DETERMINATION OF THE ATMOSPHERIC-WATER-VAPOR CONTENT IN THE 940-NM ABSORPTION-BAND BY USE OF MODERATE SPECTRAL-RESOLUTION MEASUREMENTS OF DIRECT SOLAR IRRADIANCE

We have analyzed three methods that can be used to determine the integ rated water vapor of the atmosphere in the 940-nm band by means of mod eled and measured direct solar spectral irradiance. The experimental i rradiance data were obtained with a commercial LI-COR 1800 spectroradi ometer, based on a monochromator system, of high to moderate spectral resolution (6 nm) in the 300-1100-nm range. The modeled data are based on monochromatic approaches to determine atmospheric transmittance co nstituents; for those of water vapor we used the LOWTRAN7 model. The f irst method is a curve-fitting procedure that makes use of the entire shape band absorption information to retrieve a unique water-vapor val ue. The second method makes use of the monochromatic approach of the a bsorption transmittance formula to determine the amount of water vapor at each wavelength of the absorption band, and the third method is th e classic differential absorption technique suitably applied to our da ta. Spectral analysis showed the advantages and disadvantages of each method, such as problems linked to the various spectral resolutions of the experimental and the modeled data, the width of the spectral rang e used to define the water-vapor absorption band, and the dependence o f the retrieval on the choice of the two selected wavelengths in the l ast-named technique. All these problems were considered so they could be avoided or minimized and the associated errors estimated. We used t he methods to determine water-vapor values for the period from March t o November 1995 at a rural station in Vallodolid, Spain, allowing for the evaluation of the differences in real monitoring conditions. Final ly, the contribution of continuum absorption was also evaluated, yield ing lower water-vapor values between 13 and 30%. These differences wer e considerably greater than those that were due to the problems that w e have just enumerated. (C) 1998 Optical Society of America.