Smoke's Surprising Secret

When crops, rangeland, and managed forests come down with diseases, farmers and ranchers all over the world sometimes choose to burn the diseased fields in an attempt to sterilize the area, giving little thought to the smoke that can spread hundreds of miles away. A high school student working on a back yard science project now has pretty clear evidence that such fires may not kill everything we thought. In samples of the smoke from massive fires in Central America that reached Texas in the spring of 2002, she found living fungal spores that were perfectly able to start new colonies. This surprising discovery from a young, amateur scientist has the potential to change the prevailing wisdom on the benefits of burning diseased crops or timber.

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  Photograph of Sarah Mims

Rainbows of Dust

When Sarah Mims discovered that smoke drifting across the Gulf of Mexico from fires in Central America was bringing fungal spores to the air around her home Seguin, Texas, she was definitely surprised. When Mims set out microscope slides and a continuous-operation air filter on a deer stand in her family's yard, she expected to get a collection of dust samples that she could check for the presence of fungus and bacteria.


Sarah Mims, a teenager from Seguin, Texas, is studying the smoke and dust that arrive over her hometown from thousands of miles away. By her senior year in high school, she had already made some surprising discoveries. (Photograph by Forrest M. Mims III)


“I was initially just planning to follow up on my project from the previous summer,” says Mims. In 2001, when she was in the 10th grade, Mims had decided to study the aerosols in her hometown. When most people think of aerosols, they think of hairspray or deodorant. But aerosols can be any small liquid or solid particle suspended in the air, including sea salt, volcanic ash, smoke, and dust. Mims had read that dust from the Sahara Desert, more than 4,000 kilometers away in North Africa, could be blown all the way over the Atlantic Ocean to North and South America. In the summer of 2001 numerous dust events in Florida and Texas created colorful sunsets and layered a thin blanket of dust on car windshields. Of course, Texas itself can be a dusty place. “I wanted to find out whether the dust was local, or whether some of it could be coming from the Sahara,” says Mims.

  Photograph of Red Sunset

Spectacular sunsets are caused by small particles in the air called aerosols. In the summer of 2001, dust blown from the Sahara Desert frequently contributed to brilliantly colored evening skies over Texas. (Photograph by Sarah A. Mims)


Mims analyzed the particles caught on her microscope slides using a cross-polarized light source that makes dust particles made out of quartz—a characteristic of Saharan dust—glitter with all the colors of the rainbow. Since the local dust, from a kind of soil called caliche that is common in South Texas, doesn’t change color under polarized light, Mims knew that the sparkling, shimmering dust she had collected wasn’t from Texas. Her description of the dust particles carries a sense of wonder and appreciation for nature’s magic that often seems missing from the world of grown-up, professional science. “They were really pretty,” she says, “like rainbows.” Mims also measured the size of the particles and took pictures of them, which she compared to pictures of Saharan Dust particles that she found in books. “They looked just alike,” she said.

  Micrograph of Dust Particles

Using a microscope Mims found that the particles she captured in Texas matched published images of Saharan dust. (Micrograph by Sarah A. Mims)

  Model Data of Aersols July 26, 2001  

Dust, smoke, and other aerosols move all around the world. This map of aerosol optical depth (calculated by computer models) shows the dust storms in the Sahara Desert that brought dust all the way to the United States on July 26, 2001. Click on the image to see the movements of aerosols in June, July, and August of 2001.

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(Image and animation by Robert Simmon, based on GOCART Data)


As her final piece of evidence, Mims used satellite observations of aerosols from the Sea-viewing Wide Field-of-View Sensor (SeaWiFS), the Total Ozone Mapping Spectrometer (TOMS), and the Advanced Very High Resolution Radiometer (AVHRR), combined with models of the atmosphere from the National Oceanic and Atmospheric Administration that showed the “back trajectory” of the air that was sitting over Texas on the days her samples and the satellite images were captured—in other words, where the air had been before it got to Texas. On the days when her filters were dirtiest and her microscope slides were covered with the most particles, the satellite images showed lots of aerosols over the region, and the back trajectory maps showed the air was coming from North Africa. Putting all these pieces together, says Mims, “It was obvious that the dust was Saharan Dust.”

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The data used in this study are available in one or more of NASA's Earth Science Data Centers.

  Back Trace of Air Parcel

Mims used back traces—specialized computer models—to track where the air over Seguin, Texas, came from on the days she gathered samples. On July 26, 2001, the air over her home had arrived from the Gulf of Mexico—the classic track of air currents arriving from Africa. Red triangles indicate the position of the air every 6 hours, to a maximum of 72 hours. (Image courtesy NOAA Air Resources Laboratory)


A Smoky Surprise


Since disease-causing bacteria and fungi have been known to hitch a ride across the Atlantic on Saharan Dust particles (they have probably played a role in the decline of Caribbean coral reefs over the past two decades), Mims decided to see whether she might be able to find bacteria or fungal spores along with dust from Asian deserts that sometimes reaches Texas in the spring. In the spring of 2002, she set out her microscope slides along with Petrifilms—strips of gel and nutrients that can be used to grow microorganisms like bacteria and mold.

“The strips come dry, and you have to squirt them with water before you expose them to the air. Once you set them out in the air for 15 minutes, you cover them up with a clear sheet of plastic, and after a few days, if you caught anything, you’ll see stuff starting to grow,” says Mims.

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  Photographs of Petrifilm Samples  

Sarah Mims exposed Petrifilms outside of her Texas home in April and May of 2002, expecting to find micro-organisms carried by Asian dust. Instead, colonies of bacteria and mold (right) grew on days when the air was filled with smoke, not dust. (Digital Scans by Sarah A. Mims)


The technique was a success. Within a few days, Mims’ gel strips were practically a jungle of different colored splotches, each one a separate fungal colony. Large numbers of spores were also visible on the microscope slides she had exposed to the air almost every day between April 25 and May 17. There was just one problem, at least from her “Asian-dust-carrying-microbes” hypothesis: the slides didn’t show any Asian dust!

“There were a lot of fungal spores that I could identify on the slides,” says Mims,“even at the beginning of the period. But there really wasn’t much dust.” What she did see on the slides was a lot of black carbon particles, in other words, soot. To help solve the puzzle, Mims turned to satellite images. Observations from the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) during that time showed huge plumes of smoke from forest fires in southern Mexico and Central America crossing the Gulf of Mexico and blowing over Texas. Could the Central American smoke be bringing the fungal spores with it?

  Micrograph of Carbon and Fungal Spores Under the microscope Mims found fungal spores along with chunks of black carbon—individual particles of smoke. Once she knew the micro-organisms were associated with smoke, not dust, she needed to find their source. (Micrograph by Sarah A. Mims)
  MODIS Image of Smoke: May 2, 2002

“I counted the number of spores and the number of carbon particles on the slides,” explains Mims. “If the spores were coming from fungus that lived in our area, then there shouldn’t be any relationship between the amount of black carbon soot and the number of fungal spores.”

Instead what Mims found was that when the number or carbon particles went up, the number of certain kinds of spores—but not all of them—went up, too. This is exactly what you would expect to see if some of the spores came from local sources and some came along with the smoke. It certainly seemed that the airborne fungus was related to the Central American smoke.


Satellite images from the days Mims conducted her experiments revealed smoke from fires in Central America flowing over the Gulf of Mexico and into the Texas sky. Blue-gray smoke stretches from the bottom center of the image toward Texas at the upper left. The bright region extending towards the top of the image from the Yucatan Peninsula is the sun glinting off the ocean’s surface. This image was acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the morning of May 2, 2002. (Image by Mark Gray, GSFC)

  Photograph of Sarah Mims Testing Smoke for Microorganisms

As a final demonstration that smoke can harbor fungal spores, Mims set fire to several different kinds of organic material and held Petrifilms out in the smoke to collect any spores that might be released. Smoke from fires made from live grass, dead grass, flood debris, dry leaves, dry twigs and even newspaper released fungal spores that colonized the Petrifilms. Each sample showed lots more fungal growth than the reference films—films exposed to plain air—taken before, after, and in between each of the experimental burns. Clearly, fires don’t destroy all kinds of fungal spores, and those spores can be released in a fire’s smoke.

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Mims confirmed that micro-organisms could survive in smoke with a backyard experiment. She burned piles of brush and debris, and exposed Petrifilms to the smoke. Sure enough, mold and bacteria grew on the films. (Photograph by Forrest M. Mims III)


Unpredictable Fungi


“I think that’s a fascinating possibility,” says Gene Shinn, a scientist with the U.S. Geological Survey’s Coastal and Marine Geology Program, based in Florida. Shinn and his colleagues have been culturing bacteria from African Dust blowing over the Caribbean for many years, linking the decline of Caribbean coral reefs in the 1970s, 80s, and 90s to bacterial infections brought over the sea with Saharan dust. While the air samples they collect almost always contain some bacteria that have hitched a ride across the Atlantic on the dust particles, Shinn says the presence or absence of fungi is unpredictable.

“Sometimes there are fungi in the samples, and sometimes not. We really had no idea why this was the case. We never considered smoke as a possibility, but there is periodic widespread biomass burning that takes places in the African Congo, and that smoke could be coming across the Atlantic mixed with dust. Here in Florida, we also get a lot of the smoke from fires in Central America. We have felt all along there had to be some additional factor explaining the fungi, and it’s intriguing to think this could be it.”

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  Model Aerosol Data from May 2, 2002

The possibility of the microbes traveling with the smoke isn’t farfetched, says Shinn. “That dark smoke will act as an ultraviolet shield for the fungi, protecting them from UV damage.” Even if the spores went high into the atmosphere, it’s quite possible they could remain sufficiently protected from the Sun’s ultraviolet rays to still be viable when they come back to Earth.

The presence of viable fungal spores in smoke certainly raises the possibility that burning pastures, crops, and trees that are infected with fungal diseases might not be a good idea. Such burning is practiced all over the world. If spores are released in smoke, diseases could spread as far as the smoke does.

On the other hand, many kinds of fungi are beneficial—even essential—for vegetation. They recycle nutrients into the soil by decomposing dead vegetation. Certain plants and trees have developed a partnership with fungi that “infect” their roots, with each one providing the other with nutrients that the other can’t get on its own (or can’t get as well), while doing each other no harm. If spores from these beneficial soil fungi could escape the reach of the flames with smoke and then settle back down after the fire dies back, they might speed up the recovery of the burned landscape.

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Dust and smoke travel all over the world. This global view shows aerosol optical depth on May 2, 2002, when smoke from Central America was blowing over Texas. The animation shows the movement of aerosols from their sources. Dust frequently originates in northern Africa and Asia, while smoke (less dense than dust) comes from seasonal fires in Central America and southern Africa.

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(Image and animation by Robert Simmon, based on GOCART Data)


A Scientific Family


That such an important discovery has been made by a high school student provides a wonderful contrast to the rather depressing statistics about the decline in scores of American students on standardized science and math tests that we hear so much about. Mims is the kind of student who gives us reason to believe there is hope for another generation of promising scientists.

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  Photograph of Sarah Working on top of Tower

In the Mims’ household, science is a family activity, with Sarah’s dad as the instigator. Himself an amateur scientist, Forrest Mims’ passion for tinkering with electronics and science equipment led to a career writing books and designing electronics and science kits for Radio Shack. Amateur became professional, though, when a homemade device he built provided ground-based feedback about the accuracy of one of the first satellite sensors NASA launched to monitor the ozone layer. Since then, he has been involved in several NASA projects, including studies of smoke, ozone, and ultraviolet radiation in the Amazon. He’s also one of the lead scientists in NASA’s GLOBE program, a worldwide, hand-on science education program, in which students collect Earth science observations and analyze them in conjunction with scientists.

Having a scientist for a father clearly motivated Sarah and her older brother and sister. Sarah’s older brother Eric developed a seismometer that detected two underground nuclear tests in Nevada from his bedroom back home in Texas, and her sister Vicki measured the rotation of the sun by tracking sunspots. As far back as first or second grade, Forrest Mims was encouraging Sarah to conduct basic science experiments. She remembers that one of her first science projects was a science-in-your-home standard. “The very first one I can remember doing was measuring the acidity of common household products. Every year I would take on a new project,” she says.

As she grew up, she began to realize the practical advantages to her science projects. “I like discovering new things about the world, and I like the challenge of answering questions from the scientists who judge the fairs,” she says. She must have impressed quite a few of them because she has already been offered numerous college scholarships, and hopes to win more as a result of her senior year project, which is a follow-on to her pervious work.

Senior Year

“I know that the weakest part of my last project is the possibility that the fungal spores are local,” explains Mims. Even though the samples were collected on an observation tower that was 3 meters (about 10 feet) above the ground, and even though the spores did increase when smoke was present, it’s still possible, she says, that the spores she collected on her slides and her Petrifilms were local fungi stirred up from the ground. There could be some other reason why spores increased at the same time as the Central America smoke events. Perhaps the winds that drove the smoke north, for example, kicked up more spores than usual as the air traveled the short distance between Texas’ Gulf Coast and the town of Seguin.

Mims hopes her senior year project, carried out this summer, will provide conclusive evidence that Central American smoke is the source of the spores. To get the best evidence, she knew she needed to reduce as much as possible the chances that the spores were from other places in Texas. She needed to collect samples as close to the open waters of the Gulf of Mexico as possible, and high up in the air. To solve the first problem, she chose to collect samples at Padre Island, on the Texas coast.

To solve the second problem, she followed in her father’s creative footsteps, making her own low-tech device for collecting samples. Mims hesitates a little as she describes it, as though she’s afraid it won’t sound very scientific. “I took a plastic cup and cut holes in the rim, where you drink, and then attached it to the kite with string. Then I pressed my microscope slides into the bottom of the cup.”


Sarah Mims used the observation tower her father set up in the back yard to capture aerosols—and the bacteria and mold that hitchhiked their way across the ocean. (Photograph by Forrest M. Mims III)

  Photograph of Sarah Sampling with a Kite

It might seem low-tech, but it did the job, collecting plenty of good air samples. “Well, except for the time when I quit paying attention for a second, and the kite dove into the ocean,” she says, laughing. “That was kind of a drag. I had to wipe everything off and start all over.”

In the summer of 2003, Mims began analyzing the two main kite samples and found both spores and soot. The NOAA back-trajectories show that the air over Padre Island the day she was flying her kite had originated in the Yucatan two days earlier, where plenty of fires were burning. These latest observations—collected far from land and way up in the air—once again show a link between smoke and spores, making Mims a lot more confident that many of the spores she has been collecting are coming with the smoke, and not from local sources. Her amazing discovery is sure to be the start of this young scientist’s promising career.

  • Sarah A. Mims and Forrest M. Mims III, Fungal spores are transported long distances in smoke from biomass fires, Atmospheric Environment 38, 651-655 (2004).

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Using simple tools Mims is refining her experiments from 2002. Steady ocean breezes help keep her kite flying and deliver air direct from Mexico. By testing samples collected beneath the kite, Mims can confirm that the spores she’s finding come from far-away fires and not local sources. (Photograph by Forest M. Mims III)