The Total Carbon Column Observing Network (TCCON) is a ground-based network of Fourier transform infrared (FTIR) spectrometers that measure precise and accurate columns of greenhouse gases. Column abundances of gases are converted to column-averaged dry air mole fraction (Xgas) by dividing them by the column of O2 and multiplying by the known dry air mole fraction of O2 (0.2095). This is done because errors that are common to both the target gas and O2, (like tracker mis-pointing and zero-level offsets) will cancel out in the column ratio. However, the air-mass dependent artefact that causes XCO2 and XCH4 to vary as a function of solar zenith angle still remains due to the deficiency in the calculation of absorption coefficients by the retrieval algorithms forward model. Recently, O2 laboratory spectra measured using frequency?stabilized cavity ring-down spectroscopy was acquired from the National Institute of Standards and Technology. In this presentation, I will use these spectra to show that the Voigt spectral line shape (used to calculate O2 absorption coefficients) is insufficient to model the spectral line shape of O2. Rather, a spectral line shape that takes into account speed-dependence and line mixing is more appropriate for O2.