According to a team of researchers led by Prof Allison Steiner of the University of Michigan, tiny pollen particles may make it rain.
Small pollen particles can hold condensation for cloud formation. Image credit: Brin Weins.
Pollen has been largely ignored by climatologists who study aerosols, particles suspended in the air that scatter light and heat and play a role in cloud formation.
“The grains were thought to be too large to be important in the climate system, too large to form clouds or interact with solar radiation. And also the large particles don’t last in the atmosphere. They tend to settle out relatively quickly,” said Prof Steiner, who is the lead author on the study published in the journal Geophysical Research Letters.
But Prof Steiner and co-authors weren’t sure that was the whole story.
“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,” Prof Steiner explained.
Smaller grains could have big implications. The 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,” Prof Steiner said.
In the lab experiments, the scientists tested pollen from oak, pecan, birch, cedar and pine trees, as well as ragweed.
They soaked 2 g from each source in pure water for an hour and used an atomizer to make a spray of the moist pollen fragments. They then sent the spray into a cloud-making chamber.
The scientists found that three different sizes of particles – 50, 100 and 200 nanometers – 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. If a sample is an effective cloud activator, droplets will rapidly grow on the sample fragments, forming large cloud droplets,” said co-author Prof Sarah Brooks of Texas A&M University.
Cloud droplets are ten times larger than the pollen fragments, and the team 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.
The team plans 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 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,” Prof Steiner said.
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Allison L. Steiner et al. Pollen as atmospheric cloud condensation nuclei. Geophysical Research Letters, published online May 04, 2015; doi: 10.1002/2015GL064060
