Temperature dependent absorption cross-sections of O2-O2 collision pairs between 340 and 630 nm and at atmospherically relevant pressure

The collisions between two oxygen molecules give rise to O-4 absorption in the Earth atmosphere. O-4 absorption is relevant to atmospheric transmission and Earth's radiation budget. O-4 is further used as a reference gas in Differential Optical Absorption Spectroscopy (DOAS) applications to infer properties of clouds and aerosols. The O-4 absorption cross section spectrum of bands centered at 343, 360, 380, 446, 477, 532, 577 and 630 nm is investigated in dry air and oxygen as a function of temperature (203-295 K), and at 820 mbar pressure. We characterize the temperature dependent O-4 line shape and provide high precision O-4 absorption cross section reference spectra that are suitable for atmospheric O-4 measurements. The peak absorption cross-section is found to increase at lower temperatures due to a corresponding narrowing of the spectral band width, while the integrated cross-section remains constant (within <3%, the uncertainty of our measurements). The enthalpy of formation is determined to be Delta H-250 = 0.12 +/- 0.12 kJ mol(-1), which is essentially zero, and supports previous assignments of O-4 as collision induced absorption (CIA). At 203 K, van der Waals complexes (O2-dimer) contribute less than 0.14% to the O-4 absorption in air. We conclude that O2-dimer is not observable in the Earth atmosphere, and as a consequence the atmospheric O-4 distribution is for all practical means and purposes independent of temperature, and can be predicted with an accuracy of better than 10(-3) from knowledge of the oxygen concentration profile.