The self- and foreign-absorption continua of water vapor by cavity ring-down spectroscopy near 2.35 μm

Abstract

The room temperature self- and foreign-continua of water vapor have been measured near 4250 cm⁻¹ with a newly developed high sensitivity cavity ring down spectrometer (CRDS). The typical sensitivity of the recordings is α_min ≈ 6 × 10⁻¹⁰ cm⁻¹ which is two orders of magnitude better than previous Fourier transform spectroscopy (FTS) measurements in the spectral region. The investigated spectral interval is located in the low energy range of the important 2.1 μm atmospheric transparency window. Self-continuum cross-sections, C_S, were retrieved from the quadratic dependence of the spectrum base line level measured for different water vapor pressures between 0 and 15 Torr, after subtraction of the local water monomer lines contribution calculated using HITRAN2012 line parameters. The C_S values were determined with 5% accuracy for four spectral points between 4249.2 and 4257.3 cm⁻¹. Their values of about 3.2 × 10⁻²³ cm² molecule⁻¹ atm⁻¹ are found 20% higher than predicted by the MT_CKD V2.5 model but two times weaker than reported in the literature using FTS. The foreign-continuum was evaluated by injecting various amounts of synthetic air in the CRDS cell while keeping the initial water vapor partial pressure constant. The foreign-continuum cross-section, C_F, was retrieved from a linear fit of the spectrum base line level versus the air pressure. The obtained C_F values are larger by a factor of 4.5 compared to the MT_CKD values and smaller by a factor of 1.7 compared to previous FTS values. As a result, for an atmosphere at room temperature with 60% relative humidity, the foreign-continuum contribution to the water continuum near 4250 cm⁻¹ is found to be on the same order as the self-continuum contribution.