The Sweet Smell of Rain
Most of you are probably familiar with the earthy smell that spring rains often release into the air, dubbed "petrichor" by Australian scientists in a 1964 Nature paper. At the time, it was thought that the smell came from accumulated oils in the soil that plants had secreted during arid conditions. When it rained, those oils were released. Recent research out of MIT and published in Nature Communications gives a new understanding on how those oils and other soil based compounds are released into the atmosphere. Using slow motion filming techniques, they discovered that when a raindrop falls on a porous surface it traps air bubbles underneath. These air bubbles shoot up through the water droplet, releasing aerosols that can be blown by the wind and spread. The paper suggests that these air bubbles not only carry the smell, but can also carry bacteria, such as E. Coli and other soil based compounds.
They started by testing the permeability of 28 types of surfaces (12 engineered materials and 16 soil samples) by timing how long it took for water to seep through long tubes filled with each material. Then, they made it rain; they tested rain at different intensities from varying heights and captured the results on film.
It has been known for some time that raindrops create aerosols when they fall on water, but this is the first time that this effect has been discovered on soil. The research also showed that the aerosol effect is greater when the rain is light, perhaps because there is more time for aerosols to form and float.
This is pretty exciting news; this particular mechanism might help explain why soil based compounds are often found floating high up in the atmosphere.
For video of the aerosols in action, visit: http://www.sciencealert.com/watch-raindrops-releasing-aerosols-during-slow-motion-impact
For further reading on the paper published in Nature in 1964 on plant oils, visit: https://www.nature.com/articles/201993a0
For the new research from MIT, visit:http://www.nature.com/ncomms/2015/150114/ncomms7083/full/ncomms7083.html
Picture Credit: www.tOrange.us
-Colter