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CRUISING
INTO AN OZONE PARADOX ACROSS THE ATLANTIC OCEAN
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A trans-Atlantic
research voyage has helped NASA scientists solve the long-standing
puzzle of how high levels of ozone occur off the west coast
of Africa far from known sources of air pollution.
Anne M. Thompson
of NASA's Goddard Space Flight Center and Bruce Doddridge
of the University of Maryland, College Park, report in the
October 15 issue of the American Geophysical Union's Geophysical
Research Letters that trade winds mixing near the equator
and lightning caused the tropospheric ozone high in the southeastern
Atlantic while the source of most of the pollution - biomass
burning - raged in northern equatorial Africa.
The "Aerosols99"
oceanographic voyage in January and February 1999 was the
first opportunity to examine ozone levels from the northern
to southern hemispheres via a northwest-to-southeast Atlantic
Ocean crossing.
Scientists from
NASA Goddard, the University of Maryland, and the National
Oceanic and Atmospheric Administration (NOAA), conducted their
2 month long mission aboard the NOAA research vessel Ronald
H. Brown between Norfolk, Va., and Cape Town, South Africa.
Scientists aboard the ship sampled atmospheric ozone across
the Atlantic using ozonesondes, which are sensors launched
by balloon into the atmosphere. Those data validated and were
complemented by the Total Ozone Mapping Spectrometer (TOMS)
instrument onboard the TOMS-Earth Probe satellite. Both showed
high levels of tropospheric ozone throughout the south Atlantic
Ocean during that period.
"What's
interesting," Thompson said, "is that the data explain
the long-standing paradox between the ozone levels in the
Atlantic Ocean of the two hemispheres because of what was
happening in the environment." Biomass burning is a known
source of ozone, but mid-tropospheric ozone was greatest in
the Southern Hemisphere, thousands of miles from where the
seasonal agricultural burning occurred in northern equatorial
Africa.
One of the factors
that causes this paradox is the way in which ozone moves in
the atmosphere, specifically in the lower and middle troposphere.
Another contributor to the higher ozone levels is lightning,
because it causes a chemical reaction in the atmosphere that
leads to the formation of ozone.
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Doddridge said
that the Inter-Tropical Convergence Zone (ITCZ), where the
southeast trade winds come together with the northeast trade
winds near the equator, act like cogs in a wheel. The converging
winds mix the lower level ozone and other pollutants from
north to south, and are responsible in part for this paradox.
The northern trade winds transported the pollutants from northern
equatorial Africa and contributed to the tropical Southern
Hemisphere ozone maximum. Doddridge added, "We saw carbon
monoxide levels triple when we crossed the Inter-Tropical
Convergence Zone."
NASA's Tropical
Rainfall Measuring Mission (TRMM) satellite provided data
that helped researchers look at the role of lightning as an
ozone contributor. The Lightning Imaging Sounder onboard the
TRMM satellite detected the frequency of lightning in the
Southern Hemisphere where ozone levels were highest. Lightning
produces nitric oxide that in turn leads to rapid ozone formation.
Significant lightning activity was observed in the Southern
Hemisphere over Africa, which contributed to the higher ozone
concentration in the troposphere.
Looking at aerosols
and tropospheric ozone over the entire TOMS satellite record
(from 1978 to the present), it becomes clear that the January/February
"paradox" observed last year is normal for the tropical
Atlantic, according to Thompson. Furthermore, throughout the
tropics, TOMS revealed that biomass burning is strongest in
the dry season (December through March north of the equator,
July through October to the south), but maximum ozone always
occurs between July and October.
TRMM is a joint
U.S.-Japanese mission and part of NASA's Earth Science Enterprise;
a long-term research program designed to study the Earth's
land, oceans, air, ice and life as a total system.
TOMS is an instrument on NASA's Earth Probe (TOMS-EP) satellite.
NASA instruments have been measuring ozone levels since the
early 1970s. TOMS has been a key instrument for monitoring
ozone levels over the Earth.
The ozonesonde data collected on NOAA's Ronald H. Brown vessel
are part of a TOMS validation project called SHADOZ (Southern
Hemisphere Additional Ozonesondes).
EDITOR'S NOTE:
The paper referenced is entitled "A Tropical Atlantic
Ozone Paradox: Shipboard and satellite views of a Tropospheric
Ozone Maximum and Wave-one in January-February 1999."
Geophysical Research Letters, October 15, 2000, vol. 27, no.
20, p. 3317.
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