A practical guide to coding line-by-line trace gas absorption in Earth's atmosphere

• Despite the existence of numerous computer programs for absorption spectroscopy, the code development process is poorly covered in literature. • Our paper shows how to create a code for line-by-line molecular absorption spectroscopy from scratch. • We explain the process in three steps: a) simulation of absorption by a molecule using HITRAN database; b) simulation of absorption in a gas cell by a group of molecules; c) simulation of absorption in Earth atmosphere using MODTRAN profiles. • The goals of the paper are the simplification of software development, documentation, and support, and ultimately the transfer of knowledge between generations of scientists. • Our paper comes with an open-source code, in C-language, and numerical data for unit testing, thus insuring independent reproducibility of our results. 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.