SRI Stories

SRI Stories: Smoke Plumes


Western wildfire smoke plumes are getting taller.

In recent years, the plumes of smoke crawling upward from Western wildfires have trended taller, with more smoke and aerosols lofted up where they can spread farther and impact air quality over a wider area. The likely cause is climate change, with decreased precipitation and increased aridity in the Western U.S. that intensifies wildfire activity.

“Should these trends persist into the future,” says Kai Wilmot, a postdoctoral researcher in the College of Science's Science Research Initiative and in the Department of Atmospheric Sciences at the University of Utah, “it would suggest that enhanced Western U.S. wildfire activity will likely correspond to increasingly frequent degradation of air quality at local to continental scales.”

The study is published in Scientific Reports and supported by the iNterdisciplinary EXchange for Utah Science, or NEXUS, at the University of Utah.

 

“Given climate-driven trends towards increasing atmospheric aridity, declining snowpack, hotter temperatures, etc. We’re seeing larger and more intense wildfires throughout the Western U.S., and this is giving us larger burn areas and more intense fires.”

 

Smoke height

To assess trends in smoke plume height, Wilmot and U colleagues Derek Mallia, Gannet Hallar and John Lin modeled plume activity for around 4.6 million smoke plumes within the Western U.S. and Canada between 2003 and 2020. Dividing the plume data according to EPA ecoregions (areas where ecosystems are similar, like the Great Basin, Colorado Plateau, and Wasatch and Uinta Mountains in Utah) the researchers looked for trends in the maximum smoke plume height measured during August and September in each region in each year.

In the Sierra Nevada ecoregion of California, the team found that the maximum plume height increased, on average, by 750 ft (230 m) per year. In four regions, maximum plume heights increased by an average of 320 ft (100 m) per year.

Why? Wilmot says that plume heights are a complex interaction between atmospheric conditions, fire size and the heat released by the fire.

“Given climate-driven trends towards increasing atmospheric aridity, declining snowpack, hotter temperatures, etc., we’re seeing larger and more intense wildfires throughout the Western U.S.,” he says. “And this is giving us larger burn areas and more intense fires.”

The researchers also employed a smoke plume simulation model to estimate the mass of the plumes and approximate the trends in the amount of aerosols being thrown into the atmosphere by wildfires . . . which are also increasing.

The smoke simulation model also estimated the occurrence of pyrocumulonimbus clouds—a phenomenon where smoke plumes start creating thunderstorms and their own weather systems. Between 2017 and 2020, six ecoregions experienced their first known pyrocumulonimbus clouds and the trend suggests increasingly frequent pyrocumulonimbus activity on the Colorado Plateau.

Taller plumes send more smoke up into higher elevations where it can spread farther, says John Lin, professor of atmospheric sciences.

“When smoke is lofted to higher altitudes, it has the potential to be transported over longer distances, degrading air quality over a wider region,” he says. “So wildfire smoke can go from a more localized issue to a regional to even continental problem.”

Are the trends accelerating?

Some of the most extreme fire seasons have occurred in recent years. So does that mean that the pace of the worsening fire trend is accelerating? It’s too early to tell, Wilmot says. Additional years of data will be needed to tell if something significant has changed.

“Many of the most extreme data points fall within the years 2017 -2020, with some of the 2020 values absolutely towering over the rest of the time series,” he says. “Further, given what we know of the 2021 fire season, it appears likely that analysis of 2021 data would further support this finding.”

In Utah’s Wasatch and Uinta Mountains ecoregion, trends of plume height and aerosol amounts are rising but the trends are not as strong as those in Colorado or California. Smoke from neighboring states, however, often spills into Utah’s mountain basins.

“In terms of the plume trends themselves, it does not appear that Utah is the epicenter of this issue,” Wilmot says. “However, given our position as generally downwind of California, trends in plume top heights and wildfire emissions in California suggest a growing risk to Utah air quality as a result of wildfire activity in the West.”

Wilmot says that while there are some things that people can do to help the situation, like preventing human-caused wildfires, climate change is a much bigger and stronger force driving the trends of less precipitation, higher aridity and riper fire conditions across the West.

“The reality is that some of these [climate change] impacts are already baked in, even if we cut emissions right now,” Wilmot adds. “It seems like largely we’re along for the ride at the moment.”

Find the full study at Nature.com.

 

by Paul Gabrielsen, first published in @theU.


SRI Stories is a series by the College of Science, intended to share transformative experiences from students, alums, postdocs and faculty of the Science Research Initiative. To read more stories, visit the SRI Stories page.

Arctic Adventures

Arctic Adventures


Julie and Rebecca on the ice.

Adventures in the Canadian Arctic.

Rebecca Hardenbrook and Julie Sherman, both graduate students in the Math Department, participated in the Biogeochemical Exchange Process at Sea Ice Interfaces (BEPSII) Sea Ice School May 14-23, 2022, at the Canadian High Arctic Research Station (CHARS) in Cambridge Bay, Canada.

The purpose of the BEPSII program is to provide early-career polar researchers an opportunity to learn field work methods for understanding and analyzing polar sea ice firsthand, as well as building a community in the sea ice research world. Competition for acceptance in the program is competitive—nearly 100 applications were received for 30 spots.

Recently, the Math Department asked Hardenbrook about her adventures in the Canadian High Arctic.

How did you become interested in sea ice research?

I started my college-level educational journey at the U as an undergrad in 2014. I knew that I wanted to pursue a career that would allow me to do something related to studying climate change in some way, but I also found my passion in studying math. I began working with Dr. Ken Golden in my junior year. He works right in that intersection of climate change, specifically sea ice and math. I was lucky enough to be accepted to the U for my Ph.D., which I am grateful for because being able to continue in this research direction has opened my eyes to a lot of really important research questions about things—such as the fact that all living things depend on sea ice to survive, including humans.

Approaching Cambridge Bay.

How did you travel to Cambridge Bay?

We left Salt Lake, flew to Seattle, and then to Edmonton in Canada. After spending the night in Edmonton, we flew to Yellowknife and then to Cambridge Bay. Yellowknife is a beautiful town in the Northern Territories. On all of our flights, I couldn't stop looking outside the window on the plane as the landscape changed slowly from the familiarity of the Wasatch mountains to the flattened landscape surrounding Edmonton to the frozen lakes and dense woods surrounding Yellowknife to the endless snowy and icy terrain of the Canadian High Arctic, which includes the area in the Northwest Territories, Yukon, and Nunavut.

What was it like meeting the other fellow scientists and colleagues?

Meeting other blooming scientists was equally as exciting as actually getting to be on the sea ice for the first time. I now have 30 friends all around the world who are working on exciting and relevant problems relating to polar sea ice, who I can potentially work with in the future. I certainly have never had that sort of network before! The relationships I made with other early-career researchers at the BEPSII Sea Ice School left me with a renewed passion for my own work and for asking questions I haven't thought of before.

Drilling ice cores.

What was a typical day like?

The activities really varied day-by-day, but we did have several lectures from experienced polar researchers that ranged among topics. For example, we heard from experts studying biophysical processes of the ecosystems and organisms living within the Arctic Sea ice. The researchers are investigating the movement and transport of critical nutrients and trace metals in the Chukchi Sea, the optical properties of sea ice, and how snow on the surface comes into play. We did have a few days of field work, the first two primarily were practice days for learning how to drill ice cores, dig snow pits, take snow hardness measurements, make sack holes, and more. We had a lot of free time to explore the area surrounding Cambridge Bay, although we didn't venture too far away from the town itself. You only have to go out 3/4 of a mile or so before you really understand how remote the area is.

What were your living quarters like? What about your meals?

We lived in apartments of eight people each, and within the apartments we shared a room with one person. Our apartments were part of the Canadian High Arctic Research Station (CHARS) campus, and they were very nice. Because the sun was out for most of the day (or for several hours the entire day), our apartments were pretty warm despite the outside temperature being below freezing. Our lunch and dinner were catered by a local business, and our breakfast foods were purchased from the local grocery store. The price of foods that I really take for granted, like fresh produce and even things like peanut butter, in Cambridge Bay are incredibly expensive. As Julie mentioned in her profile, we heard that a single watermelon costs $75, which is an  extreme example, but it was still shocking to me.

Walking on sea ice.

What did you enjoy most about the experience? What was the environment like?

I truly enjoyed the entire experience, but I think selfishly finally getting to be able to walk on the sea ice, see the algae at the bottom of the ice core we took, feel the cold summer Arctic air on my face, and experience that environment was life changing for me. I did get a little emotional when I first stepped out onto the ice, because I've wanted to be able to do that now for the last six years. I am also so grateful to be able to make the friends that I did. The people I met there are so  passionate about their work, and that drives me to continue doing research in this field. The environment was like nothing I've experienced, and it's kind of hard to put into words. We got to be there for several days with no sunset. Even though it was hard to sleep sometimes, I didn't mind because it was so beautiful on the lucky days when the clouds would clear out and the snow would stop falling. The air was incredibly dry despite us being right near the ocean—I mean it's technically a desert up there—so I think the cold felt a little less intense unless it was windy (which it often was). I think the most notable thing for me was just how quiet and flat it is. I could see many miles on a clear day.

There is an Inuit legend about a family of giants who died while crossing Victoria Island looking for food. These giants are the three eskers (a ridge of stratified sand and gravel, deposited by meltwater from a retreating glacier or ice sheet) nine miles outside of Cambridge Bay. They are named Uvayuq (after the father), Amaaqtuq (after the mother, who was pregnant), and Inuuhuktu (after the son). It is so flat that you can see Uvayuq clearly from the town. In fact, some of us actually considered running to it, but we got too nervous about potentially meeting a bear on our way, so we didn’t do it. Luckily, we didn't see any bears (polar or grizzly), but we heard that there was a polar bear 30 miles out from the town somewhere. We did see a few Arctic fox, which was really exciting because early on in our time there, their fur was completely white and they are hard to see. As time went on, we saw Arctic fox that were starting to shed their winter coats. Their summer fur is short and black, so they’re much more visible. We also saw a few Arctic hare, but they are very good at hiding so we didn’t see too many. There were also a lot of birds—unfortunately, I’m not much of a birder at the moment so I couldn't identify them.

What are your plans after you receive your Ph.D.?

I am hoping to get a postdoctoral research and teaching position at a college or university. I love my research, and I also love teaching undergraduate students about math, about sea ice, and about the environments around us. A life where I can continue on with both of my passions would be a good one, and so I hope to do that.

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Air Tracker

Air Tracker


New tool shows air pollution’s path.

On June 13, 2022, Environmental Defense Fund unveiled Air Tracker, a first-of-its-kind web-based tool that allows users to plot the likely path of air pollution. Run on real-time, trusted scientific models and coupled with air pollution and weather data and developed in partnership with the University of Utah and the CREATE Lab at Carnegie Mellon University, Air Tracker helps users learn more about the air they’re breathing, including pollution concentrations and its potential sources.

U professor John Lin, of the Department of Atmospheric Sciences, adapted his research group’s atmospheric model (the Stochastic Time-Inverted Lagrangian Transport model, or STILT) to run as part of Air Tracker.

John Lin

“Air Tracker is designed to trace our potential source regions for pollution. Users can make use of Air Tracker to investigate emission sources with a research-grade atmospheric model at their fingertips.”

 

“Air quality monitors can show us how polluted our air is, but they aren’t equipped to tell us what is causing the pollution,” says Tammy Thompson, Senior Air Quality Scientist and creator of the tool. “With Air Tracker, we’re able to see likely sources of pollution hotspots, which is especially helpful in cities where a variety of emitters contribute to overall air quality.”

Users can click anywhere on maps of Houston, Salt Lake City and Pittsburgh to create a “source area,” which shows the most likely origin of the air they’re breathing at any given time. They can also click on locations of individual air quality sensors to show real-time and historical fine particle (PM2.5) pollution readings, wind speed and direction.

Relying on STILT, Air Tracker incorporates a variety of weather forecasting models to show how particles move through the atmosphere, allowing the tool to map the probability of pollution’s path. Air Tracker goes beyond common source identification models–which are unable to capture fine-scale air pollution variability–to identify pollution sources at the city block level.

In Houston, for example, where a lack of zoning has allowed industrial sources to operate near communities with homes, schools, churches and hospitals, Air Tracker uses both real-time and historical data to show how different sources contribute to poor air quality at different dates and times.

“Breathing dirty air is bad for our health, and these health effects are not distributed equally,” said Sarah Vogel, EDF Senior Vice President, Healthy Communities. “The poorer and more disadvantaged groups disproportionately suffer the greater exposures and health impacts from air pollution. We hope community leaders and individuals will use this pollution data to hold polluters accountable and advocate for clean air policy change.”

In addition to learning more about the sources likely influencing the air they breathe, Air Tracker users can also use the real-time source area identification to help speed mitigation and help spot and control emissions resulting from accidents and unusual events. Through its “Share” feature, users can take screenshots of source areas to send to regulators and local officials.

Air Tracker is part of EDF’s ongoing work to better understand local air pollution, its behavior and its impacts. Air Tracker can be adapted to include additional pollutants and used in other cities around the world, including those that may not yet feature extensive, hyper-local air quality monitoring programs.

Learn more about Air Tracker, EDF’s Global Clean Air efforts and the project partners here.

One of the world’s leading international nonprofit organizations, Environmental Defense Fund creates transformational solutions to the most serious environmental problems. To do so, EDF links science, economics, law, and innovative private-sector partnerships. With more than 3 million members and offices in the United States, China, Mexico, Indonesia and the European Union, EDF’s scientists, economists, attorneys and policy experts are working in 28 countries to turn our solutions into action. Connect with us on Twitter @EnvDefenseFund.

 

by Paul Gabrielsen, first published in @theU. Adapted from a release by the Environmental Defense Fund

 

Cellular Crosslinking

Cellular Crosslinking


Structural signatures of Escherichia coli chemoreceptor signaling states revealed by cellular crosslinking

Motile bacteria are capable of swimming efficiently toward favorable chemical environments and away from inhospitable ones. This behavior–called “chemotaxis”–is frequently used by unicellular organisms for finding food.

Not surprisingly, such behaviors play important roles in establishing beneficial host symbioses and pathogenic infections. The value of understanding in detail this mechanism of directed cell migration in response to extracellular chemical signals cannot be over-stated, and Escherichia coli, commonly referred to as E. coli, has become the paradigm molecular model.

“Despite their tiny size, bacteria have evolved amazingly sophisticated protein machines for detecting and responding to changes in their environment,” says John “Sandy” Parkinson, principal investigator in the School of Biological Sciences. Caralyn Flack, a research associate in the Parkinson Lab, developed a technique for monitoring the behavior of living bacteria before and after she changed the structure of a critical signaling protein inside the cell.

Flack’s new findings were recently published in Proceedings of the National Academy of Sciences (PNAS), a peer reviewed journal of the National Academy of Sciences (NAS). The paper, titled “Structural signatures of Escherichia coli chemoreceptor signaling states revealed by cellular crosslinking,” makes an important advance in elucidating the molecular mechanism(s) of signal propagation through chemoreceptor molecules.

Caralyn Flack

"This new cellular crosslinking approach delivers unprecedented insight into receptor structural properties as the receptors function within cells"

 

“Bacteria are amazing at sensing and responding quickly to changes in their environment,” states Flack. “This makes them a great model for trying to understand transmembrane signaling.” But it turns out that changes that accompany signaling events in chemoreceptors are difficult to follow with traditional structural methods, and those techniques that do work cannot replicate the native cellular environment. To remedy that, the technique Flack developed does three things. First, it assesses behavior. Second, it changes the structure of a protein inside living bacteria, and third, it then watches the signaling behavior change in real-time. The technique, which involves “crosslinking,” she says, has proven to be “really powerful for structure-function analyses of signaling proteins.”

In essence, what Flack did was create receptor proteins with a special amino acid at a position of interest. That amino acid, cysteine, has unique chemical properties that, under the right conditions, allow it to form a covalent bond (to “crosslink”) to a nearby cysteine in the same protein or in another protein. Such crosslinks constrain the structural movements of the proteins and can change its behavior. This is how Flack was able to change the structure, and in consequence, the function of a receptor within a living cell.

The crosslinking reporter sites “enabled us to evaluate receptor output states before and after crosslink formation,” she writes. This new cellular crosslinking approach “delivers unprecedented insight into receptor structural properties as the receptors function within cells” stated one reviewer of the PNAS manuscript. Flack suggests that “similar crosslinking approaches could serve to follow signal transmission in other regions of the chemoreceptor molecule and perhaps in other signaling proteins, as well.”

Work on those topics is ongoing.

Photo: Colorized scanning electron micrograph of Escherichia coli, grown in culture and adhered to a cover slip by NIAID on Flickr

 

by David Pace, first published in biology.utah.edu.

 

Biomimetic Cephalopods

Biomimetic Cephalopods


Bringing ancient animals back to life—as robots.

In a university swimming pool, scientists and their underwater cameras watch carefully as a coiled shell is released from a pair of metal tongs. The shell begins to move under its own power, giving the researchers a glimpse into what the oceans might have looked like millions of years ago when they were full of these ubiquitous animals.

This isn’t Jurassic Park, but it is an effort to learn about ancient life by recreating it. In this case, the recreations are 3-D-printed robots designed to replicate the shape and motion of ammonites, marine animals that both preceded and were contemporaneous with the dinosaurs.

 

David Peterman

"Evolution dealt them a very unique mode of locomotion after liberating them from the seafloor with a chambered, gas-filled conch. These animals are essentially rigid-bodied submarines propelled by jets of water."

 

The robotic ammonites allowed the researchers to explore questions about how shell shapes affected swimming ability. They found trade-offs between stability in the water and maneuverability, suggesting that the evolution of ammonite shells explored different designs for different advantages rather than converged toward a single best design.

“These results reiterate that there is no single optimum shell shape,” says David Peterman, a postdoctoral fellow in the University of Utah’s Department of Geology and Geophysics.

The study is published in Scientific Reports and supported by the National Science Foundation.

Bringing ammonites to “life”

For years, Peterman and Kathleen Ritterbush, assistant professor of geology and geophysics, have been exploring the hydrodynamics, or physics of moving through the water, of ancient shelled cephalopods, including ammonites. Cephalopods today include octopuses and squid, with only one group sporting an external shell—the nautiluses.

Before the current era, cephalopods with shells were everywhere. Although their rigid coiled shells would have impacted their free movement through the water, they were phenomenally successful evolution-wise, persisting for hundreds of millions of years and surviving every mass extinction.

“These properties make them excellent tools to study evolutionary biomechanics,” Peterman says, “the story of how benthic (bottom-dwelling) mollusks became among the most complex and mobile group of marine invertebrates. My broader research goal is to provide a better understanding of these enigmatic animals, their ecosystem roles, and the evolutionary processes that have shaped them.”

Peterman and Ritterbush previously built life-sized 3-D weighted models of cone-shaped cephalopod shells and found, through releasing them in pools, that the ancient animals likely lived a vertical life, bobbing up and down through the water column to find food. These models’ movements were governed solely by buoyancy and the hydrodynamics of the shell.

But Peterman has always wanted to build models more similar to living animals.

Diagram of a Biometic Cehalapod.

“I have wanted to build robots ever since I developed the first techniques to replicate hydrostatic properties in physical models, and Kathleen strongly encouraged me as well,” Peterman says. “On-board propulsion enables us to explore new questions regarding the physical constraints on the life habits of these animals.”

Buoyancy became Peterman’s chief challenge. He needed the models to be neutrally buoyant, neither floating nor sinking. He also needed the models to be water-tight, both to protect the electronics inside and to prevent leaking water from changing the delicate buoyancy balance.

But the extra work is worth it. “New questions can be investigated using these techniques,” Peterman says, “including complex jetting dynamics, coasting efficiency, and the 3-D maneuverability of particular shell shapes.”

Three kinds of shells

The researchers tested robotic ammonites with three shell shapes. They’re partially based on the shell of a modern Nautilus and modified to represent the range of ancient ammonites’ shell shapes. The model called a serpenticone had tight whorls and a narrow shell, while the sphaerocone model had few thick whorls and a wide, almost spherical shell. The third model, the oxycone, was somewhere in the middle: thick whorls and a narrow, streamlined shell. You can think of them occupying a triangular diagram, representing “end-members” of different shell characteristics.

“Every planispiral cephalopod to ever exist plots somewhere on this diagram,” Peterman says, allowing the properties for in-between shapes to be estimated.

Once the 3-D-printed models were built, rigged and weighted, it was time to go to the pool. Working first in the pool of Geology and Geophysics professor Brenda Bowen and later in the U’s Crimson Lagoon, Peterman and Ritterbush set up cameras and lights underwater and released the robotic ammonites, tracking their position in 3-D space throughout around a dozen “runs” for each shell type.

No perfect shell shape

By analyzing the data from the pool experiments, the researchers were looking for the pros and cons associated with each shell characteristic.

“We expected there to be various advantages and consequences for any particular shapes,” Peterman says. “Evolution dealt them a very unique mode of locomotion after liberating them from the seafloor with a chambered, gas-filled conch. These animals are essentially rigid-bodied submarines propelled by jets of water.” That shell isn’t great for speed or maneuverability, he says, but coiled-shell cephalopods still managed remarkable diversity through each mass extinction.

“Throughout their evolution, externally shelled cephalopods navigated their physical limitations by endlessly experimenting with variations on the shape of their coiled shells,” Peterman says.

So, which shell shape was the best?

David Peterman

“The idea that one shape is better than another is meaningless without asking the question—‘better at what?’” Peterman says. Narrower shells enjoyed less drag and more stability while traveling in one direction, improving their jetting efficiency. But wider, more spherical shells could more easily change directions, spinning on an axis. This maneuverability may have helped them catch prey or avoid slow predators (like other shelled cephalopods).

Peterman notes that some interpretations consider many ammonite shells as hydrodynamically “inferior” to others, limiting their motion too much.

“Our experiments, along with the work of colleagues in our lab, demonstrate that shell designs traditionally interpreted as hydrodynamically ‘inferior’ may have had some disadvantages but are not immobile drifters,” Peterman says. “For externally shelled cephalopods, speed is certainly not the only metric of performance.” Nearly every variation in shell design iteratively appears at some point in the fossil record, he says, showing that different shapes conferred different advantages.

“Natural selection is a dynamic process, changing through time and involving numerous functional tradeoffs and other constraints,” he says, “Externally-shelled cephalopods are perfect targets to study these complex dynamics because of their enormous temporal range, ecological significance, abundance, and high evolutionary rates.”

Find the full study @ Nature.com.

 

by Paul Gabrielsen, first published in @TheU.

 

Moiré Magic

Moiré Magic


Highly tunable composite materials—with a twist.

The above animation shows the patterns created as two circles move across each other. Those patterns, created by two sets of lines offset from each other, are called moiré (pronounced mwar-AY) effects. As optical illusions, moiré patterns create neat simulations of movement. But at the atomic scale, when one sheet of atoms arranged in a lattice is slightly offset from another sheet, these moiré patterns can create some exciting and important physics with interesting and unusual electronic properties.

Mathematicians at the University of Utah have found that they can design a range of composite materials from moiré patterns created by rotating and stretching one lattice relative to another. Their electrical and other physical properties can change—sometimes quite abruptly, depending on whether the resulting moiré patterns are regularly repeating or non-repeating. Their findings are published in Communications Physics.

The mathematics and physics of these twisted lattices applies to a wide variety of material properties, says Kenneth Golden, distinguished professor of mathematics. “The underlying theory also holds for materials on a large range of length scales, from nanometers to kilometers, demonstrating just how broad the scope is for potential technological applications of our findings.”

 

Ken Golden

"We observe a geometry-driven localization transition that has nothing to do with wave scattering or interference effects, which is a surprising and unexpected discovery."

 

With a twist

Before we arrive at these new findings, we’ll need to chart the history of two important concepts: aperiodic geometry and twistronics.

Aperiodic geometry means patterns that don’t repeat. An example is the Penrose tiling pattern of rhombuses. If you draw a box around a part of the pattern and start sliding it in any direction, without rotating it, you’ll never find a part of the pattern that matches it.

Aperiodic patterns designed over 1000 years ago appeared in Girih tilings used in Islamic architecture. More recently, in the early 1980s, materials scientist Dan Shechtman discovered a crystal with an aperiodic atomic structure. This revolutionized crystallography, since the classic definition of a crystal includes only regularly repeating atomic patterns, and earned Shechtman the 2011 Nobel Prize in Chemistry.

Okay, now onto twistronics, a field that also has a Nobel in its lineage. In 2010, Andre Geim and Konstantin Novoselov won the Nobel Prize in Physics for discovering graphene, a material that’s made of a single layer of carbon atoms in a lattice that looks like chicken wire. Graphene itself has its own suite of interesting properties, but in recent years physicists have found that when you stack two layers of graphene and turn one slightly, the resulting material becomes a superconductor that also happens to be extraordinarily strong. This field of study of the electronic properties of twisted bilayer graphene is called “twistronics.”

Two-phase composites

In the new study, Golden and his colleagues imagined something different. It’s like twistronics, but instead of two layers of atoms, the moiré patterns formed from interfering lattices determine how two different material components, such as a good conductor and a bad one, are arranged geometrically into a composite material. They call the new material a “twisted bilayer composite,” since one of the lattices is twisted and/or stretched relative to the other. Exploring the mathematics of such a material, they found that moiré patterns produced some surprising properties.

“As the twist angle and scale parameters vary, these patterns yield myriad microgeometries, with very small changes in the parameters causing very large changes in the material properties,” says Ben Murphy, co-author of the paper and adjunct assistant professor of mathematics.

Twisting one lattice just two degrees, for example, can cause the moiré patterns to go from regularly repeating to non-repeating—and even appear to be randomly disordered, although all the patterns are non-random.  If the pattern is ordered and periodic, the material can conduct electrical current very well or not at all, displaying on/off behavior similar to semiconductors used in computer chips. But for the aperiodic, disordered-looking patterns, the material can be a current-squashing insulator, “similar to the rubber on the handle of a tool that helps to eliminate electrical shock,” says David Morison, lead author of the study who recently finished his Ph.D. in Physics at the University of Utah under Golden’s supervision.

This kind of abrupt transition from electrical conductor to insulator reminded the researchers of yet another Nobel-winning discovery: the Anderson localization transition for quantum conductors. That discovery, which won the 1977 Nobel Prize in Physics, explains how an electron can move freely through a material (a conductor) or get trapped or localized (an insulator), using the mathematics of wave scattering and interference. But Golden says that the quantum wave equations Anderson used don’t work on the scale of these twisted bilayer composites, so there must be something else going on to create this conductor/insulator effect. “We observe a geometry-driven localization transition that has nothing to do with wave scattering or interference effects, which is a surprising and unexpected discovery,” Golden says.

The electromagnetic properties of these new materials vary so much with just tiny changes in the twist angle that engineers may someday use that variation to precisely tune a material’s properties and select, for example, the visible frequencies of light (a.k.a. colors) that the material will allow to pass through and the frequencies it will block.

“Moreover, our mathematical framework applies to tuning other properties of these materials, such as magnetic, diffusive and thermal, as well as optical and electrical,” says professor of mathematics and study co-author Elena Cherkaev, “and points toward the possibility of similar behavior in acoustic and other mechanical analogues.”

Find the full study in Communications Physics.

 

by Paul Gabrielsen, first published in @TheU.

 

Interactive Forest Maps

Wildfire, Drought & Insects


Dying forests in the western U.S.

Threats impacting forests are increasing nationwide.

Planting a tree seems like a generally good thing to do for the environment. Trees, after all, take in carbon dioxide, offsetting some of the emissions that contribute to climate change.

But all of that carbon in trees and forests worldwide could be thrown back into the atmosphere again if the trees burn up in a forest fire. Trees also stop scrubbing carbon dioxide from the air if they die due to drought or insect damage.

The likelihood of those threats impacting forests is increasing nationwide, according to new research in Ecology Letters, making relying on forests to soak up carbon emissions a much riskier prospect.

“U.S. forests could look dramatically different by the end of the century,” says William Anderegg, study lead author and associate professor in the University of Utah School of Biological Sciences. “More severe and frequent fires and disturbances have huge impacts on our landscapes. We are likely to lose forests from some areas in the Western U.S. due to these disturbances, but much of this depends on how quickly we tackle climate change.”

 

William Anderegg

"We’ve seen devastating fire seasons with increasing severity in the past several years. Generally, we expect the western U.S. to be hit hardest."

 

The researchers modeled the risk of tree death from fire, climate stress (heat and/or drought) and insect damage for forests throughout the United States, projecting how those risks might increase over the course of the 21st century.

See their findings in an interactive map at carbonplan.org.

By 2099, the models found, that United States forest fire risks may increase by between four and 14 times, depending on different carbon emissions scenarios. The risks of climate stress-related tree death and insect mortality may roughly double over the same time.

But in those same models, human actions to tackle climate change mattered enormously—reducing the severity of climate change dramatically reduced the fire, drought and insect-driven forest die-off.

“Climate change is going to supercharge these three big disturbances in the U.S.,” Anderegg says. “We’ve seen devastating fire seasons with increasing severity in the past several years. Generally, we expect the western U.S. to be hit hardest by all three of these. And they’re somewhat interconnected too. Really hot and dry years, driven by climate change, tend to drive lots of fires, climate-driven tree mortality and insect outbreaks. But we have an opportunity here too. Addressing climate change quickly can help keep our forests and landscapes healthy.”

The study is published in Ecology Letters and was supported by the National Science Foundation, U.S. Department of Agriculture, David and Lucille Packard Foundation and Microsoft’s AI for Earth.

Find the full study at Ecology Letters.

 

by Paul Gabrielsen, first published at @TheU.

 

How Trees Grow

How Trees Grow


William Anderegg

What we’re still learning about how trees grow.

What will happen to the world’s forests in a warming world? Will increased atmospheric carbon dioxide help trees grow? Or will extremes in temperature and precipitation hold growth back? That all depends on whether tree growth is more limited by the amount of photosynthesis or by the environmental conditions that affect tree cell growth—a fundamental question in tree biology, and one for which the answer wasn’t well understood, until now.

A study led by University of Utah researchers, with an international team of collaborators, finds that tree growth does not seem to be generally limited by photosynthesis but rather by cell growth. This suggests that we need to rethink the way we forecast forest growth in a changing climate and that forests in the future may not be able to absorb as much carbon from the atmosphere as we thought.

“A tree growing is like a horse and cart system moving forward down the road,” says William Anderegg, an associate professor in the U’s School of Biological Sciences and principal investigator of the study. “But we basically don’t know if photosynthesis is the horse most often or if it’s cell expansion and division. This has been a longstanding and difficult question in the field. And it matters immensely for understanding how trees will respond to climate change.”

The study is published in Science and is funded by the U.S. Department of Agriculture, the David and Lucille Packard Foundation, the National Science Foundation, the U.S. Department of Energy and the Arctic Challenge for Sustainability II.

Growth rings - oldest growth is at the top.

Source vs. sink

We learned the basics in elementary school—trees produce their own food through photosynthesis, taking sunlight, carbon dioxide and water and turning it into leaves and wood.

There’s more to the story, though. Converting carbon gained from photosynthesis into wood requires wood cells to expand and divide.

So trees get carbon from the atmosphere through photosynthesis. This is the trees’ carbon source. They then spend that carbon to build new wood cells—the tree’s carbon sink.

If the trees’ growth is source-limited, then it’s limited only by how much photosynthesis the tree can carry out and tree growth would be relatively easy to predict in a mathematical model. So rising carbon dioxide in the atmosphere should ease that limitation and let trees grow more, right?

But if instead the trees’ growth is sink-limited, then the tree can only grow as fast as its cells can divide. Lots of factors can directly affect both photosynthesis and cell growth rate, including temperature and the availability of water or nutrients. So if trees are sink-limited, simulating their growth has to include the sink response to these factors.

The researchers tested that question by comparing the trees’ source and sink rates at sites in North America, Europe, Japan and Australia. Measuring carbon sink rates was relatively easy—the researchers just collected samples from trees that contained records of growth. “Extracting wood cores from tree stems and measuring the width of each ring on these cores essentially lets us reconstruct past tree growth,” says Antoine Cabon, a postdoctoral scholar in the School of Biological Sciences and lead author of the study.

Measuring carbon sources is tougher, but doable. Source data was measured with 78 eddy covariance towers, 30 feet tall or more, that measure carbon dioxide concentrations and wind speeds in three dimensions at the top of forest canopies, Cabon says. “Based on these measurements and some other calculations,” he says, “we can estimate the total forest photosynthesis of a forest stand.”

Decoupled

The researchers analyzed the data they collected, looking for evidence that tree growth and photosynthesis were processes that are linked, or coupled. They didn’t find it. When photosynthesis increased or decreased, there was not a parallel increase or decrease in tree growth.

“Strong coupling between photosynthesis and tree growth would be expected in the case where tree growth is source limited,” Cabon says. “The fact that we mostly observe a decoupling is our principal argument to conclude that tree growth is not source-limited.”

Surprisingly, the decoupling was seen in environments across the globe. Cabon says they did expect to see some decoupling in some places, but “we did not expect to see such a widespread pattern.”

The strength of coupling or decoupling between two processes can lie on a spectrum, so the researchers were interested in what conditions led to stronger or weaker decoupling. Fruit-bearing and flowering trees, for example, exhibited different source-sink relationships than conifers. More diversity in a forest increased coupling. Dense, covered leaf canopies decreased it.

Finally, coupling between photosynthesis and growth increased in warm and wet conditions, with the opposite also true: that in cold and dry conditions, trees are more limited by cell growth.

Cabon says that this last finding suggests that the source vs. sink issue depends on the tree’s environment and climate. “This means that climate change may reshape the distribution of source and sink limitations of the world forests,” he says.

A new way to look forward

The key takeaway is that vegetation models, which use mathematical equations and plant characteristics to estimate future forest growth, may need to be updated. “Virtually all these models assume that tree growth is source limited,” Cabon says.

For example, he says, current vegetation models predict that forests will thrive with higher atmospheric carbon dioxide. “The fact that tree growth is often sink limited means that for many forests this may not actually happen.”

That has additional implications: forests currently absorb and store about a quarter of our current carbon dioxide emissions. If forest growth slows down, so do forests’ ability to take in carbon, and their ability to slow climate change.

Find the full study @ science.org.

Other authors of the study include Steven A. Kannenberg, University of Utah; Altaf Arain and Shawn McKenzie, McMaster University; Flurin Babst, Soumaya Belmecheri and David J. Moore, University of Arizona; Dennis Baldocchi, University of California, Berkeley; Nicolas Delpierre, Université Paris-Saclay; Rossella Guerrieri, University of Bologna; Justin T. Maxwell, Indiana University Bloomington; Frederick C. Meinzer and David Woodruff, USDA Forest Service, Pacific Northwest Research Station; Christoforos Pappas, Université du Québec à Montréal; Adrian V. Rocha, University of Notre Dame; Paul Szejner, National Autonomous University of Mexico; Masahito Ueyama, Osaka Prefecture University; Danielle Ulrich, Montana State University; Caroline Vincke, Université Catholique de Louvain; Steven L. Voelker, Michigan Technological University and Jingshu Wei, Polish Academy of Sciences.

 

- by Paul Gabrielsen, first published in @theU

 

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Carbon Nanotubes

Carbon Nanotubes


Vikram Deshpande

Long carbon nanotubes reveal subtleties of quantum mechanics.

Vikram Deshpande had a hunch that carbon nanotubes held a lot of promise as a building block. He suspected that their unusual electrical and thermal properties and extraordinary strength could be modified for specific purposes by adding nanofabricated structures.

Working with nanotubes more than a micron long, the University of Utah physicist and his team found that the nanotubes held surprises, even without being adorned with those structural bells and whistles. “We started seeing all this richness in the data and had to investigate that before making the experiment more complicated,” Deshpande says. “Because they are only a nanometer or so in diameter, they are excellent playgrounds for studying the quantum mechanics of electrons in one dimension.”

But thin walls also mean little shielding. Impurities on the surface scatter electrons in the nanotube, and that initially prevented Deshpande from getting clean data.

His solution was to both clean the nanotubes and run his experiments in a DRY ICE 1.5K 70 mm cryostat made by ICEoxford. The UK-based company’s cryostat allows him to suspend nanotubes between supports and run a current through them. The nanotubes heat up to several hundred degrees, and the impurities are knocked off the surface.

ICEoxford cryogenic equipment.

The setup is cooled by pumped helium-4 at around 1.5 K, which is important, says Deshpande. “A lot of cryogenic equipment is vacuum-based, but the heat injected into the nanotube has no way out except along the tube, which is very ineffective.” Another boon is the fact that the cryostat is top loading so it’s easy to access. Within 12 hours of installing a new sample, the entire system is cooled and ready for testing.

With a good nanotube in place and thoroughly clean, Deshpande applies voltage to inject electrons and explore their quantum behavior.

A major influence on electron behavior inside the nanotube is the quality of the end contacts. The electrons travel unimpeded within the tube, known as the ballistic regime. But the ease at which they can escape the tube affects their behavior radically.

Using low-conductivity contacts, Deshpande’s team measured the energy required to add individual electrons to the tube. Subtle changes in the energy showed that the electrons were falling into an ordered pattern called a Wigner crystal—effectively a solid made of pure electrons—which occurs only at very low density. “Lower electron density is obtained with longer lengths, which make our experimental signature possible,” Deshpande says. His team reported their results in Physical Review Letters (volume 123, page 197701, 2019).

Last year the team published another paper in Physical Review Letters (volume 126, page 216802, 2021) with results from high-conductance contacts. They found the electrons’ wave-functions spread along the tube, creating quantum interference, analogous to light in an interferometer. There was not only interference similar to the Fabry-Perot effect between electrons bouncing back and forth, but also a more subtle interference caused by slight variations in the nanotubes, such as chirality. “These are exquisite measurements of delicate quantum effects that we can only see because our long nanotubes accumulate measurable phase difference between these modes,” Deshpande says.

He has also made use of the DRY ICE cryostat’s ability to apply magnetic fields up to 9 teslas. “If you thought the data so far were rich, you should see what happens in a magnetic field!” he says.

Phil Dooley is a freelance writer and former laser physicist based in Canberra, Australia.

 

- by Phil Dooley, first published in Physics Today

 

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Galápagos Letters

Galápagos Letters


by Nora Clayton, first published @ biology.utah.edu

January 17, 2022

Adventure Awaits

Getting ready for a field expedition is always an adventure. After packing, COVID testing, homework, buying supplies, planning travel, and coordinating a study, we are very ready to be on our way. Read More >

1

January 24, 2022

El Garrapatero

Our first day in the field we went to “El Garrapatero.” Meaning “Tick Eater,” both the site and its namesake, which happens to be a beach, are named after a common bird called an Ani. Read More >

2

January 31, 2022

Philornis Downsi

Philornis downsi is well known because its larvae are harmful to bird nestlings. The larvae suck blood, hence the “vampire” part of their name. Read More >

3

February 7, 2022

Lions, Iguanas, and Birds

People and wildlife share the space, which means you have to be careful not to step on an iguana or sit too close to a sea lion! Read More >

4

February 14, 2022

Field Observations

Readers should note that biologists, grad students, and intrigued 13-year-olds may stop frequently to observe things. Read More >

5

February 21, 2022

Galápagos Portraits

If it’s true that a picture paints a thousand words, you will have several thousand to read today! Read More >

6

February 28, 2022

Plumber’s Camera

We’re beginning to look inside the nests with a camera, taking notes on the contents of each. Read More >

7

March 7 2022

It’s a Small World

Every day, walking around town, to the beach, the station, or on our patio outside, our group constantly points out ants, carpenter bees, geckos, millipedes, katydids and grasshoppers. Read More >

8

March 14, 2022

Galápagos Penguins

We woke up early to get ready, and catch a bus to the dock on the other side of the island. The drive went through the highlands, where it was beginning to rain. Read More >

9

March 14, 2022

Old Town Quito

The city, full of people, is so different from Puerto Ayora. The streets are packed with shops and each big hill is covered in bright colorful buildings. Read More >

10

Originally published @ biology.utah.edu