The Influence of Water Vapor Absorption in the 290-350 nm Region on Solar Irradiance: Laboratory Studies and Model Simulation

Water vapor is the most abundant greenhouse gas in the earth's atmosphere. Absorption of the solar radiation by water vapor in the near UV region may partially account for the up to 30% discrepancy between the modeled and the observed solar energy absorbed by the atmosphere. But the magnitude of water vapor absorption in the near UV region at wavelengths shorter than 384 nm was not known prior to this study. We have determined absorption cross sections of water vapor at 5 nm intervals in the 290-350 nm region, by using cavity ring-down spectroscopy. Water vapor cross section values range from 2.94×10-24 to 2.13×10-25 cm2/molecule in the wavelength region studied. The effect of the water vapor absorption in the 290-350 nm region on the modeled radiation flux at the ground level has been evaluated using radiative transfer model.