YU. I. BARANOV, W. J. LAFFERTY AND G. T. FRASER, Optical Technology Division, NIST, Gaithersburg, MD 20899-8441, USA.
The water-vapor continuum and selective absorption in the atmospheric windows strongly affect the radiative balance of the Earth. We have previously reported preliminary experimental results on the water-vapor continuum absorption between 8 µm and 12.5 µm. About 200 spectra were recorded at 0.1 cm-1 resolution for six temperatures between 311 K and 363 K. The spectra were acquired at pathlengths from 76 m to 116 m using a 2 m long White cell coupled to a BOMEM DA8.002 FTIR spectrometer. Water-vapor pressures varied from 2.8 kPa (21 torr) to 15.1 kPa (113 torr). A special spectral processing program calculates, fits, and removes ro-vibrational structure from the spectrum. Regions freed from spectral structure were used to retrieve averaged and smoothed binary absorption coefficients between 8 µm to 12.5 µm and 4.5 µm to 5.2 µm. Our continuum data extrapolated to room temperature are in reasonable agreement with the CKD continuum model. However, at high temperatures the CKD model provides values up to 50% less than experimentally measured. While processing the spectra we have found that the intensities of several ro-vibrational lines in the HITRAN database need to be corrected. Also, at the relatively high vapor pressures mentioned above a self-induced pressure shift of several lines has been detected.