Atmospheric absorption of near infrared and visible solar radiation by thehydrogen bonded water dimer

Based on the physico-chemical properties of water dimers, their near infrar ed and visible absorption of solar radiation in the earth's atmosphere is c alculated. The calculation uses equilibrium constants determined by statist ical mechanics, and a vibrational absorption spectrum determined by a coupl ed oscillator quantum mechanics model and ab initio quantum chemistry. The resulting total atmospheric absorption was calculated using a line-by-line radiative-transfer model, and depends significantly on the dimer abundance, as well as on the frequency and line width given to vibrational transition s. The best estimate achieved for the possible range of total absorption fr om 400 nm to 5000 run by water dimer in the tropics is 1.6-3.3 W m(-2). In a global-warming scenario, the increased temperature and water vapour parti al pressure result in a nonlinear increase in the absorption of solar radia tion by water clusters. Most of the energy from water dimer absorption is d eposited in the lower troposphere, particularly in the tropics, tending to make it more convective.