Spectral dependence of the self-continuum cross-section: measured in ref. 3 (black circles). [3] I. V. Ptashnik, R. A. McPheat, K. P. Shine, K. M. Smith and R. G. Williams, J. Geophys. Res., 2011, 116, D16305.
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 αmin ≈ 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, CS, 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 CS 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, CF, was retrieved from a linear fit of the spectrum base line level versus the air pressure. The obtained CF 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.
PCCP publishes new, original research, covering the areas of physical chemistry, chemical physics and biophysical chemistry, and includes papers on: biophysical chemistry, chemisorption and heterogeneous catalysis, clusters, colloid and interface science, computational chemistry and molecular dynamics, electrochemistry, energy transfer and relaxation processes, gas-phase kinetics and dynamics, laser-induced chemistry, liquids and solutions, materials science, molecular beam kinetics and spectroscopy, photochemistry and photophysics, physical chemistry of macromolecules and polymers, physisorption and chromatographic science, quantum chemistry and molecular structure, radiation chemistry, reactions in condensed phases, solid-state chemistry (microstructures and dynamics), spectroscopy of molecules and gas-phase complexes spectroscopy, statistical mechanics and quantum theory of the condensed phase, statistical mechanics of gaseous molecules and complexes, surface science, thermodynamics, zeolites and ion-exchange phenomena
The RSC is the largest organisation in Europe for advancing the chemical sciences. Supported by a worldwide network of members and an international publishing business, our activities span education, conferences, science policy and the promotion of chemistry to the public.
n most near‐infrared atmospheric windows, absorption of solar radiation is dominated by the water vapor self‐continuum, and yet there is a paucity of measurements in these windows. We report new laboratory measurements of the self‐continuum absorption at temperatures between 293 and 472 K and pressures from 0.015 to 5 atm in four near‐infrared windows between 1 and 4 μm (10000–2500 cm−1); the measurements are made over a wider range of wavenumbers, temperatures, and pressures than any previous measurements. They show that the self‐continuum in these windows is typically one order of magnitude stronger than given in representations of the continuum widely used in climate and weather prediction models. These results are also not consistent with current theories attributing the self‐continuum within windows to the far wings of strong spectral lines in the nearby water vapor absorption bands; we suggest that they are more consistent with water dimers being the major contributor to the continuum. The calculated global average clear‐sky atmospheric absorption of solar radiation is increased by ∼0.75 W/m2 (which is about 1% of the total clear‐sky absorption) by using these new measurements as compared to calculations with the MT_CKD‐2.5 self‐continuum model.
