Cynthia M. O’Carroll

Goddard Space Flight Center, Greenbelt, Md.

(Phone: 301/614-5563)

Lynn Chandler

Goddard Space Flight Center, Greenbelt, Md.

(Phone: 301/614-5562)

Jana Goldman

National Oceanic and Atmospheric Administration

(Phone: 301/713-2483)

Lee Tune

University of Maryland

(Phone: 301/405-4679)

Release No: 00-125

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 in Greenbelt, Md., 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.

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:

OMS 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:

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: