B. Cros

Identification of Widespread Pollution in the Southern Hemisphere Deduced from Satellite Analyses

Vertical profiles of ozone obtained from ozonesondes in Brazzaville, Congo (4�S, 15�E), and Ascension Island (8�S, 15�W) show that large quantities of tropospheric ozone are present over southern Africa and the adjacent eastern tropical South Atlantic Ocean. The origin of this pollution is widespread biomass burning in Africa. These measurements support satellite-derived tropospheric ozone data that demonstrate that ozone originating from this region is transported throughout most of the Southern Hemisphere. Seasonally high levels of carbon monoxide and methane observed at middle- and high-latitude stations in Africa, Australia, and Antarctica likely reflect the effects of this distant biomass burning. These data suggest that even the most remote regions on this planet may be significantly more polluted than previously believed.

Precipitation chemistry in the Mayombé forest of equatorial Africa

An automatic wet-only precipitation collector was operated in the coastal forest of equatorial Congo for a complete seasonal cycle (November 1986 to September 1987). Inorganic (Na+, K+, NH4+, Ca++, NO3−, Cl−, SO4=) and organic (HCOO−, CH3COO−) ions were determined in 169 samples which represent 51 rain events. An average precipitation pH of 4.74 based on the volume weighted of H+ was obtained. Rain from stratiform clouds showed higher acidity (pH = 4.62) than convective rainfall (pH = 4.81). This acidity results from a mixture of mineral acids (64%, of which about 42% is HNO3) and organic acids (36%). Most of the HNO3 component can be attributed to the emission of nitrogen oxides from vegetation burning. To study the influence of variation in rainwater ion concentrations resulting from the differences in atmospheric liquid water content, rainfall events were stratified based on rainfall amount into convective and stratiform events. The seasonal variation in the chemical composition of these types of rain events allowed us to compare the relative seasonal importance of the different sources (terrestrial biogenic, marine, soils, and biomass burning). Comparison between precipitation chemistry in Congo and in Amazonia shows that the African equatorial forest is influenced by local fires and savanna fires in the southern hemisphere during the dry season and by fires in the northern hemisphere during the wet season. In Amazonia, on the other hand, the influence of biomass burning on rainwater chemistry appears to occur predominantly in the dry season. Since the precipitation collector subdivides rainfall events into 10 sequential samples, we examined the evolution in chemical composition and deposition during four large convective events. The results demonstrate the washout of ions at the onset of precipitation producing higher rainwater concentrations and their dilution as the rainfall intensity increases.

Observations of reduced ozone concentrations in the tropical stratosphere after the eruption of Mt. Pinatubo

The eruption of Mt. Pinatubo (15oN, 122oE) on June 15 and 16, 1991, placed a large amount of SO2 and crustal material in the stratosphere. Based on measurements of decreases of stratospheric ozone after previous volcanic eruptions, it was expected that the aerosols deposited into the stratosphere (both directly and as a result of SO2 conversion into particulate sulfate) by this eruption would give rise to significant ozone depletions. To check for such an effect, ozone profiles obtained from ECC sondes before and after the eruption at Brazzaville, Congo (4oS, 15oE), and Ascen- sion Island (8oS, 14oW), are examined. Aerosol profiles determined from a lidar system in the western Pacific (4 o- 6o1,,1, 125oE) show that most of the material injected into the stratosphere is located between 18 and 28 km with highest mounts at 24-25 km. For the period 3-6 months after the eruption, decreases in ozone are found at 16 to 29 km, with peak decreases as large as 20% found at 24 km. Integrated between 16 and 28 km, a decrease of 13-20 Dobson units is observed when the ozonesonde data after the Pinatubo eruption are compared with those prior to the eruption. The altitude at which the most pronounced ozone decrease is found strongly correlates with peak aerosol loading deter- mined by the lidar. In addition, a small increase in ozone density is found above about 28 kin. Mechanisms that might explain the results such as heterogeneous chemistry, radiative effects, and dynamics are discussed.

Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO): An overview

This paper presents an overview of the Experiment for Regional Sources and Sinks of Oxidents (EXPRESSO) including the objectives of the project, a detailed description of the characteristics of the experimental region and of field instrumentation deployed, and a summary of the main results of all components of the experiment. EXPRESSO is an international, multidisciplinary effort to quantify and better understand the processes controlling surface fluxes of photochemical precursors emitted by vegetation and biomass burning along a tropical forest to savanna gradient in central Africa. The experiment was conducted at the beginning of the dry season in November-December 1996. Three main research tools were deployed during this period: (1) the French research aircraft (Avion de Recherche Atmospherique et de Teledetection, Fokker 27), instrumented for chemistry and flux measurements (CNRS- France), (2) two satellite receivers for in situ acquisition of National Oceanic and Atmospheric Administration-advanced very high resolution radiometer (NOAA-AVHRR) imagery for fire detection (EC-JRC, Ispra, Italy), and (3) a 65-m walkup tower installed at a tropical forest site in the Republic of Congo (National Center for Atmospheric Research, Boulder, Colorado). Average dynamic and turbulence characteristics over savanna and forest ecosystems were retrieved from aircraft measurements. They illustrate the complex atmospheric circulation occurring in this region in the vicinity of the Intertropical Convergence Zone. Satellite receivers were operated three times a day to produce maps of fire distribution. Statistics and mapping of burned surfaces from NOAA-AVHRR and ERS-Along Track Scanning Radiometer space systems have been developed. The influence of biogenic and biomass burning sources on the chemical composition of the lower atmosphere was studied through both aircraft and tower measurements. The EXPRESSO field campaign was followed by modeling efforts (regional and global scales) in which model components are evaluated using the experimental data.

Methane emission from flooded forest in central Africa

In this report we give results of methane flux measurement in the flooded forest zone of the Congo River basin in central Africa. Methane fluxes are measured by the static chamber method above three types of soils characterized by various water contents. High methane emission is recorded on flooded soils (4.59 × 1012 molecules/cm2/s) while methane uptake occurs in dry soil (−8.38 × 1010 molecules/cm2/s). Methane flux is also derived from variations of surface concentrations of methane related to the variations of air stability and from vertical profiles in the lower troposphere. The four methods used to determine the average methane emission from this wetland type yield compatible values. The average yearly flux is estimated at 2 to 4 × 1012 molecules/cm2/s or 45 to 90 mg/m2/d The total wetland area being about 105 km2, global methane emission from the flooded forest zone of the Congo River basin would represent 1.6 to 3.2 Tg (CH4) per year.

Sources and sinks of methane in the African savanna. CH4 emissions from biomass burning

Sources and sinks of atmospheric methane are studied in savanna regions of west and central Africa. Flux measured over dry savanna soils, using static chambers, is always negative the average uptake rate being 2×1010 molecules/cm2/s. In these regions, sources are linked to biomass burning. Methane and CO2 emission from combustion of savanna plants and wood is studied by both field experiments and laboratory experiments using a combustion chamber. For savanna plants most of the carbon (85%) contained in the biomaterial is volatilized as CO2 and 0.1 to 0.25% as methane. For graminaceous plants like loudetia simplex the ratio C-CH4/C-CO2 is 0.11%; it is 0.28% for hyparrhenia the other main type of savanna plants and it attains 1.4% for the combustion of wood. In natural fire plumes this ratio is around 0.26% for savanna fires and 0.56 to 2.22% for forest fires. These results show that methane release is highly dependent on the type of combustion. Methane to CO2 ratios are also studied in vertical profiles in the troposphere taken during the TROPOZ I campaign, an aerial research expedition carried out over west Africa during the bushfire period. Within polluted layers, the average ratio of CH4 to CO2 excess over ambient air concentration is 0.34%. These results show that biomass burning in tropical Africa constitutes an important source of atmospheric methane estimated to about 9.2×106 T(CH4)/yr.

Analysis of the distribution of ozone over the southern Atlantic region

Tropospheric ozone data measured by ozonesondes during the Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) field mission and the multiyear pre-TRACE A program are analyzed jointly with tropospheric ozone amounts derived from remote satellite data (“residuals”). We present here the first detailed analysis of the entire Ascension Island pre-TRACE A data set. Data from the three pre-TRACE A ozonesonde sites are used to establish a coherent spatial and temporal climatology of ozone in the southern tropical Atlantic region. This analysis shows a significant ozone seasonality over the Atlantic region, with a period of maximum values that extends from the austral winter through at least October at Natal, Brazil, and Ascension Island. Concentrations begin to decline somewhat earlier at Brazzaville, Congo, especially at lower altitudes. Although Natal exhibits a significantly lower annual average than Ascension Island or Brazzaville by about 4 Dobson Units (DU), the magnitude of the seasonal amplitude at Natal is the largest of the three stations. Additionally, more of the seasonal amplitude at Natal is due to a contribution from ozone in the middle and upper troposphere than at either Ascension Island or Brazzaville. Amplitudes as large as 15 DU are measured at individual sites, and the residuals show an average amplitude over the southern tropical Atlantic region of 10–12 DU. Statistical comparison of the residuals to the ozonesonde climatology show that while the residuals tend to underpredict both the means and the seasonal amplitudes compared to the in situ data, they provide a good representation of the variance of ozone in this region and predict the local annual and seasonal means to within better than 10% and seasonal amplitudes to within 15%.

Sources and sinks of methane and carbon dioxide exchanges in mountain forest in equatorial Africa

Sources and sinks of methane were studied in the Mayombe forest, a tropical evergreen forest located in a mountainous region in central Africa. Important methane emissions, reaching 6×1013 molecules/cm2/s, were measured in flooded lowlands where soil characteristics: pH and redox potential, favor the growth of methanogenic bacteria. However, basically, soils of this region constitute a sink of atmospheric methane with uptake rates ranging from 1010 to 1011 molecules/cm2/s. Methane emission from termite nests was also studied; it appeared to be a minor component of the methane budget. CH4 concentrations were measured inside the forest and in the surrounding atmosphere, CO2 being used as a qualitative tracer of air exchanges. In spite of intense but scattered and size-limited sources this environment seems to be a net sink of atmospheric methane.

Distribution of tropospheric ozone at Brazzaville, Congo, determined from ozonesonde measurements

An analysis of 33 ozonesonde launches in Brazzaville, Congo (4°S, 15°E), between June 1990 and May 1991 is presented. The data indicate highest tropospheric amounts between June and early October, coincident with the dry season and with the presence of enhanced widespread biomass burning. The seasonal cycle of ozone derived from the ozonesonde measurements is in good agreement with the climatological seasonal cycle inferred from the use of satellite data and both seasonal cycles peak in September. Averaged throughout the year, the integrated amount of ozone derived from the ozonesondes is 44 Dobson units (DU) and is 39 DU using the satellite data. Within the troposphere the highest partial pressures are generally found at pressure levels near 700 mbar (∼3 km). Using simultaneous ozonesonde data from Ascension Island (8°S, 15°W), examples are presented illustrating that differences in the troposphere are primarily responsible for the observed spatial gradients of total ozone observed by TOMS. Calculation of correlation coefficients suggests that total ozone measurements may be a better indicator of the amount of ozone in the troposphere than are surface measurements.

Carboxylic monoacids in the air of mayombe forest (Congo): Role of the forest as a source or sink

In the tropical rain forests of the Congo during the dry season, from June to September 1987, carboxylic acid partial pressures (Pgas) in the air above the canopy, at ground level, and at the boundary layer, were estimated from water samples such as fog and rainwater. The concentrations of these acids were also measured in the sap of tree leaves. Tree leaves act as a sink or as a source if the acid Pgas is greater of lower than the acid concentrations in molecular form in sap. For each of these soluble gases, there is a value of Pgas where the exchange is nul. This is called the compensation point. Values of the compensation point for some tree leaves were evaluated according to Henrys law. Henrys law coefficients at ppm levels were redetermined for formic (HCOOH), acetic (CH3COOH), propionic (CH3CH2COOH), and isobutyric (CH3CH(CH3)COOH) acids.By comparison of Pgas and compensation points, it is concluded that the forest was a potential source for these acids. The soil-or the litter-acts as a significant source of a carboxylic acid of C3 or C4 atoms in the aliphatic chain. This carboxylic acid, not yet fully characterized, could play an important role in the rain acidity in forested zones of the equatorial areas.The direct emission of these carboxylic acids by vegetation was the main source in the boundary layer above the forest. The average emissions were 3.1×109, 7.8×109, and 8.4×109 molecules cm-2 s-1 for HCOOH, CH3COOH, and CH3CH2COOH, respectively. The savanna is an exogenous source of HCOOH and CH3CH2COOH during moderately rainy days for 30% of the time. The ozonolysis of isoprene seems to be a small source of HCOOH.