Rain Falling on Farm Land Flings up Millions of Microscopic Organic Particles

May 5, 2016 | Joanne Kennell

Field of grain under dark storm clouds
Photo credit: James Watkins/flickr (CC BY 2.0)

Promoting even more rain.

Rain, whether you like it or not, is a vital and natural process on the planet. It nourishes planets, replenishes important aquifers, and can even act as a backdrop for some pretty romantic moments. Rain also cleans the air of soot, sulfates, and organic particles.

Or so we thought.

According to new research, published in the journal Nature Geoscience, in regions where there are ploughed fields, rainfall can fling up millions of microscopic organic particles — the remains of dead plants and animals. This not only affects air quality, but may help seed clouds and generate even more rain.

It is assumed that most particles are lifted into the air by either the wind, sea spray, or human activities, such as emissions from transportation and power plants. But last year, by filming artificial rain in a lab setting, scientists were able to show that rainfall could stir up particles from the soil. And until now, no one had evidence to prove it happens in the real world.

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Alexander Laskin, from the Pacific Northwest National Laboratory in Richland, Washington, and colleagues traveled to Oklahoma’s Southern Great Plains — a massive agricultural region — back in 2014 to collect airborne particles. After analyses using high-resolution microscopic techniques, they found that tiny, 0.5-micrometer-wide spherical particles containing carbon, oxygen, and nitrogen made up between one-third to two-thirds of all the airborne dust.

Rainfall events occurred prior to each sampling session, supporting the idea that the rain, rather than the wind, flung these organic particles into the air. Why? During each session, the wind was coming from a different direction. Different masses of air contain distinct properties, so a common occurrence in the particles suggests that they came from the soil itself.

You may be wondering how falling rain lifts particles into the air. It has to do with puddles. Once rainfall starts puddling, it dissolves organic matter from the soil. “Splashing of subsequent raindrops creates air bubbles, which rise upwards and burst, ejecting a fine mist of organic matter, which then dries into tiny solid spherical balls,” explained Laskin to New Scientist.

However, this process works most effectively for light or moderate rain. If the rain is too heavy, it hits the puddle too hard and doesn’t produce as many air bubbles.

Many studies have pointed out the importance of organic particles in helping clouds to rain. They promote the freezing of cloud condensation nuclei — small particles on which water vapor condenses — and speed up the growth of rain droplets. And according to data from southern Australia, over agricultural land, the probability of rainfall increases following a rainstorm — meaning rain can spawn more rain.

Laskin even found that the same kind of organic particles became airborne after watering with a simple garden hose. “It is likely that cropland irrigation is helping to release more organic particles from soil, and potentially enhancing rainfall in irrigated regions,” he said.

“This type of particle is not considered in things like climate models, and yet in some places they could have a significant effect,” explained Laskin. Clouds are an important part of the climate system, yet they are one of the most difficult things to model. Understanding all of the processes that affect how they form is useful for climatologists and will ultimately lead to improved climate simulations.

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