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. |
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NOAA's RONALD
H. BROWN, Seattle, WA June 2000. Click on pic to
enlarge |
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.
Information and images are available at:
http://trmm.gsfc.nasa.gov/
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. Real-time maps of tropical tropospheric ozone from
TOMS are available on the Internet at: http://metosrv2.umd.edu/~tropo
The ozonesonde data collected on NOAA's Ronald H. Brown vessel are
part of a TOMS validation project called SHADOZ (Southern
Hemisphere Additional Ozonesondes) and can be obtained from: http://code916.code916.gsfc.nasa.gov/Data_services/shadoz
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.
Last Revised: 13 October 2000
