Atmospheric water vapor complexes and the continuum

Abstract

[1] Estimates of absorption optical depths for the bound complexes H 2 O-H 2 O and the sum of H 2 O-N 2 , H 2 O-O 2 , and H 2 O-Ar at visible and near-infrared wavelengths are compared to the same quantities calculated from a frequently used water continuum parameterization (MT_CKD) and from a theoretical far wing water vapor lineshape theory. The temperature dependences of some of these optical depths are also compared. The comparisons suggest qualitatively that water complexes may contribute to the continuum at these wavelengths, and show that the temperature dependence of the continuum might provide insight into the role of the complexes in the atmosphere. Because of the dearth of laboratory measurements of the continuum at these wavelengths, and because the current estimates for the equilibrium constants of these water vapor complexes remain highly uncertain, more observations are needed before the importance of water complexes can be accurately quantified.

Water Dimers in the Atmosphere III: Equilibrium Constant from a Flexible Potential

“…In recent years, it has been proposed that the water dimers may be important in several atmospheric processes, such as excess absorption of solar radiation, [1][2][3][4][5] water continuum absorption in the far infrared, 6,7 homogeneous nucleation of water into droplets and ice, 8,9 and catalysis of important chemical reactions, 10,11 such as acid rain formation. However, from a quantitative viewpoint, its influence remains a controversial issue, as its concentration and variation with conditions have not yet been precisely established.…”

Complexes and Clusters of Water Relevant to Atmospheric Chemistry: H2O Complexes with Oxidants

“…Because the absorption of the H 2 O·O 2 complex has a large effect on the water vapor continuum in the near-infrared region of the Earth's atmosphere, there was a recent attempt to model the form and optical depths of the continuum on the basis of known (mostly quantum chemical) estimates of absolute IR intensities and frequencies of water complexes with oxygen, nitrogen, and argon. [29] However, these authors concluded that "the comparisons are largely qualitative rather than quantitative due to the large uncertainty in the complexes' equilibrium constants…". Thus, there is obvious motivation to continue studying the H 2 O·O 2 and other water complexes, in order to obtain more reliable estimates for both thermodynamic properties and spectral parameters.…”

Overtone Spectroscopy: A Sensitive Probe of Hydrogen Bonding

“…Although there are no distinct boundaries for the near infrared region, typically it is considered to lie within 4000-14,000 cm -1 . The calculated changes in absorbance of hydrated complexes H 2 O•M, (M is a molecule present in the atmosphere) in the NIR and visible regions have supported the suggestion [8][9][10] that the complexes are part of the explanation for the so called water vapour continuum, an empirical continuum used in radiative transfer modelling.…”