A practical guide to coding line-by-line trace gas absorption in Earth's atmosphere
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Korkin, Sergey, Andrew M. Sayer, Amir Ibrahim, and Alexei Lyapustin. “A Practical Guide to Coding Line-by-Line Trace Gas Absorption in Earth’s Atmosphere.” Journal of Quantitative Spectroscopy and Radiative Transfer 337 (May 1, 2025): 109345. https://doi.org/10.1016/j.jqsrt.2025.109345.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract
We present two new open-source codes, in the C language, for simulation of the line-by-line molecular (gas) absorption in the solar spectral region with wavelengths up to ∼2500 (nm). The first one, gcell, simulates absorption spectroscopy in a gas cell for a given length of the cell, temperature, and pressure. The second one, aspect, is for spectroscopy in Earth's atmosphere - a common need for remote sensing applications. Both use the HITRAN database for line shape (Voigt) modeling. Aspect adapts height variations of the thermodynamic parameters (profiles) from MODTRAN. Separate discussion of the gas cell and the atmospheric modes simplifies software development, documentation, and support, and ultimately the transfer of knowledge between generations of scientists. These are the main goals of the current paper. Despite the existence of numerous computer programs for absorption spectroscopy, the code development process is poorly covered in literature. As a result, it is difficult for a non-developer to confidently modify an existing code or create a new tool within a reasonable amount of time.