Particles in air help fatten clouds’ water droplets

By making their own clouds, scientists have figured out how some of the fattest water droplets form. And, they find, it’s what’s on the outside that matters.

Climate scientists need to understand how water droplets assemble into colossal clouds. That’s the only way they can reproduce cloud formation in the computer programs that they use to model climate change. Right now, that’s something that these computer models struggle with.

“A cloud model needs to capture the underlying chemistry correctly,” says Kevin Wilson. “It appears that the current one does not.”

Wilson is a physical chemist at Lawrence Berkeley National Laboratory in California. He was part of a team that did the new research. Tiny, carbon-containing molecules can coat the outside of a developing droplet, helping it grow. In fact, they showed, these carbon-based molecules can lead to droplets that are 50 percent wider than had been expected.

The water droplets are bigger because the carbon-containing (or organic) molecules decrease the water’s surface tension. That allows more water molecules to condense out of the air, latching onto the droplet. And larger water droplets are more likely to form clouds, Wilson notes.

His team’s findings appear in the March 25 Science.

The role of chemistry

The water droplets that make up clouds form around aerosols. These are tiny airborne particles onto which water vapor can condense. Sometimes, molecules from these aerosols also can mix into the water droplets. This may help them grow larger than those made of pure water.

Scientists had assumed that this mixing process controlled the size only of water droplets that form around organic (carbon-based) aerosols. Examples of such organic aerosols include pollution from burning fossil fuels or the smoke spewed by wildfires.

Wilson and his colleagues wanted to test that assumption. They filled a cylindrical tube that was about 1.2 meter (4 feet) tall with humid air. Then they dropped organic aerosols into it. As water condensed and formed droplets around the aerosols, the researchers used lasers to monitor the droplets’ growth.

Aerosols that are used to prompt cloud growth are called “seed” particles. Like all aerosols, they’re tiny. But these seed particles led to big droplets. Indeed, they were 40 to 60 percent bigger than researchers would have expected if the organic molecules had simply mixed into the water.

So the researchers started thinking about why this might be. Rather than mixing into the interior of the droplet, Wilson and his colleagues now think that the organic molecules collect on its surface. There, they change the interaction between the droplet and the surrounding air. They lower its surface tension, which makes it easier for more water to condense onto the outside of the growing droplet.

Bigger is better, Wilson says. That’s because droplets need to reach a certain size to remain intact long enough to form clouds.

The new data show that surface tension can’t be ignored, says Leo Donner. He is an atmospheric scientist. He works in the Geophysical Fluid Dynamics Laboratory in Princeton, N.J., which is part of the National Oceanic and Atmospheric Administration.

“This is important,” Donner says, “for understanding the role of clouds in climate and how clouds can change as the composition of the atmosphere changes.” This can help people forecast what to expect as climate changes. If scientists know how certain aerosols are linked to clouds, they might be able to figure out where and how clouds will form.

It will take time and lots of research to improve the computers that attempt to forecast how climate might change, Donner says. “This isn’t the only piece of the problem, but it’s an important piece.” 

Power Words

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aerosol   A group of tiny particles suspended in air or gas. Aerosols can be natural, such as fog or gas from volcanic eruptions, or artificial, such as smoke from burning fossil fuels.                 

atmosphere   The envelope of gases surrounding Earth or another planet.

atmospheric physics      This field of meteorology is related to climate science. People who work in this field, called atmospheric physicists, use computers and math to model the properties of Earth’s atmosphere that drive weather and climate.

chemistry   The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties.

climate  The weather conditions prevailing in an area in general or over a long period.

climate change   Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

cloud    A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Their movement is driven by winds.

computer model    A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

computer program    A set of instructions that a computer uses to perform some analysis or computation. The writing of these instructions is known as computer programming.

condense      To change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.

fossil fuel    Any fuel — such as coal, petroleum (crude oil) or natural gas —  that has developed in the Earth over millions of years from the decayed remains of bacteria, plants or animals.

humidity     A measure of the amount of water vapor in the atmosphere. (Air with a lot of water vapor in it is known as humid.)

model   A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.

molecule    An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O₂), but water is made of two hydrogen atoms and one oxygen atom (H₂O).

nano    A prefix indicating a billionth. In the metric system of measurements, it’s often used as an abbreviation to refer to objects that are a billionth of a meter long or in diameter.

National Oceanic and Atmospheric Administration, or NOAA      A science agency of the U.S. Department of Commerce. Initially established in 1807 under another name (The Survey of the Coast), this agency focuses on understanding and preserving ocean resources, including fisheries, protecting marine mammals (from seals to whales), studying the seafloor and probing the upper atmosphere.

organic    (in chemistry) An adjective that indicates something is carbon-containing; a term that relates to the chemicals that make up living organisms. (in agriculture) Farm products grown without the use of non-natural and potentially toxic chemicals, such as pesticides.

particle     A minute amount of something.

physical chemistry   The area of chemistry that uses the techniques and theories of physics to study chemical systems. A scientist who works in that field is known as a physical chemist.

simulate    To deceive in some way by imitating the form or function of something. A simulated dietary fat, for instance, may deceive the mouth that it has tasted a real fat because it has the same feel on the tongue — without having any calories. A simulated sense of touch may fool the brain into thinking a finger has touched something even though a hand may no longer exists and has been replaced by a synthetic limb. (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.

surface tension     The surface film of a liquid caused by the strong bonds between the molecules in the surface layer.