Cool high speed video: Rainfall can release aerosols, study finds

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.

Aerosol generation after drop impingement on porous media is a three-step process, consisting of bubble formation, bubble growth, and bubble bursting. Image courtesy of Youngsoo Joung

Aerosol generation after drop impingement on porous media is a three-step process, consisting of bubble formation, bubble growth, and bubble bursting. Image courtesy of Youngsoo Joung

From the Massachusetts Institute of Technology:

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.”

###

Written by Jennifer Chu, MIT News Office

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95 thoughts on “Cool high speed video: Rainfall can release aerosols, study finds

    • Damn ! my eyes must be really quick.
      I’d swear, that I have actually seen raindrops splash on the ground and on other surfaces as well; and all without the benefit of high speed flash photography, just by bare unaided eyeballs.
      I have no idea what all kinds of crud; animal, vegetable and mineral, that splashes up with all that, but I do smell stuff, and often ozone as well.
      Can I get a grant to buy a high speed flash camera to check some of this out ??

      • The point made Pedro is probably relevant. In the northern hemisphere most flus jump from animal contact initial. Birds, often migratory birds including ducks and geese are a common source. The priniciple migratory periods are the fall and spring. Soils are driest in the fall and saturated in the spring. It would follow that the easiest time during the year for transfer via aerosol as shown in the video is during the fall. Once the jump is made, then the transfer is primarily person to person.

  1. Hmm… no models.. per-chance to keep an eye on ..and no I am not against models ..just alot more history needs to be compiled
    michael

  2. When big tropical raindrops fall onto a dry dusty surface such as open, bare ground they hit with such force they kick up dust. At the beginning of the storm, the air is noticeably dusty as a result of this.

    • Yeah, plenty of existing observational evidence made the basic claim a commonplace before the study was conducted. Coming up with the math to describe it would be an important contribution. But then, how do we use this information, eliminate rain?

      • OK, kidding. Potentially a researcher could discover which soil conditions would give rise to more, or more harmful, aerosols.

      • eliminate rain?

        No, but it may help to design or choose a better irrigation system depending on the circumstances (like sprinkler vs. drip irrigation).

    • I spent a week in Salvador Brasil which has red soil. There was a day of heavy rain and my white tropical hat had become red in the seams and stitching.

  3. When the ground gets wet, no more bubbles, no more aerosols, and how long do the aerosols remain in a rainstorm?

    • Very interesting, and a great example of good observational science. It would be interesting to have heard more about the effect on the various surfaces such as tarmac or concrete – do these produce more or less aerosols compared to soil? I know the focus was on picking up dirt and viruses etc for the study of how diesease occurs.
      But, given that urbanisation has had a great effect on run-off and therefore flooding, is there perhaps an effect on the production of aerosols and therefore cloud formation and climate? Is there an equivalent to the UHI for aerosols?

      • Yes and maybe compare the aerosol production from a light rain versus 18 each 42″ tires throwing up a mist of air. I know my windshield picks up a bunch of aerosols when one passes me after a rain.

    • Agreed. I assumed that was a known element for producing aerosols. Can I now assume that no one has observed the aerosols produced by waves crashing both onshore and offshore?? Can I get grant money to investigate…?
      Why didn’t I go into climate research, the money is so plentiful… Instead I opted for neuro/physio research and it isn’t the grant cake walk climate seems to be.

    • When I was a kid—almost 40 years ago—my grandma would tell me to stay indoors after light/moderate showers because the exhalation of the earth after the rain was bad for me and would make me sick. Guess it wasn’t just superstition or old wives’ tales, huh.

    • The principles for this phenomenon have been known for some time, although this may be a first for discovering it in falling raindrops hitting a porous surface. See http://onlinelibrary.wiley.com/store/10.1111/j.2153-3490.1954.tb01085.x/asset/j.2153-3490.1954.tb01085.x.pdf;jsessionid=1239DB565FFB56B571A904CE2FA62D8A.f03t04?v=1&t=i4xopvhe&s=e0ed661d7a787be3ca141cfc818f1def253fd6c4
      One of the authors of the linked paper, Doug Blanchard, continued with studies on aerosols from 1954 well into the late ’80s, maybe early 90’s. He published a few papers on the aerosols generated by waves folding over into the ocean and entrapping air which was then released into tiny air bubbles. The air-liquid interface of a rising bubble makes for a good scavenging surface, especially for hydrophobic constituents, like certain bacteria, viruses, and even mycoplasma. There has to be enough contact time (bubble rise time through the water column) to collect enough of the material of concern to be significant. This paper shows that even in the volume of a raindrop an aerosol can be generated but the contact time with the soil or a shallow puddle is certainly limited so I would question how significant of a contribution these aerosols may be in transmitting pathogens. It is an interesting phenomenon; the rising bubble reaches the surface and bursts. This sends waves of energy down into the spherical cavity of the bubble where they all meet at the bottom. The energy has no where to go but up and this sends a little geyser upward where the tip of the water spout becomes unstable and breaks off into 1 or 2 jet droplets. These are catapulted into the air where a slight breeze can make them airborne. Think of a rising bubble collecting particles (molecules, bacteria, viruses) and as it rises; these collected particles are pushed to the bottom of the bubble where all the energy from the burst culminates to form the geyser and jet droplets. For this reason the jet droplets can carry a many fold increase in concentration of the particle as compared to an equal volume of the water it came from. The aerosols generated from bursting bubbles in a river downstream of a waste treatment facility can have higher counts of mycoplasma.

      • I don’t think the link worked. Search for Kientzler, C. F., et.al. (1954) “Photographic Investigation of the Projection of Droplets by Bubbles Bursting at a Water Surface.” Tellus 6:1, pp. 1-7.

  4. Have these kids never actually eaten dirt ?
    It builds up the immune system.
    Maybe it just gives you more grit, for those times one might need grit.
    Can’t think of any grit lacking times, but it sure tasted like dirt.

      • Ok, but when it comes down to bacteria vs virus, in a host, .. which wins ?
        Just playing, like when I was a kid.

    • The Old proverb is “eat a peck of dirt before you die” – only recently I realized a peck is a volume round abouts a gallon….

  5. That earthy smell also is produced when dew forms in the evening–and the air can be completely still.

  6. Nothing new to me, or anyone who live in the Sonoran Desert.
    Here in the Sonoran desert of Southern Arizona, the desert smell after a rain shower is unique.
    It is the smell of the creosote bushes getting wet and solubilizing the lighter aromatic fractions. It is a wonderful fresh smell. Unique to the Sonoran desert.

    Larrea tridentate’s common name, creosote bush, often makes people think it must smell like the petroleum-based product that is used as a wood preservative. Those who know the Mojave, Sonoran, and/or Chihuahuan Deserts, however, know that this is an unfortunate mis-association. The smell of creosote is quite strong and distinctive, but it is what gives our desert a characteristic refreshing smell after rains.
    Source: http://ag.arizona.edu/maricopa/garden/html/pubs/1104/sonoran.html

    • Ah, Joel, I was looking for a response like this before replying. My mother, who was raised in the CA deserts (Desert Center) used to tell us about the smell of ‘greasewood’ as she said, which occurred after a rain. It would have a distinct aroma you say.

    • Creosote is also know as chaparral. I loved the smell of chaparral and sage after a rainstorm in the Southern Utah desert where I grew up. It rarely rains there, so I have always wished I could bottle that smell.

  7. http://media-1.web.britannica.com/eb-media/55/155-004-1096806E.jpg
    Edgerton at MIT did some really really cool high speed strobe stuff back in the day – trick was to match that shutter with the bullet going through the light bulb.
    Glad to see the Media Lab back in action, but watch out for the nasty E. Coli, and sorry to see disease and pestilence as well as climate strange is required for grant money.
    Now, with images, we can upscale this information to model flu progress across this season . . .

    • “Papa Flash” did some very cool stuff. I had a teacher that was at MIT when he was and says he actually covered the underside wing surface of a Douglas DC-3 with strobe tubes and took flash nighttime aerial photos of Cambridge. Flying lightning. I would have loved to see that. The best one I ever did was a still of a rock hammer going through a light bulb at 1/64,000’s of a second with a 4″x5″ view camera. There were still blurred shards of glass from velocity. The timing was the hard part… lots of busted light bulbs.

      • I was fortunate to meet the man himself when I took his high-speed photography course one summer. We did the cut the playing card in half trick. The secret to all the frozen bullet images was a microphone used to trigger the camera. We varied the point where the bullet wound up in the photo by moving the mic closer to or further from the muzzle.

  8. The Apollo astronauts reported that lunar dust off their suits in the lunar module, smelled like arc welding and hot burning brakes. The steady solar wind charged particles and UV strip electrons off the surface soil dust, giving it that arc welding “charged” smell.

    • Nah, that smell was leftover from the movie special effects and scenic crew’s welders and trucks from when they built that set they faked all those landings on. The later “mission” astronauts noticed the smell of moldy pizza, old coffee and stale beer from the production crews. (Do I need the </sarc> tag?)

  9. Thank you Buie and Joung, one more little piece fits nicely into Pete’s jigsaw – the one that’s putting together a picture of how ‘The Farmers Did It’ and also showing up Western White Man’s dumbness.
    Pete’s theory simply says that says water controls the climate, water enable plants to live and dead plants (in the soil) control the water. Dead plants enable and feed live plants and it all creates a nice climate which we have evolved to fit into. Nice feedback and why plants ALWAYS do their damndest to cover bare soil whenever and wherever it occurs. That’s why annual plants evolved, to grow quickly and cover any bare patches that naturally occur. They are nature’s sticky plaster yet we rely on them for what are referred to as ‘staples’
    Bare soil should NEVER – (no compromise here, never ever ever) be visible, hence should NEVER be getting hit directly by falling rain, unless its quite dead as in a desert and there are no plants to shelter it.
    Do Buie and Joung realise that – native Americans certainly did. Which of the many tribes were quoted as saying they’d sooner take a knife to their mother’s breast than put a plow into the ground?
    But of course, the settlers, pioneers and John Deere knew better and the natives were best herded into reserves. This cannot end well, not in the long term.

    • Not questioning that it won’t end well for the Native Americans, in the long term.
      But ploughing?
      That’s been going on for a while now. When does the long-term start?

    • “Which of the many tribes were quoted as saying they’d sooner take a knife to their mother’s breast than put a plow into the ground?”
      What percent of the tribes? Were they the more technologically advanced tribes? Did they try farming and wisely decide to discontinue the abhorrent process? Were they known for extreme rhetoric? Were they plastic surgeons? This is fascinating.

  10. Pet’ri’chor (n) a smell that frequently accompanies the first rain after a long period of warm, dry weather.
    ————
    Learnt a new word – ty WUWT!
    Great etymology too!:
    petros, meaning ‘stone’ + ichor, the fluid that flows in the veins of the gods in Greek mythology.

  11. The same earthy smell occurs just before a thunderstorm. The mechanism, I have read, is the approaching low pressure system sucks up gases from the soil into the air,

  12. After watching the high-speed vid (and it was pretty cool), youtube recommended “What causes the smell after rain” by How Stuff Works. It had a more informative explanation for “pretrichor”:

    “One of the more pleasant rain smells is actually caused by bacteria. Yep. A type of bacteria called actinomycetes grow in soil when conditions are damp and wet. When the soil dries this bacteria produces spores, and the wetness and force of rain kicks these tiny spores into the air. So we breathe them in and we sense the distinctive earthy scent so often associate with everything from a drizzle to a storm. Because the spores release when the soil dries out, the scent is strongest after a rain that breaks a dry spell.
    Now this bacteria is extremely common, and people around the world will recognize the aroma. I just want you to know that you are snorting bacteria when you smell that petrichor.”

    • I was a Soils and Crops major in college and learned this. I always tell people after a rainfall – “ah, smell the actinomycetes.” I find it amusing over the years how people in other fields of study are always “discovering” things that are general knowledge to anyone trained in agronomy.

  13. I am a little confused by the description of the smell after a rain shower. I have noticed and commented on the strong dry smell that is strongest as the beginning of the rain while the ground is still mostly dry. This fades as the rain continues. The smell after an extended rain shower is much different and not as strong. There is more physics going on as the surface becomes wet.
    I suggest an experiment to capture particulate on filters would should quite a change as a rain shower starts and progresses. I suspect much large particles are driven into the air at the beginning of the rain and this may be different than the air bubble explanation.

    • There are a few smells that accompany a storm. 1) A bacterial secretion from the dirt after a rain. 2) Plant secretions during dry periods (palmitic and stearic acid) are released when oxidized, and 3) the formation of o3 (ozone) which will precede a significant storm due to high electrical charge in the atmosphere. The strong smell is the ozone – and is worse with more significant storms.
      Its interesting to me this was even done – Geosmin itself is an aerosol that is created when the rain hits – so I guess I need to read this study more in depth to understand why this study is novel – at first glance it seems to be a picture of what is already understood…

  14. About 15 years ago, I took a course at work, related to this subject. I forgot a lot of the subject matter due to non use (and I’m retired now). The unique smell, when it first starts raining, is caused by “fugacity” (the relative tendencies of molecules to flee various situations). At boundaries where different substances meet (such as air and water, water and sediment, sediment and air, etc.) a lot of pretty interesting things go on very close to the interface. If anyone is interesting in following up on this subject, read “Environmental Organic Chemistry”, Schwarzenbach, Gschwebd, and Imboden, 1993.

  15. 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.

  16. 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!

  17. 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.

  18. “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.

  19. 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.

  20. The questions are: “How does rain form?” “How do cloud form?” This is great research, but it doesn’t explain “rain.”

  21. “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?”

  22. 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!

  23. 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!

  24. 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.

  25. 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.

    • 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

  26. 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.

  27. 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.

  28. I recall in the midst of a terrible drought that when it finally did rain you could see the dust fly up.

  29. This may be one reason my wife’s allergies are no better after a light rain, and sometimes worse.

  30. 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.

  31. 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.

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