Pollen Grains Have Newfound Role: Seeding Rain Clouds

Mixed pollen  grains
Pollen grains under the electron microscope reveal intricate shapes. A new study shows that when each of these grains disintegrates into hundreds of smaller pieces, the fragments can capture water droplets and seed clouds. This entire field of view is about half a millimeter across. (Dartmouth College Electron Microscope Facility/Wikipedia)

Allison Steiner, a researcher at the University of Michigan, was sweeping her porch one day and pondering the abundance of dusty pollen. What does this fine-grained material do during the time it's in the air, she wondered, as any atmospheric scientist would. That train of thought led her to discover that pollen has an unexpected role in seeding clouds and possibly affecting the climate.

Clouds are centers of energetic activity, all kinds of it—lightning, precipitation and winds—powered by the exchange of heat between air and water. Melting ice and evaporating water absorb heat energy while condensation and freezing of water release it.

Clouds also reflect sunlight back into space to keep the ground cool, while at the same time blanketing the ground to keep it warm. In all their energetic, contradictory complexity, clouds are central actors in the world's climate.

But computer models of global climate stumble when they handle clouds. One problem is calculating how water droplets form in humid air. Water will condense out of air that is saturated in water vapor, but outside the laboratory, real droplets grow on seed particles of some sort. The droplets are extremely tiny, about 20 microns across, or 50 to a millimeter. Seed particles, called cloud condensation nuclei (CCNs), are a hundred times smaller, or 200 nanometers.

Pollen grains, being larger than that, always seemed like poor CCN candidates. But they're very common in the environment, as anyone with hay fever can tell you. Seed-bearing plant species, from the tiniest grasses to the tallest trees, release pollen from male plants to fertilize female plants.

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Pollen grains are about the same size as cloud droplets. They do attract water and act as CCNs, but atmospheric scientists never thought pollen was important for clouds because it settles out of the air in a matter of minutes to hours.

Looking deeper into the subject, Steiner learned that in water, or even humid air, pollen grains shatter into hundreds, or even thousands, of pieces. To see whether these "subpollen particles" are significant CCNs, she and five Texas A&M researchers soaked fresh pollen in water for an hour. They used pollen from several common tree species (including birch trees from Palo Alto), plus ragweed.

The team found that this treatment makes subpollen particles of the right size to be effective CCNs. Moreover, the protein, starch and carbohydrates of the subpollen particles are a rough match for the organic materials found in cloud droplets. (Rainwater is not especially pure, being full of living organisms from viruses to insects, not just pollen.) The results were published in the journal Geophysical Research Letters this week.

Steiner is not yet sure exactly how important pollen pieces are for the world's clouds, but her paper notes that in forested regions like much of the United States, pollen could account for millions of CCNs in each cubic meter of air. Whatever the exact number is, Steiner says pollen is part of "a new pathway for a coupled vegetation-climate feedback" -- one climate models will have to incorporate.

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The paper used pollen data from midwestern and eastern U.S. locations with abundant forests and grasslands. In the Bay area, our air comes fresh off the ocean, and the haze and fog of the familiar marine layer is affected by aerosols like sea-salt nanoparticles. As the sea breeze rises up our forested hills, tree pollen might be the trigger causing clouds to form on the ridge tops. And as the pollen breaks down in that moist setting, it may affect cloud formation farther downwind all the way to the Sierra Nevada.

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