ATMOSPHERIC MEASUREMENTS OF WATER-VAPOR IN THE 442-NM REGION

In recent years much interest has been generated in the atmospheric co mmunity concerning low resolution water vapor cross sections in the bl ue spectral region. Proper removal of water absorption from long path tropospheric and zenith sky stratospheric measurements has posed a sig nificant problem for recovery of absorption spectra of low concentrati on molecular species which overlap the water vapor spectrum. The 7-qua nta 4 nu(1)+3 nu(3) and 5 nu(1)+2 nu(3) water vapor overtone bands abs orb light near 442.6 nm, and were found to have an integrated band int ensity of 4.05 x 10(-24) cm(-1) cm(2) molecule(-1) with a measured acc uracy of 4%. This cross section was synthesized from high resolution ( 0.001 nm or 0.055 cm(-1)) solar spectra taken on the Fourier transform spectrometer at the National Solar Observatory (NSO). This cross sect ion was convolved with the instrument function of a low resolution spe ctrometer and used to simultaneously measure tropospheric water vapor and nitrogen dioxide over a folded 20.6 km optical path. A comparison of this spectroscopic measurement and a frost point hygrometer located at one end of the path agree to within 4%+/-17% with the spectroscopi c measurement generally lower. Some of the observed discrepancy can be explained by the proximity of the hygrometric measurement to local su rface sources of water, and by the tendency of the elevated long path measurement to sample drier air coming from aloft.