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Spring 1998

EXPRESSO data yield richer understanding of African atmosphere


by Bob Henson
UCAR Communications

A new, broad-scale picture of African air chemistry is coming into place with analysis of data from EXPRESSO, the Experiment for Regional Sources and Sinks of Oxidants (see UCAR Quarterly, Fall 1995). The picture is more complex than scientists had previously guessed. For example, more tropospheric ozone was observed above the equatorial rain forest's boundary layer than above the nearby savanna, even though the burning of savanna grasses across Chad and the Sudan is a major contributor to ozone production (see diagram).

NCAR's Jim Greenberg (Atmospheric Chemistry Division, or ACD) and Robert Delmas, the French principal investigator for EXPRESSO, both presented results from the experiment at the American Geophysical Union's (AGU) December meeting.

EXPRESSO's main field campaigns took place in March and November-December 1996. During the latter period, dry-season burning was widespread to the north of the study region. EXPRESSO's observations were collected across the south-to-north transition zone from rain forest to savanna, which spans the Central African Republic and the Republic of Congo. The contrast in vegetation is mirrored by a contrast between two low-level air masses: warm, moist Atlantic breezes (the monsoon) from the south and hot, dry Saharan winds (the harmattan) from the north. The convergence zone where these air masses meet and mix was found to be hundreds of kilometers wide--much larger than expected, according to ACD's Paul Ginoux. The harmattan's north winds typically override the monsoon at a height of several kilometers, so the mixing is both horizontal and vertical.

Where the rain forest and savanna meet across central Africa, air masses meet as well. The EXPRESSO team is studying a convergence zone--several hundred kilometers wide--that separates warm, moist monsoon flow (left) from hot, dry harmattan winds (right). As the harmattan flows across the burning savanna and overrides the monsoon, its ozone levels appear to increase. The reason could be forest-produced isoprenes, which react with other chemicals to produce ozone. Mixing across the convergence zone may be providing the harmattan air with enough isoprenes to boost its ozone content.

The widespread mixing helps explain some of the air-chemistry findings. Methane and carbon monoxide (hydrocarbons that fuel ozone production) were plentiful in both monsoon and harmattan air masses, as were nitrogen oxides (NOX), produced largely by biomass burning. Remarkably, though, according to Greenberg, the ozone levels increased in the harmattan air as it flowed from north to south over the rain forest. Researchers had expected the forest air to be relatively clean and the savanna air to be more influenced by the burning. However, the reactive potential of hydrocarbons that produce ozone over the savanna had apparently been depleted by the time that air reached the EXPRESSO study region. At that point, scientists suspect, isoprenes took over. These tree-produced hydrocarbons are much less abundant than methane and carbon monoxide, but they are far more reactive and efficient at producing ozone in the presence of NOX.

ACD's Alex Guenther created a map of biogenic hydrocarbon emissions in Africa that was unveiled at the AGU meeting as part of a poster session on global emissions inventories. "We've just recently started developing this picture and putting the data together," says Greenberg. The French EXPRESSO team is assembling the data base, while Ginoux, ACD director Guy Brasseur, and colleagues have embarked on a modeling effort to integrate the various measurements. "It'll give us a framework to explain what went on there," says Greenberg.

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