The water vapor absorption in the 195–260 GHz region is investigated using asymptotic line wing theory. A knowledge of the absorption by the H2O lines of the rotational band where the shape of the line wing provides an adequate description of the water vapor absorption data in the 300–1000 cm-1 interval is not enough to reproduce the absorption data for the microwave region. While asymptotic line wing theory treats the absorption by any colliding molecular pairs and ignores that due to bound dimers, the experimental absorption data reveal the fraction of absorption by the bound water dimers. The role of the local spectral line contribution to the evaluation of the dimer absorption is discussed.
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The physical origin of humidity-related atmospheric continuum absorption is examined. The existence of double water molecules (dimers) in equilibrium water vapor at room temperature is proved by direct spectroscopic experiments supported by ab initio calculations. It is demonstrated that diluting water vapor with air does not significantly reduce the abundance of dimers. Numerous previous studies have predicted the presence of water dimers in the atmosphere and their influence on chemical reactions, homogeneous condensation, and Earth's radiation balance. Our results provide experimental proof of the presence of dimers in the atmosphere, thus enabling a detailed study of their role in natural processes. Prospects for future research are discussed.
