by Bob Henson
Scientists have clarified the process by which ozonean essential
shield in the stratosphere, but a pollutant at lower levelsreaches
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 ResearchAtmospheres (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 (15
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.
Its 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 NASAs 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: 49 February and 1523 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,
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 tropospherea process the researchers note is far from understood.