Evaporation and Climate Change
December 13, 2015
One of the most common processes on the planet—the phenomenon of evaporation—is governed by mechanisms different than previously thought, say researchers at the Polish Academy of Sciences’ Institute of Physical Chemistry in Warsaw. The discovery could have far-reaching implications for how scientists build global climate models, where a key role is played by the evaporation of the oceans.
The process of evaporation shapes the climate of the planet. It is also one of the most important factors determining the temperature of the human body. Oceans and seas evaporate, as do microdroplets of fuel in car engines and sweat on human skin.
Science sometimes does a poor job describing processes occurring in nature, says Prof. Robert Ho≥yst, head of the Institute of Physical Chemistry. “We are very good at describing states at the beginning of a process and at its end; but what happens in between? How does a given process really take place?” he says. “For so many years we have been asking ourselves this question in relation to the phenomenon of evaporation—and we are coming to ever more interesting conclusions.”
For more than 100 years, scientists have used the so-called Hertz-Knudsen equation to measure the rate of evaporation. This is based on an intuitive prediction that, at a given temperature, the rate at which a liquid evaporates depends on the difference between the actual and hypothetical pressure that would occur if the evaporating liquid were in thermodynamic equilibrium with its environment. “The farther a system is from equilibrium, the more dynamically it should return to it. It’s so intuitive,” says the Institute of Physical Chemistry’s Marek Litniewski, Ph.D.
To test the Hertz-Knudsen equation, the Warsaw scientists made accurate computer simulations that gave them a closer look at the process of evaporation.
Advanced computer simulations using molecular dynamics—a computer simulation method for studying the physical movements of atoms and molecules—showed that some parameters describing evaporation were several times greater than those suggested by the Hertz-Knudsen equation. An interesting effect was also observed whereby the stream of gas released from the surface of a liquid during evaporation underwent little change despite significant pressure fluctuations.
“There could only be one conclusion from this observation: that, despite what was previously believed, the rate of evaporation was not closely related to the pressure of the vapor,” says Litniewski. “For more than a century we had all been making a serious mistake in our theoretical approach to the phenomenon of evaporation.”
The previous model of evaporation was based on the principle of conservation of mass: the mass of molecules released from the surface of a liquid was supposed to increase the mass of the gas in its surroundings accordingly. The Warsaw physicists noticed, however, that since the particles released from the surface have a certain velocity, the principle of conservation of momentum should be applied to describe this phenomenon.
"We realized that, to an extent, evaporation resembles shooting from a gun: the missile flies in one direction, but the overall momentum of the system must be maintained, so the gun recoils in the opposite direction,” says Ho≥yst. He says the same happens with the molecules of the evaporating liquid. “Since there is an increase in the momentum, there must be recoil, and if there is recoil, the pressure felt by the molecules on the surface of the liquid will be different.”
The Warsaw physicists used computer simulations to measure the velocities of the molecules released from the surface of the liquid. These proved to be small, on the order of hundreds of micrometers per second, or a few kilometers per hour. This means that the evaporation process can be affected by any flow naturally occurring over the surface of the liquid, the scientists say.
The scientists published their findings in the scientific journal Soft Matter.
The discovery by the Warsaw physicists is important in understanding the mechanisms responsible for global warming. Contrary to popular belief, the most common greenhouse gas in the atmosphere is not carbon dioxide, but water vapor. At the same time, it is known that the speed of air masses flowing over the oceans can significantly exceed 100 kilometers per hour and is therefore certain to affect the rate of evaporation. This means that experts were wrong in their evaluation of the rate at which oceans evaporate. Predictions contained in most models of Earth’s climate are inaccurate and should be revised, according to the Warsaw physicists.
While investigating evaporation, the scientists from the Institute of Physical Chemistry asked their colleagues at the Polish Academy of Sciences’ Institute of Physics in Warsaw to verify the accuracy of the simulations.