Effect of pressure broadening on molecular absorption cross sections in exoplanetary atmospheres

Spectroscopic observations of exoplanets are leading to unprecedented constraints on their atmospheric compositions. However, molecular abundances derived from spectra are degenerate with the absorption cross-sections which form critical input data in atmospheric models. Therefore, it is important to quantify the uncertainties in molecular cross-sections to reliably estimate the uncertainties in derived molecular abundances. However, converting line lists into cross-sections via line broadening involves a series of prescriptions for which the uncertainties are not well understood. We investigate and quantify the effects of various factors involved in line broadening in exoplanetary atmospheres - the profile evaluation width, pressure versus thermal broadening, broadening agent, spectral resolution and completeness of broadening parameters - on molecular absorption cross-sections. We use H2O as a case study as it has the most complete absorption line data. For low-resolution spectra (R <= 100) for representative temperatures and pressures (T similar to 500-3000 K, P <= 1 atm) of H-2-rich exoplanetary atmospheres, we find the median difference in cross-sections (delta) introduced by various aspects of pressure broadening to be a parts per thousand(2)1 per cent. For medium resolutions (R less than or similar to 5000), including those attainable with James Webb Space Telescope, we find that delta can be up to 40 per cent. For high resolutions (R similar to 10(5)), delta can be less than or similar to 3100 per cent, reaching less than or similar to 1000 per cent for low temperatures (T less than or similar to 500 K) and high pressures (P less than or similar to 1 atm). The effect is higher still for self-broadening. We generate a homogeneous data base of absorption cross-sections of molecules of relevance to exoplanetary atmospheres for which high-temperature line lists are available, particularly H2O, CO, CH4, CO2, HCN, and NH3.