Atmospheric measurements of water vapor in the 442‐nm region

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Abstract
In recent years much interest has been generated in the atmospheric community concerning low resolution water vapor cross sections in the blue spectral region. Proper removal of water absorption from long path tropospheric and zenith sky stratospheric measurements has posed a significant problem for recovery of absorption spectra of low concentration molecular species which overlap the water vapor spectrum. The 7‐quanta 4v1+3v3 and 5v1+2v3 water vapor overtone bands absorb light near 442.6 nm, and were found to have an integrated band intensity of 4.05×10−24 cm−1 cm2 molecule–1 with a measured accuracy 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 section was convolved with the instrument function of a low resolution spectrometer 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 spectroscopic measurement generally lower. Some of the observed discrepancy can be explained by the proximity of the hygrometric measurement to local surface sources of water, and by the tendency of the elevated long path measurement to sample drier air coming from aloft.