and results for modelling chlorinated compounds absorption in the 8-12µm atmospheric window

<p> </p><p><strong>Anusanth Anantharajah<sup>a</sup>, Fridolin Kwabia Tchana<sup>a</sup>, Jean-Marie Flaud<sup>a</sup> , Pascale Roy<sup>b</sup> and Laurent Manceron<sup>b,c</sup></strong></p><ul><li>a- Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583, <br>Université de Paris et Université Paris-Est Créteil, Institut Pierre Simon Laplace, <br>61 Avenue du Général de Gaulle, 94010 Créteil Cedex, France.</li> <li>b- Synchrotron SOLEIL, AILES Beamline, L’Orme des Merisiers, Saint-Aubin F-91192, France.</li> <li>c-  Sorbonne Université, CNRS, MONARIS, UMR 8233, 4 place Jussieu, F-75005 Paris, France. </li> </ul><p> </p><p>Nitryl chloride (ClNO<sub>2</sub>) and Chlorine Nitrate are molecules of great interest for atmospheric chemistry since these are produced by heterogeneous reactions, in the marine troposphere, between NaCl sea-salt aerosols or ClO and gaseous N<sub>2</sub>O<sub>5</sub> [1,2], and on polar stratospheric clouds, between N<sub>2</sub>O<sub>5</sub> and solid HCl [3,4].</p><p> </p><p>Many high-resolution spectroscopic studies in the microwave and mid-infrared regions are available. However, these molecules present low-lying vibrational levels and thus numerous hot bands in the regions of the NOx stretching and bending mode absorptions in the 8-12 µm atmospheric transparency window which could serve for remote sensing and quantification of these species.</p><p>Fourier Transform Spectrometry is a useful technique to observe broad band high resolution spectra (0.001 cm<sup>-1</sup>) of these molecules and a significant advantage is gained by combining interferometry with the high brightness of a synchrotron source [5]. At SOLEIL we have developed specific instrumentation to study such reactive molecules and a few results concerning chlorine-containing compounds will be presented.</p><ol><li>B. J. Finlayson-Pitts, M. J. Ezell, and J. N. Pitts Jr, Nature <strong>337</strong>, 241-244 (1989).</li> <li>W. Behnke, V. Scheer, and C. Zetzsch, J. Aerosol Sci. <strong>24</strong>, 115-116 (1993).</li> <li>. M. A. Tolbert, M. J. Rossi, and D. M. Golden, Science <strong>240</strong>, 1018-1021 (1988).</li> <li>M. T. Leu, Geophys. Res. Lett. <strong>15</strong>, 851-854 (1988).</li> <li> J-M. Flaud, A. Anantharajah, F. Kwabia Tchana, L. Manceron, J. Orphal, G. Wagner, and M. Birk, J Quant Spectrosc Radiat Transf <strong>224</strong>, 217-221 (2019).</li> </ol><p> </p>

Quantifying CH₄ emissions from hard coal mines using mobile sun-viewing Fourier transform spectrometry

Methane (CH4) emissions from coal production amount to roughly one-third of European anthropogenic CH4 emissions in the atmosphere. Poland is the largest hard coal producer in the European Union with the Polish side of the Upper Silesian Coal Basin (USCB) as the main part of it. Emission estimates for CH4 from the USCB for individual coal mine ventilation shafts range between 0.03 and 20 kt a−1, amounting to a basin total of roughly 440 kt a−1 according to the European Pollutant Release and Transfer Register (E-PRTR, http://prtr.ec.europa.eu/, 2014). We mounted a ground-based, portable, sun-viewing FTS (Fourier transform spectrometer) on a truck for sampling coal mine ventilation plumes by driving cross-sectional stop-and-go patterns at 1 to 3 km from the exhaust shafts. Several of these transects allowed for estimation of CH4 emissions based on the observed enhancements of the column-averaged dry-air mole fractions of methane (XCH4) using a mass balance approach. Our resulting emission estimates range from 6±1 kt a−1 for a single shaft up to 109±33 kt a−1 for a subregion of the USCB, which is in broad agreement with the E-PRTR reports. Three wind lidars were deployed in the larger USCB region providing ancillary information about spatial and temporal variability of wind and turbulence in the atmospheric boundary layer. Sensitivity studies show that, despite drawing from the three wind lidars, the uncertainty of the local wind dominates the uncertainty of the emission estimates, by far exceeding errors related to the XCH4 measurements themselves. Wind-related relative errors on the emission estimates typically amount to 20 %.

Multispectral incoherent holography based on measurement of differential wavefront curvature

We propose a technique of multispectral incoherent holography. The differential wavefront curvature is measured, and the principle of Fourier transform spectrometry is applied to provide a set of spectral components of three-dimensional images and continuous spectra for spatially incoherent, polychromatic objects. This paper presents the mathematical formulation of the principle and the experimental results. Three-dimensional imaging properties are investigated based on an analytical impulse response function. The experimental and theoretical results agree well.

Towards verifying CH₄ emissions from hard coal mines using mobile sun-viewing Fourier transform spectrometry

Methane (CH4) emissions from coal production are one of the primary sources of anthropogenic CH4 in the atmosphere. Poland is the largest hard coal producer in the European Union with the Polish side of the Upper Silesian Coal Basin (USCB) as the main part of it. Emission estimates for CH4 from the USCB for individual coal mine ventilation shafts range between 0.03 kt/a and 20 kt/a, amounting to a basin total of roughly 440 kt/a according to the European Pollutant Release and Transfer Register (E-PRTR, http://prtr.ec.europa.eu/, 2014). We mounted a ground-based, portable, sun-viewing FTS (Fourier Transform Spectrometer) on a truck for sampling coal mine ventilation plumes by driving cross-sectional stop-and-go Patterns at 1 to 3 km distance to the exhaust shafts. Using a mass balance approach, several of these transects allowed for estimating CH4 emissions based on the observed enhancements of the column-averaged dry-air mole fractions of methane (XCH4). Our resulting emission estimates range from 6 ± 1 kt/a for a single shaft up to 109 ± 33 kt/a for a subregion of the USCB, which is in broad agreement with the E-PRTR reports. Three wind lidars were deployed in the larger USCB region providing ancillary information about spatial and temporal variability of wind and turbulence in the atmospheric boundary-layer. Sensitivity studies show that, despite drawing from the three wind lidars, the uncertainty of the local wind dominates the uncertainty of the emission estimates, by far exceeding errors related to the XCH4 measurements itself. Wind-related relative errors on the emission estimates typically amount to 20 %.