{"id":2119,"date":"2015-01-16T06:23:07","date_gmt":"2015-01-16T06:23:07","guid":{"rendered":"http:\/\/revoscience.com\/en\/?p=2119"},"modified":"2015-01-16T06:33:40","modified_gmt":"2015-01-16T06:33:40","slug":"rainfall-can-release-aerosols","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/rainfall-can-release-aerosols\/","title":{"rendered":"Rainfall can release aerosols"},"content":{"rendered":"

High-speed imaging captures raindrops releasing clouds of aerosols on impact.<\/strong><\/em><\/span><\/p>\n

\"Aerosol<\/a>
Aerosol generation after drop impingement on porous media is a three-step process, consisting of bubble formation, bubble growth, and bubble bursting.<\/figcaption><\/figure>\n

CAMBRIDGE, Mass. —\u00a0Ever 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.<\/span><\/p>\n

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.<\/span><\/p>\n

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.<\/span><\/p>\n

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.<\/span><\/p>\n

\u201cRain happens every day \u2014 it\u2019s raining now, somewhere in the world,\u201d says Cullen R. Buie, an assistant professor of mechanical engineering at MIT. \u201cIt\u2019s a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before.\u201d<\/span><\/p>\n

Youngsoo Joung, a postdoc in Buie\u2019s 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.\u00a0<\/span><\/p>\n

\u201cUntil now, people didn\u2019t know that aerosols could be generated from raindrops on soil,\u201d Joung says. \u201cThis 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.\u201d<\/span><\/p>\n

Buie and Joung have published their results this week in the journal\u00a0Nature Communications.<\/em><\/span><\/p>\n

Capturing a frenzy, in microseconds<\/strong><\/span><\/p>\n

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\u2019s campus and along the Charles River. He also collected sandy soil from Nahant Beach in Nahant, Massachusetts.<\/span><\/p>\n

In the lab, the researchers measured each soil sample\u2019s 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.<\/span><\/p>\n

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\u2019s release, the faster its ultimate speed.<\/span><\/p>\n

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 \u201cfrenzied aerosols\u201d may be dispersed.<\/span><\/p>\n

\u201cFrenzied means you can generate hundreds of aerosol droplets in a short time \u2014 a few microseconds,\u201d Joung explains. \u201cAnd we found you can control the speed of aerosol generation with different porous media and impact conditions.\u201d<\/span><\/p>\n

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.<\/span><\/p>\n

Buie says this mechanism may explain petrichor \u2014 a phenomenon first characterized by Australian scientists as the smell released after a light rain.<\/span><\/p>\n

\u201cThey 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,\u201d Buie says. \u201cInterestingly, they don\u2019t discuss the mechanism for how that smell gets into the air. One hypothesis we have is that that smell comes from this mechanism we\u2019ve discovered.\u201d<\/span><\/p>\n

From the ground up<\/strong><\/span><\/p>\n

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 P\u00e9clet number, which is used to contrast impact velocity and surface wettability.<\/span><\/p>\n

Based on their calculations, the researchers found that aerosol generation is greatest when the ratio between the Weber and P\u00e9clet numbers is balanced, around 1 \u2014 a ratio that Buie and Joung expressed as the Washburn-Reynolds number. When this ratio is balanced, \u00a0raindrops are neither too fast nor too slow, and the surface is neither too wet nor too dry.<\/span><\/p>\n

\u201cWhen moderate or light rain hits sandy or clay soils, you can observe lots of aerosols, because sandy clay has medium wetting properties,\u201d Joung says. \u201cHeavy rain [has a high] impact speed, which means there\u2019s not enough time to make bubbles inside the droplet.\u201d<\/span><\/p>\n

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 \u2014 a finding that suggests such aerosols may also carry other contaminants, such as soil-based viruses and bacteria.<\/span><\/p>\n

\u201cAerosols in the air certainly could be resulting from this phenomenon,\u201d Buie says. \u201cMaybe it\u2019s not rain, but just a sprinkler system that could lead to dispersal of contaminants in the soil, for perhaps a wider area than you\u2019d normally expect.\u201d<\/span><\/p>\n

Adds Joung: \u201cTo 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.\u201d<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

High-speed imaging captures raindrops releasing clouds of aerosols on impact. CAMBRIDGE, Mass. —\u00a0Ever 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 […]<\/p>\n","protected":false},"author":6,"featured_media":2120,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[15,17],"tags":[],"class_list":["post-2119","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-environment","category-research"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0-150x150.jpg",150,150,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0-300x200.jpg",300,200,true],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",600,400,false],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",600,400,false],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",540,360,false],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",95,63,false],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",639,426,false],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",96,64,false],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/01\/MIT-Frenzied-Aerosols-01_0.jpg",150,100,false]},"author_info":{"info":["Amrita Tuladhar"]},"category_info":"Environment<\/a> Research<\/a>","tag_info":"Research","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/2119","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=2119"}],"version-history":[{"count":0,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/2119\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/2120"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=2119"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=2119"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=2119"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}