A new joint study by French and Russian
scientists shows in detail how carbon dioxide molecules absorb and sometimes scatter light energy not only singly but also during inter-molecular collisions. The absorption by single molecules is indeed governed by quantum laws, but absorption by molecules during collisions is, the new study shows, a process governed by classical laws of motion. The new look at this important greenhouse gas should help scientists better model greenhouse warming.
Visible light coming from the sun pours down daily and is reflected back from earth as infrared (IR) radiation.
Extra warming occurs when some of this IR is absorbed and retained in the atmosphere. CO2 is only a trace gas in the atmosphere, far outnumbered by O2 and N2 molecules, but its growing presence (partly owing to human activity) and its ability to absorb and trap infrared radiation is thought to be instrumental in producing greenhouse effects. CO2 molecules one at a time can absorb light. But molecules can also absorb light when they collide with other molecules. This collision-induced absorption, occurring at wavelengths different from those for single molecules and accounting for about 10% of overall IR absorption, is insufficiently understood.
Michael Chrysos and his colleagues at the University of Angers (France) and the University of Saint Petersburg (Russia) have now derived the first exact mathematical formulas that can be used to calculate how collisions between molecules modify the absorption spectra for those
molecules (see figure at http://www.aip.org/png/2008/300.htm). And not just between colliding CO2 molecules, but also collisions among triatomic molecules such as CO2 and diatomic molecules such as O2 and N2, or between diatomic molecules. Ordinarily, O2 and N2 don’t absorb the IR radiation, since they don’t have many of the vibrational motions of triatomic molecules, but they can absorb IR under certain collision circumstances. Thus the formulas allow researchers to look at how greenhouse warming -the capture of radiation and the subsequent sharing around of heat energy- comes about. Chrysos (firstname.lastname@example.org, 33-241-735435) says that the importance of collision-induced absorption depends on altitude. At 600 km, for example, air molecules collide at a rate of one per minute, while at sea level the rate is 10^10 per second. On Venus, where CO2 is the dominant atmospheric gas (96%) and where pressures are enormous, CO2-CO2 collisions provide the larger share of all greenhouse warming.
In conclusion, the new study improves the study of greenhouse warming in several ways: (1)
It allows us to calculate exactly how much of the IR photon energy (intercepted by a CO2
molecule during a CO2-CO2 collision) is transferred directly to the neighboring gas molecule,
where it is converted to kinetic energy of translation; it’s about half of the IR photon energy.
The other half of the IR photon energy goes to rotation of the two molecules, which will then
start spinning more quickly. (2) It shows how to introduce higher-order effects, such as the
simultaneous collision of three molecules. On Venus such collisions should add significantly
to IR absorption. (3) It provides evidence that inter-molecular interactions at close range
separations (interactions acting when the colliding molecules approach to within a few
angstroms) have no effect on absorption, a conclusion in conflict with the mainstream belief
that short range interactions should play a substantial role in collision induced molecular
absorption. Instead, the new study argues that absorption owing to CO2-CO2 collisions is
exclusively governed by long range interactions, which can be modeled and interpreted with
the known laws of classical physics alone. (Chrysos et al., Physical Review Letters, 4 April 2008)