H2O nu2 Line Strengths and Positions for MIPAS/ENVISAT

Abstract

As a first step towards the improvement of the spectroscopic database of water for the MIPAS satellite instrument, linestrengths and line positions have been measured in the range 1250 – 1750 cm-1. Pure water vapour was measured at ambient temperature with a Bruker IFS 120 HR at a spectral resolution of 0.0027 cm-1, pressures ranging from 0.2 to 5 hPa, and absorption paths from 0.25 to 80 m. In case of the short cell measurements (0.25 m absorption path) high resolution reference measurements were taken while for the White cell measurements (20 and 80 m) low resolution reference spectra were sufficient. The White cell required a continuous flow of water while for the short cell static filling with a gas reservoir attached was sufficient. Pressures were measured with capacitance manometers for number density determination. Spectra were corrected for non-linearity and thermal self emission. Spectral calibration was performed by means of N2O measurements with NIST heterodyne line positions as reference. The field stop diameter was fitted from N2O and water lines. In total nine spectra were analyzed. Self broadening was obtained from measurements at 1 and 5 hPa. Merged pressure broadening data were then used as input for line position and linestrength determination of all data sets. Linestrengths with reasonable uncertainties (<10%) were obtained for 2x10-25 < S < 1x10-19. The large redundancy among the data allowed for extensive quality checks concerning linestrengths. Averaged linestrength ratios for different measurements revealed some peculiarities which will be discussed. The status of the quantum mechanical analysis will be reported, too.