Ozone and the lack of it

Rarely before the 1980s did a scientific issue jump from the corridors of research to the halls of international diplomacy in less than a decade. Such was the case when a profound threat to the Antarctic’s protective layer of stratospheric ozone became apparent.

One of the sparkplugs for NCAR’s involvement in ozone-related research was Paul Crutzen, who directed the center’s Atmospheric Quality Division (now the Atmospheric Chemistry Division) from 1977 to 1980. Crutzen arrived with an impressive résumé: in 1970, he’d established that nitrogen oxides could help deplete stratospheric ozone. A few years later, Mario Molina and Sherwood Rowland (both at the University of California, Irvine) showed that chlorofluorocarbons (CFCs) were another threat. The three scientists shared a Nobel Prize in 1995 for their pioneering work.

Photograph of a woman dressed in a parka standing in the snow
Susan Solomon (NOAA) led a 1986 expedition to the Antarctic that confirmed the process leading to the continent’s seasonal ozone loss. (Photo courtesy NOAA.)

Under Crutzen’s 1970s leadership, NCAR chemists and instrument developers put together an impressive array of sensors designed to sample trace gases—scarce but critical components of the troposphere and stratosphere—from airborne and balloon-borne platforms. Ozone was just one of many constituents studied with these tools. Most field studies of the day focused on pollution above populated areas, while the scant data from Antarctica didn’t yet point to a dramatic loss of ozone.

At the same time, a graduate student named Susan Solomon was poring over dozens of vertical profiles of ozone from around the globe. Her doctoral work at the University of California, Berkeley, was supported in part by a fellowship from NCAR’s Advanced Study Program, which brought Solomon to NCAR in 1977–78. Working with NCAR advisers Crutzen, Jack Fishman, and Raymond Roble, Solomon hunted for differences in tropospheric ozone between the polluted Northern Hemisphere and relatively clean Southern Hemisphere. “Remarkably, we were able to find a fair amount of data, enough to really see the difference between the two hemispheres,” she said in a subsequent interview.

Later, as she embarked on a career at NOAA, Solomon collaborated with NCAR’s Rolando Garcia on modeling ozone. They built a two-dimensional model, unveiled in a 1983 paper, that combined chemistry and dynamics to simulate the northward and southward movement of stratospheric ozone around the globe. According to Garcia, “The model worked very well. In 1983, ozone was thought to be reasonably well understood, but it wasn’t. We happened to have the right tool to look at the problem that was uncovered by observations published in 1985.”

Visualization of Earth with yellow, red and green overlays
The extent of azone depletion above Antarctica in 1986 peaked on 6 October, as shown in purple on this map derived from the satellite-borne Total Ozone Mapping Spectrometer. (Courtesy NASA.)

That year, scientists from the British Antarctic Survey made a blockbuster announcement: more than a third of the ozone in the Antarctic stratosphere had begun to disappear for a few weeks each spring. Solomon was transfixed by the work, which she saw early on as a reviewer: “I found it to be the most exciting paper I had ever seen.”

The findings generated huge public interest and spurred scientists to learn more. By late 1986, Solomon was leading an NSF-funded trip to Antarctica to document the life cycle of the  “ozone hole” and the chlorine-driven chemistry feeding it. The project confirmed Solomon’s hypothesis: ozone was being destroyed on the backs of polar stratospheric cloud particles. These clouds—which form only in the intense cold above the poles in wintertime—served as a framework on which small amounts of chlorine from CFCs could eliminate vast amounts of ozone.

The case was convincing enough to push most of the world’s nations into adopting the 1987 Montreal Protocol, aimed at stemming the growth of CFCs. Even with that potential solution taking shape, there was more science to be done. Roughly 100 scientists trekked to Antarctica in late 1987 to carry out a more intensive study of the ozone hole, with an NCAR team bringing several instruments in tow.

Almost before they could catch their breath, the team headed to the Arctic in early 1989 to find out whether conditions might support a northern ozone hole. Two instruments were rushed to completion for the project: a chromatograph/spectrometer for measuring bromine, and another spectrometer that counted and sized particles within polar stratospheric clouds. The campaign found ominous signals of potential Arctic ozone loss: “The precursor stages are very similar to the Antarctic,” said NCAR’s William Mankin shortly afterward. However, the boreal landscape makes a big difference. The position of oceans, mountains, and continents across the Northern Hemisphere leads to large-scale atmospheric waves that tend to warm and disrupt the wintertime vortex that would otherwise allow ozone destruction to proceed unchecked.

With the Montreal Protocol putting the brakes on CFCs, ozone depletion has begun to level off in recent years. But scientists are still exploring the process on many fronts. For instance, the seasonal ozone hole may help explain why interior Antarctica hasn’t warmed in sync with the rest of the planet.

“With the new climate connections, Antarctic ozone is still fascinating to study and explore,” says Solomon today.

 

Today — The chemistry of northern air

Photo of Anne Thompson

"Instruments in ARCTAS measured things we hadn't been able to measure before."

—Anne Thompson, Pennsylvania State University

A key component of smog, tropospheric ozone is a recurring problem in the nation’s big cities, where calm weather can trap pollutants in a sunlight-cooked stew close to the ground. Ground-level ozone also makes its presence known in the Arctic, where pollutants drift during the dark winter, then react to form ozone as sunlight returns in the spring. Huge fires across the forested north in summer add their own influence to air quality.

The seasonal sweep of ozone production and related chemistry across the far north was clarified in a 2008 project called ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites). The largest airborne project ever to study boreal climate, ARCTAS involved three NASA aircraft on springtime flights out of Fairbanks, Alaska, and early-summer missions above western Canada. Flying near thick plumes of smoke, several of the airborne instruments captured their highest-ever concentrations of carbon monoxide, carbon dioxide, and nitrogen oxide.

Among the project’s many investigators, Anne Thompson (Pennsylvania State University) worked with NOAA’s Samuel Oltmans in planning a fleet of ground-based ozone sensors combined with ozonesondes. The latter consist of weather observing packages (radiosondes), plus ozone-measuring equipment and a balloon large enough to loft the sensors to 35 kilometers (22 miles), well into the stratosphere.

photo of man dressed in parka taking measurements in snow
James France (Royal Holloway, University of London) samples optical properties of the snow near Barrow, Alaska

As a NASA scientist, Thompson had pioneered the concept of coordinated ozonesonde networks for comparison with satellite data. In ARCTAS, these networks complemented the flight data well, says Thompson: “It gave us the same information from the same spot every single day, so we could gather statistics to verify satellite algorithms and evaluate the models used for ARCTAS analysis.” Together, the ozone data have helped researchers to analyze the atmospheric fingerprints of fires in California, Canada, and Siberia, as well as Asian pollution.

A 2009 project focused more specifically on the dramatic changes in Arctic air wrought by springtime sunlight. The OASIS project (Ocean–Atmosphere–Sea Ice–Snowpack) brought instrumented towers, lidars, and balloons to the region around Barrow, Alaska—including a station located atop the ice just offshore, with armed sentries keeping polar bears at bay.

When seasonal sunlight returns to the Arctic, complex reactions pull mercury into snow and ice. Often occurring in localized pulses, these mercury depletion events can last for as long as a few days. Concurrent with the mercury loss, ozone drops to near-zero levels. How these two depletions are chemically linked, and why they tend to spike in such distinct times and places, are among the questions being probed as scientists analyze the results from the OASIS field phase.

While OASIS isn’t focused on climate change per se, the project will shed light on chemistry shaping the future of this rapidly changing region. “We’re trying to find out how these chemicals get there, how the Arctic tolerates their intrusion, and what possible impacts there will be to the ecosystem should the Arctic Ocean melt,” said Jan Bottenheim (Environment Canada) as the fieldwork unfolded.

 

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