High-speed imaging captures raindrops releasing clouds of aerosols on impact, showing once again that we just don’t know all the sources for aerosols and other climate forcings.
Ever notice an earthy smell in the air after a light rain? Now scientists at MIT believe they may have identified the mechanism that releases this aroma, as well as other aerosols, into the environment.
Using high-speed cameras, the researchers observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact. As in a glass of champagne, the bubbles then shoot upward, ultimately bursting from the drop in a fizz of aerosols.
The team was also able to predict the amount of aerosols released, based on the velocity of the raindrop and the permeability of the contact surface.
The researchers suspect that in natural environments, aerosols may carry aromatic elements, along with bacteria and viruses stored in soil. These aerosols may be released during light or moderate rainfall, and then spread via gusts of wind.
“Rain happens every day — it’s raining now, somewhere in the world,” says Cullen R. Buie, an assistant professor of mechanical engineering at MIT. “It’s a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before.”
Youngsoo Joung, a postdoc in Buie’s lab, adds that now that the group has identified a mechanism for raindrop-induced aerosol generation, the results may help to explain how certain soil-based diseases spread.
“Until now, people didn’t know that aerosols could be generated from raindrops on soil,” Joung says. “This finding should be a good reference for future work, illuminating microbes and chemicals existing inside soil and other natural materials, and how they can be delivered in the environment, and possibly to humans.”
Buie and Joung have published their results this week in the journal Nature Communications.
Capturing a frenzy, in microseconds
Buie and Joung conducted roughly 600 experiments on 28 types of surfaces: 12 engineered materials and 16 soil samples. In addition to acquiring commercial soils, Joung sampled soil from around MIT’s campus and along the Charles River. He also collected sandy soil from Nahant Beach in Nahant, Massachusetts.
In the lab, the researchers measured each soil sample’s permeability by first pouring the material into long tubes, then adding water to the bottom of each tube and measuring how fast the water rose through the soil. The faster this capillary rise, the more permeable the soil.
In separate experiments, the team deposited single drops of water on each surface, simulating various intensities of rainfall by adjusting the height from which the drops were released. The higher the droplet’s release, the faster its ultimate speed.
Joung and Buie set up a system of high-speed cameras to capture raindrops on impact. The images they produced revealed a mechanism that had not previously been detected: As a raindrop hits a surface, it starts to flatten; simultaneously, tiny bubbles rise up from the surface, and through the droplet, before bursting out into the air. Depending on the speed of the droplet, and the properties of the surface, a cloud of “frenzied aerosols” may be dispersed.
“Frenzied means you can generate hundreds of aerosol droplets in a short time — a few microseconds,” Joung explains. “And we found you can control the speed of aerosol generation with different porous media and impact conditions.”
From their experiments, the team observed that more aerosols were produced in light and moderate rain, while far fewer aerosols were released during heavy rain.
Buie says this mechanism may explain petrichor — a phenomenon first characterized by Australian scientists as the smell released after a light rain.
“They talked about oils emitted by plants, and certain chemicals from bacteria, that lead to this smell you get after a rain following a long dry spell,” Buie says. “Interestingly, they don’t discuss the mechanism for how that smell gets into the air. One hypothesis we have is that that smell comes from this mechanism we’ve discovered.”
From the ground up
Buie and Joung looked further into the relationship among raindrop velocity, surface properties, and aerosol generation, and came up with two dimensionless parameters that can be used to describe the relationship: the Weber number, which is a function of the impact speed of a droplet, and a modified Péclet number, which is used to contrast impact velocity and surface wettability.
Based on their calculations, the researchers found that aerosol generation is greatest when the ratio between the Weber and Péclet numbers is balanced, around 1 — a ratio that Buie and Joung expressed as the Washburn-Reynolds number. When this ratio is balanced, raindrops are neither too fast nor too slow, and the surface is neither too wet nor too dry.
“When moderate or light rain hits sandy or clay soils, you can observe lots of aerosols, because sandy clay has medium wetting properties,” Joung says. “Heavy rain [has a high] impact speed, which means there’s not enough time to make bubbles inside the droplet.”
Joung and graduate student Zhifei Ge are now conducting similar experiments, with surfaces containing soil bacteria and pathogens such as E. coli, to observe whether such contaminants can be spread significantly via rainfall. In the current paper, he and Buie performed initial experiments using dyed liquid droplets on certain surfaces containing fluorescent dye. In those experiments, they observed through microscopy that the aerosols released from raindrops contained the dye — a finding that suggests such aerosols may also carry other contaminants, such as soil-based viruses and bacteria.
“Aerosols in the air certainly could be resulting from this phenomenon,” Buie says. “Maybe it’s not rain, but just a sprinkler system that could lead to dispersal of contaminants in the soil, for perhaps a wider area than you’d normally expect.”
Adds Joung: “To prevent transmission of microorganisms from nature to humans, we need to know the exact mechanism. In this work, we provide one possible way of transmission.”
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Written by Jennifer Chu, MIT News Office
It’s not just an ‘earthy’ smell, but here in Australia’s North, the first rains also smell smoky, due to lightning caused fires prior to the wet season. Chemicals in the smoke are found to start off a germination process in many indigenous seeds, especially from desert regions. I love that smell.
Maybe the answer as to how the Spanish Flu spread to all corners of the earth in the early part of the 20th century. Read one account where the scientist actually thought the Flu was from space, as it penetrated areas of the north so completely that rarely had seen flu’s before.
IO love science!
“Spanish” flu was so called because that was the country during WWI most allowed to report it. Could have originated from France,China or even Kansas. The military was mostly responsible for the rapid and wide spread of this influenza.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2862337/
http://en.wikipedia.org/wiki/1918_flu_pandemic
If interested, a gritty description of how it was to live in the Spanish flu era is John Barry’s “The Great Influenza”. Some large American cities, such as Philadelphia, had fatality rates bordering on those from the Black Plague in the middle ages with about as much care available.
How pathetic MIT has become. Perhaps they will announce the invention of the wheel next. This is not science worthy of a University but more suited to a Grade 6 Science fair. There was a time MIT was synonymous with the highest academic standards – sadly those days are long gone.
It would is interesting though.
“Until now, people didn’t know that aerosols could be generated from raindrops on soil,” Joung says.
Joung is an idiot… and never was an observant farm boy! Any kid that grew up on a farm knows from first hand observation that each rain drop hitting dry, dusty soils causes ejecta and aerosoling of powdered soil from the little impact craters.
From the “coulda told them that years ago” department. Many years ago the swedish equivalent of the EPA had plans for a general ban against any and all organic aersols above a certain level. However somebody took a measurement in a pine forest after a summer rain and ruined their plans. They would have had to put every forest in the country off-limits.
The questions are: “How does rain form?” “How do cloud form?” This is great research, but it doesn’t explain “rain.”
“We need 1M dollars to photograph raindrops at high speed”
“No way!l
“We need 1M dollars to photograph raindrops at high speed to investigate arerosols and climate change”
“Sure but are you sure 1M is enough?”
Rain drops cause the release of aerosols when impacting the ground.
Aerosols seed clouds causing rain.
This is a positive feedback loop. (Of course it is worse than we though).
If this is allowed to continue for as little as forty days and forty nights, it would flood out the whole earth!
The most deadly thing in a modern home is the low-flow showerhead. One nearly killed me in 2007 when it plus some mucus gave me a near fatal asthma attack. I spend about $300.00 a month on medicine to keep my lungs from killing me now. Ban low-flow shower heads, they cause global warming, extreme weather and spread disease!
Interesting research, but my initial thought was that it would concern the release of the aerosol nuclei around which rain drops are formed in the atmosphere. What happens to them upon impact? One anecdotal observation: Frequently, down-wind from thunderstorms that form in the dusty High Plains of the U.S., it “rains mud”. Even after relatively heavy rain from such fast moving storms, it is not uncommon to need to wash cars.
Reblogged this on the WeatherAction News Blog and commented:
Interesting but not surprising as I believe the same mechanism is used by many fungi to attack my plants!
Yep, if you have plants outdoors you have seen all sorts of detritus kicked up by rain storms.
Pine trees release aerosols which cause rain, I also imagine their needles and branches when dropped load up the ground with aerosols which also release when rain drops hit the ground. It appears pine trees are basically rain machines.
They also release clouds of pollen in the spring, I’m sure there are better videos:
In the free movie on line ‘What in The World Are They Spraying’ and ‘Why in The World Are They Spraying’ and ‘Who in The World is Spraying’ detail and outline the reason weather is totally messed up and the boreal forests, especially pine trees in the West are weakened for the pine beetle to invade and kill the trees. All forest life is effected. There are microplasms put into the spray which cause white nose syndrome and same bacteria causing the death of dolphins, other sea life, making people very sick. Scientists CANNOT understand how in the world the bacteria can thrive in colder climes (from South A.)
and how it can reach dolphins! HOW? It’s man’s devilish devices of war and control. AKA: drought in CA etcetcetcetc etc
Experiment time everyone. Open a room temperature cola, pour it quickly into a cup sitting on a white paper towel. Pour fast enough for the bubbles to come close to the top, but not overflow. Let the bubbles subside. Observe the ring of “brown/tan” around the cup, very noticeable when you remove the cup. Ta-da! Same basic experiment, you can see the little water droplets flying all around. No high-speed cameras needed.
You’d be led to believe that these MIT guys discovered the question to the answer of 42.
What happens to all those condensation nuclei in rain?
Thanks, Anthony.
WUWT is a learning resource.
petrochor – the odor produced from the first drops of rain on an asphalt road.
I always have noticed that on watering (with a spray) my tomato plants I immediately smell their scent. I had always assumed that it was due to the plants opening their leaf stomatal pores, but perhaps it is due to the process described above.
I recall in the midst of a terrible drought that when it finally did rain you could see the dust fly up.
This may be one reason my wife’s allergies are no better after a light rain, and sometimes worse.
Maybe people should lay off these guys. We all know about the dust etc kicked up by rain but it’s usually “rained out” quickly, most things are.
As has been said it may be useful WRT to irrigation but at the very least it’s one more piece in understanding the great jigsaw puzzle nature has given us.
There is also the fact that the smell involved isn’t after rain. Anyone living in dry country knows the rain is coming because you can smell it sometimes hours ahead. This has implications for the covering of dangerous waste which instead of a nice smell could generate a cloud of toxic aerosols. Remember a usual practice is to water down the topsoil to prevent dust, are there implications for that practice?
Research that increases our understanding of the natural world is good research, even if we can’t see an immediate benefit in it.
Very interesting. Finding out the mechanism of something is fundamental in understanding other things in more detail….. As for Petrichor, I found that the Western Australian Jarrah forest bushland had some of the most fragrant scents after a light rain….. It’s a spicy slightly peppery smell…. Really nice.
[HAARP, chemical droppings, and the other subjects lists are not admitted into the public discussion here. .mod]
So much for “the science is settled”!!!