In this work we extend previous studies by exploring the potential impact of spectroscopic knowledge along with atmospheric state knowledge on retrievals of carbon dioxide (XCO2) and methane (XCH4) column amounts from laser differential absorption spectrometer (LAS) measurements. This has been done for multiple CO2 absorption lines in the 1.57 and 2.05 μm regions, and for CH4 in the 1.65 μm region. One such potential source of error in performing XCO2 retrievals is modeled surface pressure. Since it has been proposed to derive surface pressure from LAS-based O2 measurements in lieu of modeled surface pressure for use in XCO2 retrievals as a means of error reduction, our past work has also attempted to characterize and quantify potential improvements in XCO2 retrieval error associated with O2-derived surface pressure for a set of CO2 and O2 absorption line combinations. All of our previous analyses have relied on a radiative-transfer-based simulation framework utilizing the Line-by-Line Radiative Transfer Model (LBLRTM), version 12.2 (release date November, 2012). LBLRTM has undergone several upgrades since version 12.2, to include updates to its line parameter database, updates to its continuum model, and bug fixes. Our current work revisits our prior assessments using the latest version of LBLRTM (version 12.8) and comparisons are provided and discussed.