Ground-based photon path measurements from solar absorption spectra of the O_2 A-band

High-resolution solar absorption spectra obtained from Table Mountain Facility (TMF, 34.38°N, 117.68°W, 2286 m elevation) have been analyzed in the region of the O_2 A-band. The photon paths of direct sunlight in clear sky cases are retrieved from the O_2 absorption lines and compared with ray-tracing calculations based on the solar zenith angle and surface pressure. At a given zenith angle, the ratios of retrieved to geometrically derived photon paths are highly precise (∼0.2%), but they vary as the zenith angle changes. This is because current models of the spectral lineshape in this band do not properly account for the significant absorption that exists far from the centers of saturated lines. For example, use of a Voigt function with Lorentzian far wings results in an error in the retrieved photon path of as much as 5%, highly correlated with solar zenith angle. Adopting a super-Lorentz function reduces, but does not completely eliminate this problem. New lab measurements of the lineshape are required to make further progress.