WORLD METEOROLOGICAL ORGANIZATION

UNITED NATIONS ENVIRONMENT PROGRAMME

CONGO

THE CAUSES OF SEASONAL ENHANCEMENT IN TROPOSPHERIC OZONE AND THE REDUCTION IN STRATOSPHERIC OZONE AFTER THE ERUPTION OF MT. PINATUBO OBSERVED IN TROPICAL AFRICA.

D. NGANGA⁽¹⁾, A. MINGA⁽¹⁾, B. CROS⁽¹⁾, C. BOUKA BIONA⁽¹⁾, J. FISHMAN⁽²⁾, W B. GRANT⁽²⁾

In seeking to reach a better understanding of the atmospheric chemistry, studies have been carried out in the Congo as part of the DECAFE (Dynamique et Chimie de l'Atmosphère en Forêt Equatoriale) and TRACE - A (Transport and Atmospheric Chemistry near the EquatorAtlantic) programs.

I -TROPOSPHERIC OZONE

The analysis of the ozone data set obtained respectively at Impfondo (01° 37N, 18° E) and Brazzaville (04° 17S, 15° 15E) shows the existence of a seasonal trends of tropospheric ozone, and the presence of an ozone enriched layer located between 1 and 4 km altitude during the dry season of each hemisphere (Cros et al., 1988; Cros et al., 1991, Fishman et al, 1992, Andreae et al., 1992: Nganga et al., 1995). The available data in the region suggest that this ozone layer spreads itself throughout the central of Africa.

This ozone is formed in the troposphere from precursors emitted by biomass burning originated in southern or northern Africa and transported by southeast (S.E.) or northeast (N.E.) trade winds. In addition during the dry season of northern hemisphere our measurements show two biomass burning periods, the first one from late May to early August, and the other one from late September to early October.

Now it is widely established that biomass burning is the dominant phenomenon that controls the tropospheric ozone in the African intertropical zone. Unfortunately, no date concerning the repartition of fires throughout the entire year and overall the continent are available. So it is difficult to explain why the dry season of N.H. has a limited influence in the troposphere at large scale.

From 1983 to 1992, surface airborne and balloon measurements of ozone were carried out in Equatorial Africa between 5°S and 2°N. These measurements show:

1-A seasonal ozone variation with maxima in dry seasons, corresponding to the biomass during periods and minima in rainy season;

2-The role of ozone sink played by the forest;

3-An ozone and particulates enriched layer which is located between 1 and 4 km altitude during the dry season of each hemisphere.

Meteorological observations, satellite imagery and air mass trajectories suggest that this layer may be formed from air mass which originates in northern or southern Africa and advected over the dry tropical regions where biomass burning emits large amounts of aerosols and ozone precursors (NGANGA, 1991)

This air mass (the southeast or the northeast) is trapped in the equatorial region between the near-surface monsoon flow, and the easterly flow above 4 km.

II -STRATOSPHERIC OZONE

The distribution of ozone in African tropical region is examined during the TRANCE-A experiment conducted at Brazzaville and Ascension Island (08°S, 14°W) between late June 1990 and early October 1992.

A set of ozone data has been obtained in the troposphere and the stratosphere using balloon borne ECC ozonesondes.
The eruption of Mt. Pinatubo (15°N;122°E) took place on June 15 and 16, 1991. The analysis of the data obtained before and after the eruption points out a significant ozone depletions. For the period 3-6 months after the eruption decreases in ozone are found at 16 to 29 km altitude with peak decreases as large as 20% found at 24 km. Integrated between 16 and 28 km, a decrease of 13 -20 Dobson units (D.U., 1DU = 2.69 x 10¹⁶ mol.O3 Cm^-3 ) is observed when the ozonesonde data after the Pinatubo eruption are compared with those prior the eruption. The altitude at which the most pronounced ozone decrease is found strongly correlates with peak aerosol loading determined by the lidar.

A possible explanation for the observed results is heterogeneous chemistry involving the sulfuric acid volcanic aerosols. Normally, chlorine is bound by nitrogen oxide compounds in the form of CLONO2. However, in presence of sulfuric acid aerosols, the chemistry can be perturbed, with, for examlple, the old nitrogen species being more readily converted from NO x (NO + NO2 ) into nitric acid (HNO3) on the surface of the aerosols. Since NO x helps bind chlorine, its reduction can shift the chlorine balance from HCL in favor of CLO, allowing CLO to destroy more ozone (Brasseur et al., 1990). Indeed, increased levels of NHO3 were found near volcanic aerosols from El Chichon (Arnold et al., 1990)

Two independent data sets, one ozone from ozonesonde measurements, and one of aerosols from an airborne lidar system suggest that significant ozone decreases my have occurred as a result of the injection of debris by the Mt. Pinatubo volcano in June 1991 (Grant et al. 1992)

References

Andreae M.O., A, Chapuis, B. Cros, J. Fontan, G. Helas;, C. Justice, Y.J. Kaufman, A. Minga, D. Nganga. Ozone and Aitken nuclei Over Equatorial Africa: Airborne Observations During DECAFE 88, J. Gesphys. Res. 97; 6137 - 6148, 1992.

Arnold. F. Th. Buhrke, and S. Qui. Evidence for stratospheric ozone-depleting heterogeneous chemistry on volcanic aerosols from El Chichon, Nature, 348, 49-50, 1990.

Brasseur, G. P., C. Granier, and S. Walters. Future changes in stratospheric Ozone and role of heterogeneous Chemistry, nature, 348, 626428, 1009.

Cros, B., R. Delmas,D. Nganga, B. Clairac. Seasonal trends of ozone in Equatorial Africa: Experimental evidence of photochemical formation, J. Geophys. Res. 93, 8355-8366, 1988.

Cros,B., D. Nganga, R. Delmas, and J. Fontan. Tropospheric Ozone and Biomass Burning in Intertropical Africa, in "Global Biomass Burning", edited by J.S. Levine, MIT Press, Cambridge, 143 -146, 1991.

Fishman, J. Fakhruzzaman, B. Cros, and Nganga. Identification of widespread pollution in the Southern Hemisphere duced from satellite Analyses, Sciences 252, 1693. 1996, 1992.

Grant. W.B.. J. Fishman, E.V. Browell, V.G. Brackett, D. Nganga, A. Minga, B. Cros, R.E. Veiga, C. F Butler. M.A. Fenn G. Nowicki. Observations of reduced ozone concentrations in the tropical Stratosphere after the eruption of Mt. Pinatubo, Geophys. es. Iett., 19, 1109-1112, 1992.

Nganga D. Contribution à l'étude de l'ozone dans la base troposphère en Afrique Equatoriale, thesis of D. Sc., N° 150/91, Abidjan, Côte -d'Ivoire, 1991.

Nganga D., A. Minga, B. Cros, C. Bouka Biona, J. Fishman, and W.B. Grant. The vertical distribution of ozone measured at Brazzaville, Congo during, TRANCE -A; accepted by J. Geophys. Res, 1995.

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1 Laboratoire de physique de l'Atmosphère - UMNG-DGRST - Congo
2 NASA Langley Research Center - USA