Absorption of Solar Radiation by Atmospheric O4

Abstract

Spectroscopic measurements of the atmospheric solar radiation attenuation reveal that the near ultraviolet?visible?near-infrared absorption of the oxygen collision complex (O2)2, thus far omitted from models, is important for the direct heating for both clear and cloudy skies. Atmospheric line-by-line radiative transfer calculations show that the absorption by (O2)2 leads to a globally averaged clear sky shortwave (SW) heating of about 0.53 W m?2. It is therefore proposed that the absorption by (O2)2 should be included in models designed to calculate the SW heating. It is shown that due to its weak absorption under clear sky, the SW heating by (O2)2 approximately increases linearly with increasing optical pathlengths for cloudy sky conditions. This is in contrast to SW heating by the molecular absorptions of H2O or O2, whose absorption lines are already partially saturated under clear sky, causing the SW heating (due to these gases) to increase rather as the square root of the optical path. From a limited number of cloudy sky optical path observations, the atmospheric (O2)2 absorption is estimated to account for an additional solar heating of about 0.57 W m?2 globally averaged.