Estimation of the Effect of Spectroscopic Information Accuracy on the Lidar Measurements of Methane with the Use of Expe
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TICAL INSTRUMENTATION
Estimation of the Effect of Spectroscopic Information Accuracy on the Lidar Measurements of Methane with the Use of Expert Line Lists I. A. Vasilenkoa, *, S. A. Sadovnikova, and O. A. Romanovskiia aV.E.
Zuev Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences, Tomsk, 634055 Russia *e-mail: [email protected] Received March 16, 2020; revised March 16, 2020; accepted March 19, 2020
Abstract—The transmission spectra and methane absorption coefficients are compared for different widths of the instrument function of the spectral distribution of laser power using different sources of spectroscopic information. An expert list of methane lines is compiled. Keywords: methane, differential absorption method, expert line lists DOI: 10.1134/S1024856020040181
The background concentration of methane in the surface air layer is about 2 ppm. Its increase enhances the greenhouse effect, since methane actively absorbs the Earth’s thermal radiation in the IR. Therefore, monitoring of the distribution and dynamics of the CH4 concentration is required. For environmental monitoring with a differential absorption lidar (DIAL) [1], the accuracy of spectral line parameters, such as the line center, intensity, halfwidth, and shift, is very important, since the errors in these data lead to significant systematic errors in the concentration of a substance under study. Well-known databases, for example, HITRAN [2] and GEISA [3], contain information on rotational-vibrational transitions of molecules, but these databases should be used very carefully, because, despite the high accuracy of all the above parameters claimed, these databases contain both experimental and calculated data for each molecule. The line center position and intensity calculated may have errors of up to several cm−1 and 100%, respectively, for certain molecules. In this case, a user, not being a specialist in high resolution spectroscopy, may erroneously use the calculation data. To avoid such errors, the scientific community suggested using the so-called expert line lists. For example, expert water line lists were published in [4, 5]. Expert line lists are detailed lists which include the centers of rotational-vibrational absorption lines of molecules estimated from known experimental data and the intensities taken from variational calculations. These lists are good because the line centers in them are determined with the best experimental accuracy, and the upper and lower vibrational-rotational energy
levels are averaged over all known experimental transitions excluding weak and distorted lines. The aim of this work is to estimate the effect of spectroscopic information accuracy on the DIAL measurements of methane with the use of expert line lists. We have reviewed literature sources from which the information necessary for compiling an expert line list can be derived. In [6], experimental data are presented in a wide spectral range from 0 to 4800 cm−1. That study included the line identification (assignment of quantum numbers) f
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