Making Stem-Cell Band-Aids for the Retina

Over the coming weeks, we’ll be highlighting several undergraduates and their summer research at Caltech. Some are Techers; others hail from schools across the country. Most are participating in the Summer Undergraduate Research Fellowships (SURF) program, a unique opportunity for undergraduates to spend 10 weeks over the summer doing original research with Caltech faculty. At the end of the project, students write a paper and present their work at SURF Seminar Day, which will take place on October 15 this year.

At the beginning of July, Caltech senior Wilson Ho found himself hiking, stargazing, and camping in Yosemite National Park with a Nobel laureate. He even joined a group of scientists for a spontaneous jump into a freezing cold stream. 

Ho was spending the summer working on a SURF project in the lab of Robert Grubbs, one of the winners of the 2005 Nobel Prize in Chemistry. Therefore, he had earned himself a spot on the Grubbs team's annual camping trip.

"It feels pretty surreal sometimes," Ho says, not just of spending five days camping with Grubbs, the Victor and Elizabeth Atkins Professor of Chemistry, but also of getting the opportunity to work for the esteemed researcher. "The experience has been exactly what I was hoping for."

Ho, a chemistry major, originally contacted Grubbs in January, searching for a summer research project that dealt with catalysis or organometallic chemistry, the study of chemical compounds that contain carbon atoms bound to metals. Grubbs told him that postdoctoral scholar Paresma Patel was looking for an intern to help with a chemical-synthesis project related to macular degeneration—a disease that is associated with aging and causes cells in a part of the retina to die. Macular degeneration is estimated to affect 1.8 million Americans, with another 7.3 million at substantial risk of developing the disease. Patel's project, funded by the California Institute for Regenerative Medicine, sounded so interesting that Ho signed up.

Ho, the 2011 Rossum Family SURF Fellow, tells his nonscientist friends and family that the goal of his project is to develop "stem-cell Band-Aids." The idea is that retinal cells, derived from human embryonic stem cells (or hESC-RPE cells), would be attached to the "Band-Aids." Eventually, the strips could be surgically applied to damaged retinas, holding new cells in place long enough to be incorporated into the eye and to restore vision. 

"You might think that you could just stick stem cells in the eye and have them work," Ho says. "But it doesn't work that way because the new cells need to be held on top of the damaged tissue for some time."

What Ho is really doing is trying to coat a thin film of a bio-inert polymer called parylene with something that will encourage retinal cells to latch on. That something is a matrix containing peptides with a repeating arginine-glycine-aspartic acid amino-acid sequence, called RGD peptides.

Previous research has shown that RGD peptides bind to certain receptors expressed in hESC-RPE cells. Patel and Ho felt that the cells might bind better to a multilayered matrix containing RGD peptides, rather than just a single layer. So Ho spent much of the summer designing and carrying out a series of chemical reactions to create such a matrix.

Now that Ho has worked out how to synthesize the matrix, he's trying to figure out a way to get it to coat the parylene film. "Making one thing stick onto another that is prized for its inertness is obviously going to be a little bit difficult," Ho says with a chuckle. But he has plenty of ideas about how to try to make that happen, and he has already started testing them.

Those problem-solving skills, combined with Ho's attention to detail and his enthusiasm for the project, have impressed Patel. "Wilson surpassed the goals set out for him over the summer and will continue doing research in the Grubbs lab throughout the academic year," she says.

For his part, Ho says that he's looking forward to starting his senior year but that he truly enjoyed his SURF experience. "I love learning and being in the lab around such incredibly brilliant people—Grubbs himself, and really everyone in the lab," he says. "I've learned so much from all of them."

Kimm Fesenmaier

Caltech Researchers Find That Disorder Is Key to Nanotube Mystery

PASADENA, Calif.—Scientists often find strange and unexpected things when they look at materials at the nanoscale—the level of single atoms and molecules. This holds true even for the most common materials, such as water.

Case in point: In the last couple of years, researchers have observed that water spontaneously flows into extremely small tubes of graphite or graphene, called carbon nanotubes. This unexpected observation is intriguing because carbon nanotubes hold promise in the emerging fields of nanofluidics and nanofiltration, where nanotubes might be able to help maintain tiny flows or separate impurities from water. However, no one has managed to explain why, at the molecular level, a stable liquid would want to confine itself to such a small area.

Now, using a novel method to calculate the dynamics of water molecules, Caltech researchers believe they have solved the mystery. It turns out that entropy, a measurement of disorder, has been the missing key.

"It's a pretty surprising result," says William Goddard, the Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics at Caltech and director of the Materials and Process Simulation Center. "People normally focus on energy in this problem, not entropy." 

That's because water forms an extensive network of hydrogen bonds, which makes it very stable. Breaking those strong interactions requires energy. And since some bonds have to be broken in order for water to flow into small nanotubes, it would seem unlikely that water would do so freely. 

"What we found is that it's actually a trade off," Goddard says. "You lose some of that good energy stabilization from the bonding, but in the process you gain in entropy."

Entropy is one of the driving forces that determine whether a process will occur spontaneously. It represents the number of ways a system can exist in a particular state.  The more arrangements available to a system, the greater its disorder, and the higher the entropy. And in general, nature proceeds toward disorder.

When water is ideally bonded, all of the hydrogen bonds lock the molecules into place, restricting their freedom and keeping water's entropy low.  What Goddard and postdoctoral scholar Tod Pascal found is that in the case of some nanotubes, water gains enough entropy by entering the tubes to outweigh the energy losses incurred by breaking some of its hydrogen bonds. Therefore, water flows spontaneously into the tubes.

Goddard and Pascal explain their findings in a paper recently published in the Proceedings of the National Academy of Sciences (PNAS). They looked at carbon nanotubes with diameters between 0.8 and 2.7 nanometers and found three different reasons why water would flow freely into the tubes, depending on diameter.

For the smallest nanotubes—those between 0.8 and 1.0 nanometers in diameter—the tubes are so minuscule that water molecules line up nearly single file within them and take on a gaslike state.  That means the normal bonded structure of liquid water breaks down, giving the molecules greater freedom of motion. This increase in entropy draws water into the tubes.

At the next level, where the nanotubes have diameters between 1.1 and 1.2 nanometers, confined water molecules arrange themselves in stacked, icelike crystals. Goddard and Pascal found such nanotubes to be the perfect size—a kind of Goldilocks match—to accommodate crystallized water. These crystal-bonding interactions, not entropy, make it favorable for water to flow into the tubes.

On the largest scale studied—involving tubes whose diameters are still only 1.4 to 2.7 nanometers wide—the researchers found that the confined water molecules behave more like liquid water. However, once again, some of the normal hydrogen bonds are broken, so the molecules exhibit more freedom of motion within the tubes. And the gains in entropy more than compensate for the loss in hydrogen bonding energy.

Because the insides of the carbon nanotubes are far too small for researchers to examine experimentally, Goddard and Pascal studied the dynamics of the confined water molecules in simulations. Using a new method developed by Goddard's group with a supercomputer, they were able to calculate the entropy for the individual water molecules. In the past, such calculations have been difficult and extremely time-consuming. But the new approach, dubbed the two-phase thermodynamic model, has made the determination of entropy values relatively easy for any system.

"The old methods took eight years of computer processing time to arrive at the same entropies that we're now getting in 36 hours," Goddard says. 

The team also ran simulations using an alternative description of water—one where water had its usual properties of energy, density, and viscosity, but lacked its characteristic hydrogen bonding. In that case, water did not want to flow into the nanotubes, providing additional proof that water's naturally occurring low entropy due to extensive hydrogen bonding leads to it spontaneously filling carbon nanotubes when the entropy increases.

Goddard believes that carbon nanotubes could be used to design supermolecules for water purification. By incorporating pores with the same diameters as carbon nanotubes, he thinks a polymer could be made to suck water out of solution. Such a potential application points to the need for a greater understanding of water transport through carbon nanotubes.

The paper, "Entropy and the driving force for the filling of carbon nanotubes with water," appeared in the July 19 issue of PNAS. Yousung Jung of the Korea Advanced Institute of Science and Technology (KAIST) also contributed to the study. Yousung completed a postdoctoral fellowship at Caltech under Nobel Prize winner Rudy Marcus before joining the faculty at KAIST, where he and Goddard are participating in the World Class University program of Korea. They are developing practical systems as part of the Energy, Environment, Water, and Sustainability Initiative, which provided the supercomputers used in this research.

Kimm Fesenmaier

Zewail Honored with the Royal Society's Davy Medal

Nobel Laureate Ahmed Zewail, Pauling Professor of Chemistry and professor of physics, has received the Royal Society's Davy Medal "for his seminal contributions to the study of ultrafast reactions and the understanding of transition states in chemistry, and to dynamic electron microscopy."

First awarded in 1877, the medal is named after the 19th-century British chemist and inventor Sir Humphry Davy, who was a Fellow of the Royal Society and became its president in 1820. The medal is of bronze, is accompanied by a gift of £1,000, and is awarded annually "for an outstandingly important recent discovery in any branch of chemistry." The Royal Society—of which Zewail was elected a foreign member in 2001—is the United Kingdom's independent academy for science and was founded in 1660.

A member of the U.S. National Academy of Sciences and a fellow of the American Academy of Arts and Sciences, Zewail received the Nobel Prize in Chemistry in 1999. In 2009 he was named to the President's Council of Advisors on Science and Technology, as well as an envoy in the new U.S. Science Envoy Program, created to foster science and technology collaborations between the United States and nations throughout the Middle East, North Africa, and South and Southeast Asia. His many other honors include the 2011 Priestley Medal, the American Chemical Society's most prestigious award.

Zewail received his BSc from Alexandria University in 1967 and his PhD from the University of Pennsylvania in 1974. He joined Caltech's faculty in 1976 as an assistant professor, becoming professor in 1982 and Pauling Professor in 1990.

Michael Farquhar
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Alternative Energy Expert Frances Arnold Profiled in the LA Times

For biochemist and chemical engineer Frances Arnold, the road to success has not been straight and narrow. In fact, she has often bucked the academic tradition of rigorous, time-consuming pre-experiment methodology for a more fast and furious approach to research.

"I said 'OK, if one experiment doesn't work I'm going to do a million experiments, and I don't care if 999,999 don't work. I'm going to find the one that does,'" said Arnold, the Dickinson Professor of Chemical Engineering, Bioengineering and Biochemistry at Caltech, in a profile published online and in the July 3 print edition of the Los Angeles Times.

Her unconventional approach has paid off. She is co-founder of a company that develops liquid fuel from plants and oversees a lab of 20 students and researchers dedicated to alternative energy.

To learn more about Arnold's career path, including a stint as a cab driver, read the full profile here

Katie Neith
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Improving Health Assessments with a Single Cell

Caltech researchers develop quick, low-cost, and portable microchip for immune monitoring and clinical applications

PASADENA, Calif.—There's a wealth of health information hiding in the human immune system. Accessing it, however, can be very challenging, as the many and complex roles that the immune system plays can mask the critical information that is relevant to addressing specific health issues. Now, research led by scientists from the California Institute of Technology (Caltech) has shown that a new generation of microchips developed by the team can quickly and inexpensively assess immune function by examining biomarkers—proteins that can reflect the response of the immune system to disease—from single cells.

The scientists reported on their advanced technology in the May 22 online issue of Nature Medicine.

"The technology permits us for the first time to quantitatively measure the levels of many functional proteins from single, rare immune cells," says James Heath, the Elizabeth W. Gilloon Professor and professor of chemistry at Caltech and corresponding author of the study. "The functional proteins are the ones that are secreted by the cells, and they control biological processes such as cell replication and inflammation and, specific to our study, tumor killing."

In 2008, Heath—an expert in molecular electronics and personalized medicine—led the development of a "barcode chip" that, using just a pinprick's worth of blood, could measure the concentrations of dozens of proteins, including those that herald the presence of diseases like cancer and heart disease. This latest single-cell barcode chip (SCBC) device builds upon the success of that initial design, which is currently being utilized in diagnostic medical testing of certain cancer patients.

The researchers tested the chip by measuring a cancer patient's response to a type of cell-based immunotherapy designed to target and kill tumor cells. The only way to know if the therapy is doing its job is to measure many proteins at the same time from the individual cells that were targeting the tumor. The SCBC aced this test, generating readouts of a dozen secreted biomarkers—each of which represented a distinct cell function—and taking those readings from about a thousand single cells simultaneously.

The team was able to conduct a proof-of-concept study by looking at samples from a melanoma patient participating in the immunotherapy trials, and comparing those results to similar samples from three healthy subjects.

"This technology has the potential to be used routinely to monitor immune system performance," says Chao Ma, a graduate student in Heath's lab at Caltech's NanoSystems Biology Cancer Center and lead author of the Nature Medicine paper. "For example, it can be directly used to evaluate the effectiveness of certain classes of therapeutics, such as vaccines and other immunotherapies."

According to Ma, the technology is minimally invasive, cost-effective, and highly informative. The goal, he says, is to help physicians closely track the effectiveness of a therapy, and to rapidly alter or switch that therapy for the maximum benefit of the patient.

"The research fully demonstrates real-life clinical use of our revolutionary technology," Ma says.

The next step for the team will be to systematically apply the technology to clinical studies. The researchers have already begun to test the technology in additional patient populations, and to combine the SCBC with existing assays in order to get a more comprehensive picture of a therapy's efficacy.

In fact, the same study that showed the microchip's efficacy is already helping the researchers better evaluate the specific cancer immunotherapy trial, from which the patient in the study was drawn. "We are doing these same types of measurements on similar patients but at a significantly higher level of detail, and at many time points over the course of the cancer immunotherapy procedure," explains Heath. "It is helping us put together a 'movie' of the patient's immune system during the therapy, and it is providing us with some very surprising but also valuable insights into how the therapy works and how we might work with our UCLA colleagues to improve it."

"Application of this technology provides an unprecedented understanding of the human immune system by allowing an efficient and multiplexed functional readout of immune responses using limiting numbers of lymphocytes," says Antoni Ribas, associate professor of medicine and physician who led the clinical trial portion of the study at UCLA's Jonsson Comprehensive Cancer Center.

The other Caltech authors of the Nature Medicine paper, "A clinical microchip for evaluation of single immune cells reveals high functional heterogeneity in phenotypically similar T cells," are postdoctoral scholar Qihui Shi; Rong Fan, former postdoctoral scholar; former graduate students Habib Ahmad and Gabriel Kwong; and Chao-Chao Liu, former undergraduate student. Begonya Comin-Andiux, assistant professor of surgery; Thinle Chodon, assistant researcher of medicine; Richard C. Koya, assistant professor of surgery; and Caius G. Radu, associate professor of medical and molecular pharmacology from UCLA's Jonsson Comprehensive Cancer Center also contributed to the study. 

The work was funded by the National Cancer Institute, the Ivy Foundation, the Jean Perkins Foundation, the California Institute for Regenerative Medicine, the Caltech/UCLA Joint Center for Translational Medicine, the Melanoma Research Alliance, and the National Institutes of Health.

Katie Neith
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ACS Honors Zewail

On March 29, the world's largest scientific society will bestow its highest honor on Ahmed H. Zewail, Caltech's Linus Pauling Professor of Chemistry and professor of physics. 

At its 241st National Meeting and Exposition in Anaheim, California, the American Chemical Society (ACS) will present the 2011 Priestley Medal to Zewail. The ACS, whose membership numbers over 163,000, annually awards the Priestley Medal "for distinguished services to chemistry," as stated on the back of the gold medallion. According to ACS guidelines, no individual may be so honored more than once. 

Zewail is being recognized for his pioneering work in femtochemistry, the visual study of chemical processes occurring on a scale of millionths of billionths of a second. In that period of time, light travels only about the diameter of a large virus (300 nanometers), a distance shorter even than one wavelength of visible light. 

Femtochemistry uses extremely brief flashes of laser light to illuminate molecules in motion, exposing individual images that are then stitched together in chronological order. The resulting high-resolution, slow-motion electron microscopy "movie" provides a way for scientists to view chemical processes over time, and at an unprecedented resolution.

Zewail, winner of the 1999 Nobel Prize in Chemistry, has dubbed the new science "four-dimensional (4D) electron microscopy," because it encompasses not only the standard three spatial dimensions but also the dimension of time. Applications of the technology include improved understanding of the dynamics of chemical processes, visualization of the makeup of new materials, and insights into the function of cells and other biological structures. 

The Priestley Medal commemorates the life of British scientist Joseph Priestley, who discovered oxygen in 1774 and spent the last 10 years of his life in the United States. Previous recipients include legendary Caltech chemistry professors Linus Pauling (1984), John D. Roberts (1987), and Harry B. Gray (1991).


Dave Zobel
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President Obama's Science Advisor to Speak on "Science, Technology and Innovation Policy"

PASADENA, Calif.—Dr. John P. Holdren, assistant to the president for science and technology, director of the White House Office of Science and Technology Policy, and co-chair of the President's Council of Advisors on Science and Technology (PCAST), will be the speaker at the 2011 DuBridge Distinguished Lecture Series at the California Institute of Technology (Caltech). The event, which is free and open to the public, will take place at 8 p.m., April 19, in Beckman Auditorium, 332 S. Michigan Ave., on the Caltech campus in Pasadena.

The title of Dr. Holdren's talk is "Science, Technology and Innovation Policy in the Obama Administration." In a question-and-answer session after his speech, Dr. Holdren will be accompanied by Nobel Laureate Dr. Ahmed Zewail, Linus Pauling Professor of Chemistry and professor of physics at Caltech. Dr. Zewail is a member of PCAST and is the U.S. Science Envoy to the Middle East. Since the January 25 revolution in Egypt, he has played a critical role in aiding the transition to a democratic state and to development in his mother country.

Prior to joining the Obama administration, Dr. Holdren was Teresa and John Heinz Professor of Environmental Policy and director of the Program on Science, Technology, and Public Policy at Harvard University's Kennedy School of Government, as well as a professor in Harvard's Department of Earth and Planetary Sciences and director of the independent, nonprofit Woods Hole Research Center.

In the early 1970s, Dr. Holdren was a member of the Caltech community as a senior research fellow in the Environmental Quality Laboratory. He later joined the faculty of UC Berkeley, where he co-led until 1996 the interdisciplinary graduate-degree program in energy and resources. Dr. Holdren holds advanced degrees in aerospace engineering and theoretical plasma physics from MIT and Stanford and is a member of the National Academy of Sciences, the National Academy of Engineering, and the American Academy of Arts and Sciences, as well as a foreign member of the Royal Society of London. Dr. Holdren served as a member of the MacArthur Foundation's board of trustees from 1991 to 2005, as chair of the National Academy of Sciences Committee on International Security and Arms Control from 1994 to 2005, and as co-chair of the independent, bipartisan National Commission on Energy Policy from 2002 to 2009.

A former president of the American Association for the Advancement of Science, Dr. Holdren's awards include a MacArthur Foundation Prize Fellowship, the John Heinz Prize in Public Policy, the Tyler Prize for Environmental Achievement, and the Volvo Environment Prize. In December 1995 he gave the acceptance lecture for the Nobel Peace Prize on behalf of the Pugwash Conferences on Science and World Affairs, an international organization of scientists and public figures in which he held leadership positions from 1982 to 1997.

The Lee A. DuBridge Distinguished Lecture series brings prominent speakers of national and international importance to the Caltech campus. The series was inaugurated in 1996 in honor of Lee A. DuBridge, president of Caltech from 1946 to 1969. DuBridge, who died in 1994, was once called America's "senior statesman of science" by Time magazine, and was considered an exemplary research-university president in an era of vast scientific, societal, and educational change. He guided the growth of the modern Caltech while maintaining an understanding and interest in national affairs that was rare among university presidents. Previous DuBridge speakers include Charles "Charlie" Munger, Warren Buffett, Walter Cronkite, John Hume, Jack Valenti, and Judy Woodruff.

No tickets are necessary for the event; at least 700 seats will be available on a first-come, first-served basis. Doors open at 7:30 p.m. For more information on the lecture, call (626) 395-4652 or, outside the greater Pasadena area, call toll free, (888) 222-5832.

Kathy Svitil
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Nobel Laureate Ahmed Zewail to be Caltech Commencement Speaker

PASADENA, Calif.—Renowned chemist and Nobel laureate Ahmed Zewail, Linus Pauling Professor of Chemistry and professor of physics at the California Institute of Technology (Caltech), will be the speaker for Caltech's 117th annual commencement ceremony, which will take place at 10 a.m. on June 10 of this year.

"Professor Zewail is an esteemed scientist and statesman," says Caltech president Jean-Lou Chameau. "Our graduates will benefit greatly from his wisdom as they prepare to enter a world where scientists and engineers are increasingly called upon to provide leadership throughout the civic arena."

Zewail received the 1999 Nobel Prize in Chemistry for his groundbreaking research that established the field of femtochemistry by enabling chemical reactions to be studied in real time, on a scale of one quadrillionth of a second. More recently, he and his group have developed four-dimensional electron microscopy for direct imaging of matter in 3-D and in time, with applications spanning physical and biological sciences.

In 2009, Zewail was appointed to President Obama's Council of Advisors on Science and Technology. That same year, he was named U.S. Science Envoy to the Middle East as part of a program created by the State Department to foster science and technology collaborations between the United States and nations throughout the Middle East, North Africa, and South and Southeast Asia. Since the January 25th revolution in Egypt, he has played a critical role in his home country's development and transition to a democratic state.

Zewail has a long-standing interest in global affairs, particularly as they relate to science, education, and world peace. His commentaries on these global issues have appeared in the International Herald Tribune, the New York Times, the Los Angeles Times, and the Wall Street Journal, among other publications. He has written more than 500 articles and books and has given public addresses all over the world.

The Caltech professor's numerous honors include the Albert Einstein World Award of Science, the Benjamin Franklin Medal, the Robert A. Welch Award in Chemistry, the Leonardo da Vinci Award, the Wolf Prize, and the King Faisal International Prize. He was awarded the Grand Collar of the Order of the Nile, Egypt's highest state honor, and was featured on postage stamps issued to honor his contributions to science and humanity. He holds honorary degrees from 40 universities around the world and is an elected member of many professional academies and societies, including the National Academy of Sciences, the American Philosophical Society, the Royal Society of London, and the Swedish, Russian, Chinese, and French Academies.

Zewail completed his early education in Egypt, receiving his Bachelor of Science and Master of Science degrees in chemistry from Alexandria University. He obtained a PhD in chemical physics from the University of Pennsylvania and, after a postdoctoral fellowship at the University of California, Berkeley, joined the faculty at Caltech in 1976.

Caltech's 2010 commencement speaker was NASA Administrator Charles Bolden.

Kathy Svitil
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Arnold Wins Draper Prize

Frances Arnold has been named co-recipient of the Charles Stark Draper Prize by the National Academy of Engineering (NAE). Arnold, Caltech's Dick and Barbara Dickinson Professor of Chemical Engineering, Bioengineering and Biochemistry, was awarded the $500,000 prize—the engineering profession's highest honor—for a method called directed evolution, used worldwide to guide the creation of certain properties in proteins and cells, allowing the engineering of novel enzymes and biocatalytic processes for pharmaceutical and chemical products.

Arnold showed that randomly mutating genes of a targeted protein, especially an enzyme, would result in some new proteins having more desirable traits than they did before the mutation. She selected the best proteins and repeated this process multiple times, essentially directing the evolution of the proteins until they had properties needed for a particular use.

The Draper Prize was given jointly to Arnold and Willem P.C. Stemmer, the CEO of Amunix.

The NAE also named Caltech alumnus Leroy Hood the recipient of the Fritz J. and Dolores H. Russ Prize—a $500,000 biennial award recognizing "a bioengineering achievement that significantly improves the human condition"—for the development of an automated DNA sequencer that "revolutionized biomedicine and forensic science," according to the prize announcement. Now the president of the Institute for Systems Biology, Hood was on the Caltech faculty when he developed the sequencer in the 1980s. 

Both prizes will be presented in Washington, D.C., on February 22.

Kathy Svitil
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Caltech/JPL Experiments Improve Accuracy of Ozone Predictions in Air-Quality Models

Team says current models may underestimate ozone levels; findings made by characterizing rates of key chemical reactions

PASADENA, Calif.—A team of scientists led by researchers from the California Institute of Technology (Caltech) and NASA's Jet Propulsion Laboratory (JPL) have fully characterized a key chemical reaction that affects the formation of pollutants in smoggy air. The findings suggest that in the most polluted parts of Los Angeles—and on the most polluted days in those areas—current models are underestimating ozone levels, by between 5 to 10 percent.

The results—published in this week's issue of the journal Science—are likely to have "a small but significant impact on the predictions of computer models used to assess air quality, regulate emissions, and estimate the health impact of air pollution, " says Mitchio Okumura, professor of chemical physics at Caltech and one of the principal investigators on the research.

“This work demonstrates how important accurate laboratory measurements are to our understanding of the atmosphere,” added JPL Senior Research Scientist Stanley P. Sander, who led that team's effort.

The key reaction in question in this research is the reaction between nitrogen dioxide, NO2, and the hydroxyl radical, OH. In the presence of sunlight, these two, along with volatile organic compounds (VOCs), play important roles in the chemical reactions that form ozone.

Until the last decade or so, it was thought that NO2 and OH combine only to make nitric acid, HONO2, a fairly stable molecule with a long lifespan in the atmosphere. "HONO2, or nitric acid, dissolves in rainwater, so that the molecules get washed away," Okumura explains. "It's basically a sink for these radicals, taking them out of the ozone equation and thus slowing down the rate of ozone formation."

Chemists had suspected, however, that a second reaction might occur as well: one that creates a compound called HOONO (pronounced WHO-no), otherwise known as peroxynitrous acid. HOONO is much less stable in the atmosphere, falling apart quickly after being created, and thus releasing the OH and NO2 back for use in the ozone-creation cycle.

But what was not known with any reasonable certainty—until now—is how fast these reactions occur, and how much HONO2 is created relative to the amount of HOONO created. Those relative amounts are known as the branching ratio, so called because OH and NO2 can chemically transform, or branch, into either HONO2 or HOONO.

Enter the Caltech and JPL teams. The JPL team took the lead on measuring the rate at which the OH + NO2 reaction produces both HONO2 and HOONO. They did this using "an advanced chemical reactor built at JPL that was designed to measure reaction rates with very high accuracy," says Sander.

Once the scientists had determined the combined reaction rate for the two possible products—coming up with rates that are on the higher rather than the lower end of the scale of previous estimates—the Caltech group took the lead to try to uncover the branching ratio, or the ratio of the rates of the two separate processes.

Using a powerful laser measurement technique called cavity ringdown spectroscopy, the team was able to detect both products being created in the lab in real time, says Okumura. "We could start the reaction and watch, within microseconds, the products being formed," he says. "That allowed us to measure the species immediately after they were formed, and before they got lost in other side reactions. That is what allowed us to figure out the branching ratio."

Because HOONO was not a well-studied molecule, another key was using state-of-the-art theoretical calculations; for this, the authors enlisted Anne McCoy, professor of chemistry at The Ohio State University. “Solving this atmospheric chemistry problem required us to use many tools from modern chemical physics,” says Okumura.

"This work was the synthesis of two very different and difficult experiments," adds Andrew Mollner, the Science paper's first author and a former Caltech graduate student who is now at the Aerospace Corporation. "While neither experiment in isolation provided definitive results, by combining the two data sets, the parameters needed for air-quality models could be precisely determined."

In the end, what they found was that the loss of OH and NO2 is slower than what was previously thought—although the reactions are fast, fewer of the radicals are going into the nitric acid sink than had been supposed, and more of it is ending up as HOONO. "This means less of the OH and NO2 go away, leading to proportionately more ozone, mostly in polluted areas," Okumura says.

Just how much more? To try to get a handle on how their results might affect predictions of ozone levels, they turned to Robert Harley, professor of environmental engineering at the University of California, Berkeley, and William Carter, a research chemist at the University of California, Riverside—both experts in atmospheric modeling—to look at the ratio's impact on predictions of ozone concentrations in various parts of Los Angeles during the summer of 2010.

The result: "In the most polluted areas of L.A.," says Okumura, "they calculated up to 10 percent more ozone production when they used the new rate for nitric acid formation."

Okumura adds that this strong effect would only occur during the times of the year when it's most polluted, not all year long. Still, he says, considering the significant health hazards ozone can have—recent research has reported that a 10 part-per-billion increase in ozone concentration may lead to a four percent increase in deaths from respiratory causes—any increase in expected ozone levels will be important to "people who regulate emissions and evaluate health risks." The precision of these results reduces the uncertainty in the models—an important step in the ongoing effort to improve the accuracy of the models used by those policymakers.

Okumura believes that this work will cause other scientists to reevaluate recommendations made to modelers as to the best parameters to use. For the team, however, the next step is to start looking at "a wider range of atmospheric conditions where this reaction may also be very important."

Sander agrees. "The present work focused on atmospheric conditions related to urban smog—i.e., relatively warm temperatures and high atmospheric pressure," he says. "But the OH + NO2 reaction is important at many other altitudes. Future work by the two groups will focus on the parts of the atmosphere affected by long-range transport of pollution by high-altitude winds (the middle and upper troposphere) and where ozone depletion from man-made substances is important (the stratosphere)."

In addition to Okumura, Sander, Mollner, McCoy, Harley, and Carter, the other authors on the Science paper, "Rate of Gas Phase Association of Hydroxyl Radical and Nitrogen Dioxide," are postdoctoral fellow Lin Feng and graduate student Matthew Sprague, both from Caltech; former JPL postdoctoral researchers Sivakumaran Valluvadasan, William Bloss, and Daniel Milligan; and postdoctoral fellow Philip Martien from the University of California, Berkeley.

Their work was supported by grants from NASA, the California Air Resources Board, and the National Science Foundation, and by a NASA Earth Systems Science Fellowship and a Department of Defense National Defense Science and Engineering Graduate Fellowship.

JPL is a federally funded research and development facility managed by Caltech for NASA.

Lori Oliwenstein


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