Teeny-tiny particles in the air can make storms stronger, new research suggests. This is especially true over places where the air starts out relatively clean, such as over the oceans or in the Amazon rainforest. A passing plume of the tiny pollutants in these places can make a storm up to 50 percent more intense.
That’s the conclusion of a new computer analysis.
The smaller an air pollutant particle is, the longer it can remain aloft in the air. And the longer it stays in the air, the farther it can travel. Such small particles are known as ultrafine (meaning super small) aerosols. Each will be less than 50 nanometers — billionths of a meter — across. (For perspective, a human hair may be 80,000 to 100,000 nanometers wide.)
Such nanopollutants spew from a wide range of sources. These run the gamut from car exhaust and wildfires to printer toner. When inhaled, these tiny pollutants can cause harm to the body and brain. When drawn into the atmosphere, they can alter the weather.
Fairly large air-pollutant particles can help form clouds. But scientists used to think the ultrafines were too small to do that. New research suggests that’s wrong. Ultrafine aerosols indeed may help drive storms in the Amazon and elsewhere. That would mean they may play a big role in how water (the basis of storms) moves across the planet.
“I have studied aerosol interactions with storms for a decade,” says Jiwen Fan. She’s an atmospheric scientist who led the new study. She works at the Pacific Northwest National Laboratory in Richland, Wash. “This is the first time I’ve seen such a huge impact” from these tiniest particles, she says.
Her team shared its findings January 26 in Science.
How to make a cloud
Particles such as soot are quite tiny. Still, they’re far bigger than ultrafine aerosols. Sooty bits are more than 100 nanometers across. They can help create clouds by allowing water vapor to condense onto them. This begins the formation of tiny water droplets. When enough of them get together, they turn into a cloud that we can see.
But water vapor has a far harder time condensing around tinier particles. For that, the air must hold even more water vapor than normal. This waterlogged air is described as supersaturated.
It rarely occurs. Usually, the air hosts plenty of larger aerosols. Water droplets form around these before the air gets a chance to become supersaturated. Explains Fan, those larger pollutant bits in air remove the extra water. Yet supersaturation can be relatively common at humid sites with little air pollution, she notes. The Amazon rainforest is one such place.
Brazilian and U.S. research groups worked together to study weather and pollution in that rainforest from 2014 to 2015. As part of that, several observation sites tracked polluted air that drifted across the rainforest. Such a traveling cloud of pollution is called a plume. And the plumes, here, came from Manaus. It’s an industrial city that is home to 1.8 million people.
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During the Amazon’s warm, wet season, the rainforest’s weather seldom changes. Temperature, humidity and wind direction all stay much the same from day to day, Fan says. Here, then, an occasional pollutant plume tended to be the only major change.
The researchers observed such plumes at one research station there between March and May in 2014. They also looked at vertical winds, known as updrafts. When a large plume with lots of ultrafine aerosols passed by, the researchers saw heavier rains and stronger updrafts. Such winds tend to make storms more intense.
Next, the researchers used a computer to model, or simulate, an actual storm that had occurred on March 17, 2014. They fed the computer model data on that storm’s temperature, wind and moisture levels. They also included pre-storm data on the low level of pollution before the plume came by. Then, the team ran the simulation several different ways. In one scenario, there was no pollution plume. In another, there was a passing plume typical of what might be coming from Manaus.
The results suggested that the ultrafine aerosols not only helped clouds form but they also created water droplets. And those would greatly strengthen a gathering storm, the computer model predicted.
If the conditions are right, the plume of ultrafine particles would create a whole lot of cloud droplets very quickly. As water vapor condensed onto the particles, it released heat. The heat from all of those the droplets forming would enter the air and rise. This would fuel the updrafts, making the storm even stronger.
Clean, humid and stormy?
The Amazon isn’t the only region that’s is damp, with little pollution. Such moist, pristine conditions also exist over large swaths of the oceans. Several months back, a study showed that lightning is more common in parts of the ocean where shipping traffic is heavy. The passing ships spew a lot of exhaust. And that includes ultrafine particles. The findings appeared last September 16 in Geophysical Research Letters. “This [newfound] mechanism may have been at play there,” Fan now says.
Joel Thornton led that study on shipping exhaust. He’s an atmospheric scientist at the University of Washington in Seattle. Ultrafine particles may help cause more lightning storms over shipping routes, Thornton says. The new study by Fan’s group, he says, shows why it’s important to understand where and how ultrafine particles move through the atmosphere.
Johannes Quaas agrees. A meteorologist at the University of Leipzig in Germany, he was not involved in either study. The latest work offers “a very interesting hypothesis,” he says. But it doesn’t prove that ultrafine aerosols alone drive updrafts, he adds.
Even if the weather seems the same from day to day, systems like the Amazon rainforest do have many changing variables. Everything from wind to temperature to how the land receives the sun’s rays may be changing, he notes. “In reality,” Quaas cautions, “it’s not just the aerosols that change.”
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.
cloud A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its 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.
condense To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean 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.
exhaust (in engineering) The gases and fine particles emitted — often at high speed and/or pressure — by combustion (burning) or by the heating of air. Exhaust gases are usually a form of waste.
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.)
hypothesis (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.
lightning A flash of light triggered by the discharge of electricity that occurs between clouds or between a cloud and something on Earth’s surface. The electrical current can cause a flash heating of the air, which can create a sharp crack of thunder.
mechanism The steps or process by which something happens or “works.” It may be the spring that pops something from one hole into another. It could be the squeezing of the heart muscle that pumps blood throughout the body. It could be the friction (with the road and air) that slows down the speed of a coasting car. Researchers often look for the mechanism behind actions and reactions to understand how something functions.
meteorologist Someone who studies weather and climate events.
Pacific The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
particle A minute amount of something.
plume (in environmental sciences) The movement of some gas or liquid, under the direction of gravity, winds or currents. It may be in air, soil or water. It gets its name from the fact that it tends to be long and relatively thin, shaped like a large feather. (in geology) Fluids (air, water or magma typically) that move, largely intact, in a feather-like shape over long distances.
pristine An adjective referring to something that is in original or near-original condition. It means something is somewhat old but in a seemingly “untouched” or unaltered condition.
rainforest Dense forest rich in biodiversity found in tropical areas with consistent heavy rainfall.
scenario A possible (or likely) sequence of events and how they might play out.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
smoke Plumes of microscopic particles that float in the air. They can be comprised of anything very small. But the best known types are pollutants created by the incomplete burning of oil, wood and other carbon-based materials.
soot Also known as black carbon, it's the sometimes oily residues of incompletely burned materials, from plastics, leaves and wood to coal, oil and other fossil fuels. Soot particles can be quite small — nanometers in diameter. If inhaled, they can end up deep within the lung.
ultrafine (in environmental science) A size designation given to very small pollutant particles, typically those on the order of several tens of nanometers in diameter. These are the smallest size particles, allowing them to stay suspended in air for days or weeks. They also are small enough to be inhaled deeply into the lungs or nostrils where some may cross directly into the bloodstream and brain.
variable (in experiments) A factor that can be changed, especially one allowed to change in a scientific experiment. For instance, when researchers measure how much insecticide it might take to kill a fly, they might change the dose or the age at which the insect is exposed. Both the dose and age would be variables in this experiment.
vertical A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.
water vapor Water in its gaseous state, capable of being suspended in the air.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Journal: J. Fan et al. Substantial convection and precipitation enhancements by ultrafine aerosol particles. Science. Vol. 359, January 26, 2018, p. 411. doi: 10.1126/science.aan8461.
Journal: J.A. Thornton et al. Lightning enhancement over major oceanic shipping lanes. Geophysical Research Letters. Vol. 44, September 16, 2017, p. 9102. doi: 10.1002/2017GL074982.