GTE: TRACE-A Results

Fires Observed During TRACE-A

The TRACE-A measurements clearly demonstrate that the tropospheric ozone that accumulates over the tropical Atlantic ocean between Africa and South America is a result of the wide spread biomass burning that occurs on both of these continents during the southern hemispherical spring and summer time. Shown here are some of the fires that were observed from the DC-8 aircraft during TRACE-A. Burning in both Africa and South America is, to a large extent, associated with agricultural practicies that have been employed in these countries for generations. Shown below are AVHRR imagery over Brazil and Africa during the TRACE-A study period.

AVHRR imagery over Brazil (top left image) cover the time period during which the INPE aircraft were conducting meaurements. The bottom left image covers the time period that both the DC-8 and INPE aircraft were conducting coordinated flights in Brasil. The image in the right panel depicts fires over southern Africa for September-October, 1992. Both sets of images illustrate the extensive amount of burning that occurs during the spring and summer time in the Southern hemisphere. Measurements in the ozone pool provides definitive "finger prints" of biomass burning.

The TRACE-A mission provided the first opportunity for direct sampling within the ozone pool. The results illustrated in the right panel, along with other trace gas measurements aboard the DC-8 provided evidence clearly demonstrating that the satellite derived tropospheric ozone residuals are valid and that the origins of this ozone pollution is indeed associated with biomass burning. The top panel on the right illustrates the extent of the ozone pool observed from satellite measurements on the day of a DC-8 flight. The flight path of the DC-8 is indicated by the dotted path along the coast of Africa. The lower three panels show respectively the aerosol distribution below the aircraft flight altitude from point A to B ( from the Differential Absorption Lidar (DIAL)), in situ measurements of ozone and carbon monoxide as the DC-8 descended in altitude at points A and B, and the distribution of ozone below the aircraft flight altitude (also from the DIAL instrument). The DIAL and in situ measurements clear show the elevated concentrations of aerosols, ozone, and carbon monoxide in side the ozone pool.

The prevailing meteorology over the Atlantic ocean serves to confine the ozone resulting from the fires.

Emissions from burning in southern Africa and South America are lifted to the upper regions of the troposphere by local convective activity. At different altitudes the prevailing winds over each continent funnel ozone, which is produced locally, into a massive stagnet region over the Atlantic ocean. The combination of buring on such a massive scale and the unique metorological conditions result in the accumulation of ozone over the Atlantic ocean. The air flow from southern Africa also pushes ozone into the Indian Ocean east of the African continent and ultimately in the tropical Pacific ocean.

Results from the TRACE-A study, as well as other data from the equatorial Pacific suggest that the Walker circulation plays a major role in the ozone budget in the equatorial regions. Net ozone production in the upper troposphere balances net ozone loss in the lower troposphere. Mass exchange between the lower and the upper troposphere is a result of the vertical flow associated with the Walker circulation along the equatorial regions. Deep convection motions over South America, southern Africa, and Oceania (rising branches of the Walker circulation) inject nitric oxides from combustion, soils, and lighting to the upper troposphere, driving ozone production. Eventually, the air subsides over the oceans and net ozone loss takes place in the lower troposphere due to low nitric oxides concentrations and high humidifies, closing the ozone cycle. ( Jacob, et al, "Photochemistry of the Tropical Troposphere" JGR , vol. 101, October 30, 1996, pp 24,235-24,250.)