"It's possible," says Allison Steiner, "that when trees emit pollen, that makes clouds, which in turn makes rain and that feeds back into the trees and can influence the whole growth cycle of the plant." (Credit: Cesar Lopez Rivadeneira/Unsplash)

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Cloud chamber suggests pollen causes rain

Pollen’s main job is to create the next generation of trees and plants, but the grains might also seed clouds, a new study shows.

The unexpected findings demonstrate that pollen might have an effect on the planet’s climate. And they highlight a new link between plants and the atmosphere.

Atmospheric scientists who study aerosols—particles suspended in the air that scatter light and heat and play a role in cloud formation—have largely ignored pollen.

“The grains were thought to be too large to be important in the climate system, too large to form clouds or interact with the sun’s radiation,” says Allison Steiner, associate professor of atmospheric, oceanic and space sciences at the University of Michigan. “And also the large particles don’t last in the atmosphere. They tend to settle out relatively quickly.”

But Steiner and her colleagues weren’t sure that was the whole story. Sweeping a dusting off her porch one spring morning, she wondered what happens to the grains in the air. Steiner turned to the medical community. Pollen causes seasonal allergies, which affect between 10 and 20 percent of US residents.

“When we were looking in the allergy literature we discovered that it’s pretty well known that pollen can break up into these tiny pieces and trigger an allergic response,” Steiner says.

Smaller grains could have big implications. The research team set out to see if moisture could cause the pieces to break down.

“What we found is when pollen gets wet, it can rupture very easily in seconds or minutes and make lots of smaller particles that can act as cloud condensation nuclei, or collectors for water,” Steiner says.

In the cloud-making chamber

In the lab-based experiment at Texas A&M University, the researchers tested pollen from oak, pecan, birch, cedar, and pine trees, as well as ragweed. These are the most common sources of wind-driven pollen in the US. They soaked two grams from each source in pure water for an hour.

They used an atomizer to make a spray of the moist pollen fragments and sent the spray into a cloud-making chamber in the laboratory of in Sarah Brooks, a professor of atmospheric science at Texas A&M. The team found that three different sizes—50, 100 and 200 nanometers—of all six types began to pull in moisture and form clouds.

“Samples entering the cloud chamber are exposed to moist conditions representative of the relative humidity found in the atmosphere,” Brooks says. “If a sample is an effective cloud activator, droplets will rapidly grow on the sample fragments, forming large cloud droplets.”

Cloud droplets are 10 times larger than the pollen fragments, and the researchers used an optical particle counter to count them as they exited the cloud chamber.

For confirmation, they looked at the samples under a scanning electron microscope. They saw that grains that had begun as around 20-50 micrometers in size had been reduced to the nanometer size range—well within the size that can lead to cloud formation.

Climate change and allergies

The findings inform climate science and public health.

“What happens in clouds is one of the big uncertainties in climate models right now,” Steiner says. “One of the things we’re trying to understand is how do natural aerosols influence cloud cover and precipitation under present day and future climate.”

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And the allergy community might be interested in knowing what the particles are made of. When they ruptured, the researchers determined that they’re mostly carbohydrates and proteins.

As next steps, they plan to conduct similar studies in the field and, through computer simulations, model the potential feedback between the plant life and the atmosphere.

“It’s possible,” Steiner says, “that when trees emit pollen, that makes clouds, which in turn makes rain and that feeds back into the trees and can influence the whole growth cycle of the plant.”

A paper on the findings is available online in Geophysical Research Letters. The National Science Foundation supported the research.

Source: University of Michigan

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