Analysis of the atmospheric water vapor content determination in the 940-nm band using moderate spectral resolution measurements of direct solar irradiance

We have determined the vertical integrated water vapor of the atmosphere based on the absorption features of the 940nm band by means of ground-based measurements of direct solar spectral irradiances and modelled ones. The experimental irradiance data were performed under clear skies with a LIcor 1800 spectroradiometer, based on a monochromator system, of high to moderate spectral resolution (6nm) in the 300-1100 nm range. The modelled data are based on the monochromatic approaches for atmospheric transmittance constituents, where for water vapor we used the band-model transmittance of LOWTRAN7 Code. The method here used is a curve fitting procedure (CF method) making use of the whole shape band absorption information and the contribution of molecular and aerosol constituents retrieving a unique water vapor value. The method were used to determine water vapor values for the period from March to November of 1995 at a rural station in Valladolid (Spain) under different atmospheric conditions. The contribution of continuum absorption was also evaluated in the retrieval, obtaining lower values from 13 to 30%. This contribution appears as considerable greater than those expected.