The water vapor foreign continuum in the 8100-8500 cm−1 spectral range

Abstract

In the Earth's atmosphere, the foreign absorption continuum of water vapor is due to the interaction of water molecules with other atmospheric gases (mostly N2 and O2). Following our study of the self-continuum in the high energy edge of the 1.25 µm transparency window (Korovela et al, J. Quant. Spectrosc. Radiat. Transfer 286 (2022) 108206), we report here the first room temperature measurements of water vapor foreign continuum in the same spectral region. Foreign continuum cross-sections, CF, are derived at several selected spectral points between 8120 and 8500 cm−1 for humidified nitrogen, humidified oxygen and humidified air (10000 ppm of H2O). The absorption signal is measured by cavity ring-down spectroscopy (CRDS) using pressure ramps up to 750 Torr. While the measured total continuum absorption follows nicely the expected quadratic dependence versus the total pressure, the uncertainty on the retrieved weak foreign continuum is strongly affected by other contributions which have to be subtracted from the measured absorption (far wings of the resonance lines, O2 collision induced absorption, self-continuum, Rayleigh scattering). The obtained H2O-air and H2O-N2CF cross-section values are found to be comparable while the H2O-O2CF values appear to be significantly smaller. Overall, the retrieved CF values for H2O-air mixture validate the MT_CKD model in the considered region.