The greenhouse effect is caused by molecules that absorb infrared radiation released from the Earth's surface, trapping heat in the atmosphere. Water acts as a greenhouse gas because it is one of the molecules that can absorb infrared radiation and cause warming. "Our research supports the suggestion that in a global warming scenario higher temperatures will lead to increased absorption of solar radiation by water clusters," said lead author, George Shields, the Winslow Professor of Chemistry at Hamilton College. "The prediction that higher order water clusters (trimers, tetramers, and pentamers) are present in the atmosphere is significant because it shows that these entities must be considered as key players in atmospheric processes."
Previous research has hypothesized that water clusters (two or more water molecules held together by hydrogen bonds) could catalyze acid rain or the formation of aerosol in the atmosphere, and even lead to acceleration of the Greenhouse effect. All of these ideas depend on the presence of water clusters in the troposphere, the region of the atmosphere that is directly heated by the Earth's surface. The Hamilton group can now predict the concentration of water clusters present in the troposphere. Large water clusters have for some time been thought to catalyze reactions which have implications for the chemistry that takes place in the atmosphere. A paper in the June 27, 2003 issue of Science documented the first detection of a water dimer (two hydrogen bonded water molecules) in the troposphere.
Shields said, "Once we knew the dimers were present, we investigated whether larger water clusters might also be involved in a variety of atmospheric chemistry processes. We started by using high level quantum chemistry methods to predict dimer concentrations that would be found on a warm, humid day. The accuracy of our dimer calculation, which matched the experimentalists' detection of water dimer concentrations under the same conditions, led us to calculate the concentration of other water clusters in the troposphere." The researchers found that water clusters consisting of cyclic trimers, cyclic tetramers, and cyclic pentamers should all be detectable in the lower troposphere.
The Hamilton researchers used the documented information on water cluster structures to investigate the effectiveness of various model chemistries in modeling gas-phase water cluster formation. The performance of these chemistries was compared against previous calculations, and the Hamilton team found that thermodynamic calculations by Gaussian-2, Gaussian-3 and Complete Basis Set-APNO chemistries compared quite well to the prior calculations. (Experimentalists reported a value of 6 x 10^14 dimers per cubic centimeter at 292 K on a 100% humid day. The Hamilton study predicted a value of 4 x 10^14 dimers per cubic centimeter at 292 K.)
George Shields conducted the research with two undergraduate students, Meghan Dunn and Emma Pokon. The research was made possible through funding from the American Chemical Society/Petroleum Research Fund, Merck/AAAS, the Camille and Henry Dreyfus Foundation, and from NSF Grant CHE-0116435 for supercomputer instrumentation as part of the MERCURY supercomputer consortium (http://mars.hamilton.edu).
Journal
Journal of the American Chemical Society