North America's ozone: a closer look

by Bob Henson

Scientists have clarified the process by which ozone—an essential shield in the stratosphere, but a pollutant at lower levels—reaches its peak abundance above North America each spring. The new findings come from a comprehensive study that links computer models with airborne measurements of gases, particles, and ultraviolet radiation.

A set of papers outlining results from TOPSE, the Tropospheric Ozone Production about the Spring Equinox experiment, appears in the 28 February issue of the Journal of Geophysical Research–Atmospheres (JGR). The principal investigators are NCAR scientists Elliot Atlas, Christopher Cantrell, and Brian Ridley. TOPSE also involves scientists from NASA; the Georgia Institute of Technology; Harvard University; Rutgers, the State University of New Jersey; and the Universities of California (Berkeley and Irvine), Maryland, New Hampshire, Rhode Island, and Virginia.

In the lower to middle troposphere, about 1.5 to 8 kilometers (1–5 miles) above the United States and Canada, ozone levels peak as springtime arrives. “Understanding the sources of this ozone, and the processes that produce and destroy it, will help us determine how human-produced emissions affect air quality on a global scale,” says Atlas.

It’s been unclear whether the ozone peak develops due to seasonal intrusions of ozone-rich air from the stratosphere above or whether it forms in place through photochemical effects of the intensifying spring sun. The answer, TOPSE found, is a little of both, though photochemical effects are several times more prominent by late spring.

North with the sun

From February to May 2000, scientists from NCAR and other institutions took to the skies above North America for the TOPSE field campaign. Seven round-trip flights aboard the National Science Foundation/NCAR C-130 aircraft took scientists and instruments from Broomfield, Colorado, to northernmost Canada (up to latitude 87°N) and back. The scientists compared results to the output of two computer models that simulate air chemistry and winds over the Northern Hemisphere.

The NSF-NCAR C-130 (right) flew back and forth from Colorado to northermost Canada throughout the late winter and spring of 2000, collecting data for TOPSE. On board was NASA’s airborne differential absorption lidar, which sends pulses of laser radiation at different wavelengths into the atmosphere to measure ozone, aerosols, and clouds.. Left are the average ozone levels, in parts per billion by volume, observed by altitude and latitude during two of the seven TOPSE flights in early 2000: 4–9 February and 15–23 May. (Photo above by Jim Hannigan. Images from "Ozone, aerosol, potential vorticity, and trace gas trends observed at high-latitudes over North America," Journal of Geophysical Research 108, D4, p. 8369, are courtesy Edward Browell and the American Geophysical Union.)

Together, the data and model results paint a picture that answers some key questions about springtime ozone and air chemistry above North America. By tracing chemical reactions and following stratospheric markers through their models, the scientists found that significant amounts of ozone descend from the stratosphere throughout the spring. At the same time, the troposphere itself produces substantial amounts of ozone, especially as the spring sun intensifies.

One piece of the puzzle arrived through measurements of beryllium-7 and other tracers by a University of New Hampshire team. Beryllium-7 is a naturally occurring radionuclide that gets quickly removed from the troposphere but can descend with ozone-rich air from the stratosphere. “We used it as one clear indicator of intervals when the C-130 encountered stratospheric air,” says Jack Dibb, who led the UNH effort with Robert Talbot. The beryllium-7 data pointed to a steady rate of ozone descent from the stratosphere through the spring, while the rate of solar-driven ozone production in the troposphere ramped up. By late spring, TOPSE found, up to five times more ozone was being produced locally than delivered from aloft.

TOPSE also addressed a quite different puzzle: how ozone can disappear so quickly in wintertime from surface air across the Arctic Ocean and adjacent land areas. Previous studies suggested that Arctic surface ozone depletion appears to be due to natural halogen compounds, such as bromine and chlorine, that react with ozone and the Arctic snowpack as the spring sun arrives.

This surface ozone depletion in the north is unrelated to the better-known ozone “hole” in the Antarctic stratosphere, which also forms in the spring. That ozone thinning involves a different set of reactions with chlorine derived from industrial chemicals, including chlorofluorocarbons. Even at its peak levels, Northern Hemisphere ozone is far less prevalent in the lower to middle troposphere than in the higher stratosphere. This means that the seasonal waxing and waning studied in TOPSE should have little effect on the ultraviolet light that reaches people, animals, and plants.

“A virtual ozone hole was observed [during TOPSE] for the first time over much of Hudson Bay and over the Arctic Ocean,” write the authors. Low-level winds, they note, “can distribute ozone-depleted air over a larger region beyond the Arctic than had been previously recognized.” TOPSE mapped episodes of surface ozone depletion over much of the Arctic Ocean, northern Canada, and Greenland. The Arctic Ocean appears to be the origin of these depletions, but winds can move these chemically processed air masses to more southerly latitudes.

Still needed, according to TOPSE scientists, are more-extensive measurements of the halogens that drive ground-level Arctic ozone depletion, as well as a better understanding of the atmospheric exchange between stratosphere and troposphere—a process the researchers note is far from understood.

Also in this issue...

How random is our winter weather?

Super-sizing a community data trove

Chasing mesoscale monsters

Larry Winter: NCAR's new Deputy director

President’s Corner: University roles in the weather and climate services partnership

UCAR Community Calendar

Web Watch

Governance Update