Anne Thompson, Ph.D. ’78, chemistry, aboard the DC-8 in October 1997.

FOLLOW THE OZONE

A Bryn Mawr scientist studies the impact of human activities on the remote atmosphere.

By Alicia Bessette

It is hot on the plane. You strap yourself into your seat in the cockpit as the navigator, pilot and copilot prepare for take-off. Behind you your crew of 35—chemists, meteorologists, technicians, engineers—are setting up their instruments, a shiny collection of probes and computers.

Using a headset, you brief the scientific crew: “We’re trying to get stratospheric air today. We want to head north.” After the plane takes off from the Shannon airstrip it heads in the direction you requested, at an altitude of 33,000 feet. You are in the upper troposphere, where commercial airplanes travel in organized patterns, like a highway in the sky. It is a gray, cloudy day.

After a few minutes of sampling gases in the upper troposphere, one of your scientists taps you on the shoulder eagerly and hands you a printout of measurements to study. “Look at this! I think we just detected lightning,” she says. You shout to the navigator, “Can we go down 2,000 feet so we can see if it’s still there?”After a few status checks the navigator nods, and the pilot takes the plane downward. During the descent you get the sensation that your stomach has risen into your ribcage.

You are not an actor in a sci-fi movie, and you are not dreaming. You are Anne Thompson, Ph.D. ’78, NASA scientist and atmospheric chemist. The research aircraft is the DC-8, NASA’s “flying laboratory. It’s like walking onto an airplane, but half the seats are missing, and the rest of the space is taken up with highly specialized instruments,” Thompson said in a recent interview.

Thompson was awarded the 1998 William Nordberg Medal from the international Committee on Space Research (COSPAR) in Nagoya, Japan, last summer. COSPAR awards the medal every two years to an outstanding space scientist. Members cited Thompson’s “contributions in understanding the chemistry of tropospheric ozone.”

Good ozone, bad ozone
Ozone, the object of much concern and publicity in recent decades, is the layer of molecules in the stratosphere, the region from six to 30 miles above the earth. It absorbs ultraviolet sunlight before it reaches the earth. This is “good ozone,” said Thompson.

But you might say that ozone is two-faced, because in the troposphere—the lower region of the atmosphere extending up to six miles above the earth—“ozone is a bad guy, not a good guy,” said Thompson. While the ozone in the troposphere and the stratosphere is chemically identical, ozone in the troposphere reacts strongly with other molecules, and high levels of ozone can be toxic to plants and animals. It is a key component of urban smog and also harms crop production, forest growth and human health.

Where is all the ozone coming from? Emissions from cars are one contributor. Thompson was one of two head scientists in the NASA experiment SONEX (for NASA’s Subsonic Assessment Ozone and Nitrogen Oxide Experiment), which studied the effects of plane emissions on the atmosphere. “Airplanes burn clean just like cars now, but we have too many of them,” Thompson said. “Between North America and Europe the daily traffic is 300 to 700 planes, including the jumbo jets you go to Europe in and the freighters like FedEx. We wanted to look at the air quality at this interstate in the sky.”

The DC-8 flew in areas above Ireland, the Azores, Maine and Nova Scotia last fall while SONEX scientists on board sampled air from these flight corridors, trying to assess how aviation impacts the troposphere.

“So we’re not worrying about ozone as smog like today outside the window, but about the global impact on a much larger scale,” said Thompson. “I’m not talking about the airplane when it’s landing and tak- ing off, but what it’s doing in the major corridors over the globe. What we were looking at in the DC-8 were the chemicals coming out of the engines of these jets.”

To Thompson and her crew the most important chemicals are nitrogen oxides, an ingredient in the mixture of chemicals that makes ozone. “If you increase the amount of nitrogen oxides,” Thompson explained, “it has the most influence on whether you make more ozone.”

But airplanes are not the only thing putting nitrogen oxides in the upper troposphere. Lightning and the stratosphere are two natural sources of nitrogen oxides.

“And the other source,” said Thompson, “is pollution on the ground, like from our cars. A big thunder storm acts like a giant pump, and in 20 to 30 minutes that pollution from the ground can be up there at 30,000 feet. So what you do is you measure the nitrogen oxides. But they’re not labeled, ‘I came from a plane,’ ‘I came from the stratosphere,’ ‘I came from the ground,’ ‘I came from lightning.’ You have to measure all those other chemicals that fingerprint the source.”

The instruments on the DC-8 are one-of-a-kind, but based on the same designs that you would find in a chemistry lab in Bryn Mawr. “Plus they’ve been certified to operate on an airplane,” Thompson said. “So you bring in the air and run it through some kind of sampler, and each instrument is checking for a different chemical or set of chemicals. “It’s a puzzle that I’m trying to figure out; the contributions of the four sources are difficult to interpret. I’m still trying to work on the data from the flights out of Maine. But we can definitely see a signal.”

Biomass burning
This work is similar to Thompson’s work on biomass burning, for which COSPAR awarded her the Nordberg Medal. “The single thread that runs through both experiments is that we’re looking at the impact of human activities on the remote atmosphere. This is what distinguishes our work at NASA with this large research jet from what the EPA does, which is worry about pollution close to urban sources. We’re looking at global pollution, far from the source.”

Biomass burning is “seasonal agriculture burning, what we used to do in this country,” Thompson said. “It’s the end of the season, you have to clear a field so you light a match and you take care of it.” In 1992 Thompson and her crew flew out of Brazil and South Africa on the DC-8, trying to determine how much ozone was being created from the burning. “We much more dramatically saw this mixture of the burning pollution getting pumped up from Brazil and going out over the Atlantic. Over the African continent, the contribution of the burning—you could actually see it,” she said.

Thompson is based in Greenbelt, Maryland, at NASA’s Goddard Space Flight Research Center. She is close to the University of Maryland and local high and middle schools, where she is a frequent visitor to natural science classrooms. This involvement allows her to correct and explain common misconceptions. “For example we’ve put a lot of worry into savanna or grasslands burning and also the rain forest that’s disappearing,” said Thompson. “But they’re two different processes. One of them is a human impact that’s been going on for millennia, and the other is a rapid change in the ecological system due to current economic pressures. Another dimension is that the lightning in the tropics is also helping to make ozone. You need to understand how complicated these issues are. A simple chemical like ozone is affected by a range of processes that go from natural to traditional human activity to accelerating, contemporary activity,” she said.

“A genuine concern to me is that if people don’t understand these issues, then in the everyday decisions you make about what to buy, what movements to join, whatever your extracurricular activities may be, your activism, your political decisions and so on, you can so easily make misinformed choices.”

Young people, science careers
Thompson also hopes to interest more students in science careers. “I’m energized by the educational process of trying to transmit my enthusiasm and knowledge that I picked up on the job with the current generation of students.

“I never could have pictured what my life has turned into when I had my nose hard on the grindstone in chemistry labs,” said Thompson. “You have to be open to doing something completely different and, maybe at first glance, a little bit crazy. It’s risk taking, and I never stop taking them. I wouldn’t be where I am if I didn’t take risks. You always have to have your feet on the ground in one sense, but have willingness to go places. You know, I didn’t particularly want to live in California back in 1979 when I took a second post-doc in San Diego. But I just followed the ozone.”

Why ozone? “It’s astounding to me now when I look back,” she said. “I have an uncle who’s an oceanographer. We were at Thanksgiving dinner the year I was to graduate with my Ph.D. He asked me, ‘Have you ever thought about chemistry in the atmosphere?’ I just kind of looked at him and said, ‘Really?’ It had never occurred to me. Maybe at Bryn Mawr if I had paid attention to the other people in Park Hall, I would have known! But I don’t think the Crawfords and other people were teaching environmental science at the time. I took my uncle’s question as good advice, and I spent a year at Woods Hole Oceanographic Institution, where I won a post-doctoral fellowship in their chemistry department.

“Six weeks after I graduated from Bryn Mawr I was on an oceanographic cruise in the middle of the Pacific ocean. I had one look at blue water, and that just kind of did it for me. I got hooked by the curiosity, the fascination of studying the natural environment and putting my chemical training to work,” Thompson said.

“One thing I discovered that has stayed with me, however, is how much I owe to the sound and rigorous training I got as a physical chemistry Ph.D. student in George Zimmerman’s lab. One never loses the need to be self-critical, to ask how the job can be done better. Have we thought of all the uncertainties, in our models, in our interpretation of the data, in the satellite products we are now generating for global distribution on the world wide web? This is the approach I learned at Bryn Mawr.

“Another thing that resonates with me in this kind of work is the international aspect. Nobody owns the globe, and you can’t do the job if you don’t learn to work with different environments and cultures. At NASA, we try to be ambassadors for good science.”

But she is dismayed that many young people don’t consider careers in science. “Bryn Mawr puts out an astounding number of women scientists, and I’ve come to appreciate the school a lot better than when I went there, and what it does for women.

“But I serve on various committees and work with professors from schools all over the country, and I hear, ‘We can’t find graduate students in atmospheric and ocean sciences.’ I hear, ‘We fly them in for a weekend to recruit them, and we’re still not getting them.’

And to Thompson, that’s puzzling: “I have the most exciting career in the world!” she said. “I wake up every morning, and I’m so excited to go to work! I can’t wait to get there. That’s how I feel about what I do. I feel like the luckiest, most privileged person in the world.”

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