Jean Servant

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 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.

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.

Acid rain in the tropical forests of the Ivory Coast

Abstract This paper presents the first results concerning the acidity of the precipitation collected over a period of 1 year (February 1983–March 1984) in an undisturbed area of tropical forest in the Ivory Coast. A large proportion of the precipitation is acid, 77% bas a pH below 5.6. The average acidity is 5.0, with a temporal evolution tending towards a basic pH during the dry season, characteristic of the influence of the Sahelian terrigenous dust source and tending towards an acid pH of about 4.2 at the beginning of each humid period. This result, new for an African tropical forest, confirms the observations made in the Amazonian forests or in those of Northern Australia. An extensive research program on the acidity in the equatorial areas was started in the forests of the Ivory Coast and Congo in order to confirm this preliminary study.

Aspects of the cycle of inorganic nitrogen compounds in the tropical rain forest of the Ivory Coast

A partial balance of mineral N is given for the basins of two coastal rivers in a forest zone in the Ivory Coast. The dry and wet depositions on the basin surfaces is given for particulate matter (NO3−, NH4+). The quantity of mineral N washed away in the rivers is evaluated. The losses from leaching of the soils by rainwater are about 0.33 to 1.0% of the atmospheric depositions for NH4+−N and 2.2 to 5.8% for NO3−−N. The yearly atmospheric input of N compounds to the ecosystem, about 1.4 g N m−2 y−1, is at least 14% of mineral N formed in the soils and is therefore quite significant.

Daily variations of the H2S content in atmospheric air at ground-level in France

Variations of the natural H2S content of the atmosphere near the ground were studied during sunny periods in 3 different sites: in the urban area of Toulouse, on the Mediterranean coast at Grau de Vendres (pop. 1508) and in a pine forest at Luxey (pop. 800) near the Atlantic Ocean. For each site, on certain days, every 3 h, the H2S content and natural radioactivity in the air (222Rn + short-lived products) were measured. Ozone concentrations in the air were also recorded near the sea. The origins of the diurnal variations of the H2S content in the air are discussed along with the vertical stability of the atmosphere and OH radical oxidation. It is concluded, on the basis of these observations, that H2S oxidation by free OH radicals is hidden by rapid variations of H2S emission from the soils.

Variation des concentrations et caracteristiques physiques des noyaux d'aitken dans un site non pollue

Resume In a rural area of the South-West of France from April to June, 1971, the concentrations of Aitken nuclei are measured. The variations of these concentrations are related to the meteorological parameters, in particular to the stability of the lower atmosphere. The existence of a nocturnal source and a diurnal source is shown. The intensities of the sources are calculated. During the day photolytic reactions are taking place and the formation of the Aitken nuclei is increasing (5 × 103 particles cm−2 s−1 to 5 × 104 particles cm−2 s−1). The vertical stability of the lower atmosphere is also an important parameter to explain the variations of the concentrations. The mean radius of the particles is equal at 3.2 ± 0.6 × 10−6 cm.