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FOS/DECAFE 91 (Fire of Savannas/Dynamique et Chimie Atmosphérique en Forêt Equatoriale) was the first multidisciplinary experiment organized in Africa to determine gas and aerosol emissions by prescribed savanna fires. The humid savanna of Lamto in Ivory Coast was chosen for its ecological characteristics representative of savannas with a high biomass density (900 g m–2 dry matter). Moreover the vegetation and the climate of Lamto have been studied for more than twenty years. The emission ratios (X/CO2) of the carbon compounds (CO2, CO, NMHC, CH4, PAH, organic acids and aerosols), nitrogen compounds (NOx, N2O, NH3 and soluble aerosols) and sulfur compounds (SO2, COS and aerosols) were experimentally determined by ground and aircraft measurements. To perform this experiment, 4 small plots (100×100 m) and 2 large areas (10×10 km) were prepared and burnt in January 1991 during the period of maximum occurrence of fires in this type of savanna. The detailed ecological study shows that the carbon content of the vegetation is constant within 1% (42 g C for 100 g of vegetal dry matter), the nitrogen content (0.29 g N for 100 g of dry matter) may vary by 10% and the sulfur content (0.05 g S/100 d.m.) by 20%. These variations of the biomass chemical content do not constitute an important factor in the variation of the gas and particle emission levels. With the emission ratios characteristic of humid savanna and flaming conditions (CO/CO2 of 6.1% at the ground and 8% for airborne measurements), we propose a set of new emission factors, taking into account the burning efficiency which is about 80%: 74.4% of the carbon content of the savanna biomass is released to the atmosphere in the form of CO2, 4.6% as CO, 0.2% as CH4, 0.5% as NMHC and 0.7% as aerosols. 17.2% of the nitrogen content of the biomass is released as NOx, 3.5% as N2O, 0.6% as NH3 and 0.5% as soluble aerosols.  相似文献   
2.
Ozone has been observed in elevated concentrations by satellites over areas previously believed to be background. There is meteorological evidence, that these ozone plumes found over the Atlantic Ocean originate from vegetation fires on the African continent.In a previous study (DECAFE-88), we have investigated ozone and assumed precursor compounds over African tropical forest regions. Our measurements revealed large photosmog layers at altitudes from 1.5 to 4 km. Both chemical and meteorological evidence point to savanna fires up to several thousand km upwind as sources.Here we describe ozone mixing ratios observed over western Africa and compare ozone production ratios from different field measurement campaigns related to vegetation burning. We find that air masses containing photosmog ingredients require several days to develop their oxidation potential, similar to what is known from air polluted by emissions from fossil fuel burning. Finally, we estimate the global ozone production due to vegetation fires and conclude that this source is comparable in strength to the stratospheric input.  相似文献   
3.
Soft sediments can deform under both changes in stress and changes in strain rate, as well as during sustained loading (e.g. creep). In glacial conditions, soft subglacial sediments can contribute to glacier movements, and when the ice/sediment system is coupled, the rate-sensitivity of the sub-layer can affect the velocity of the glacier. These issues have been the object of studies by several geologists. In parallel, the effects of time and strain rate on the behaviour of soils have been studied for many years by geotechnical engineers, to estimate the long-term performance of ground structures. This paper applies the knowledge acquired in soil mechanics to a geological problem: results on two glacial sediments of different origins, obtained from advanced geotechnical laboratory tests, are presented and analysed in the light of recent advances in soil mechanics. Independently of time effects, the test data show that the pre-failure behaviour is not elastic but that the sediments develop plastic strains before reaching their peak strength, with the stiffness gradually degrading. Results from tests performed with variable strain rates suggest that the time-dependent (viscous) behaviour of the sediments may be governed by their mineralogy, in particular the presence of plastic fines. The viscous behaviour observed in the plastic tills seems to affect the pre-failure behaviour only, for example the yield surface, stiffness, strength, but the criteria of failure, e.g. critical or residual angle of failure are found to be constant and independent of strain rate. The test data also suggest that the viscous behaviour may be related to stress level, being more prevalent at low stresses. These results, obtained under laboratory-controlled conditions, over a large range of strains, pre- and post-peak strength, are a necessary step towards developing constitutive models for subglacial sediments rigorously. It is only after this has been achieved that such models should be implemented in finite element code to analyse ice/sediment systems, with the aim to understand glacier movements better.  相似文献   
4.
On the shearing behaviour of an artificially cemented soil   总被引：2，自引：1，他引：1  
Ground improvement by cementation is a technique widely used because of the easiness of application and relatively low cost. To date, however, there is no global understanding of the behaviour of artificially cemented soil with respect to its dosage, so that in practice methodologies still rely on previous used scenarios rather than scientific facts. This paper presents results of low-to-high-pressure triaxial tests performed on pure and cemented specimens of decomposed granite where the dosage was controlled, chosen as a function of the ratio of porosity to volumetric cement content or adjusted porosity/cement index. The behaviour of the cemented soil was analysed in terms of peak strength and failure envelopes, stress dilatancy and state boundary surfaces, on which the influence of the porosity/cement index was examined.  相似文献   
5.
The early Paleogene is critical for understanding global biodiversity patterns in modern ecosystems. During this interval, Southern Hemisphere continents were largely characterized by isolation and faunal endemism following the breakup of Gondwana. Africa has been proposed as an important source area for the origin of several marine vertebrate groups but its Paleogene record is poorly sampled, especially from sub-Saharan Africa. To document the early Paleogene marine ecosystems of Central Africa, we revised the stratigraphic context of sedimentary deposits from three fossil-rich vertebrate localities: the Landana section in the Cabinda exclave(Angola), and the Manzadi and Bololo localities in western Democratic Republic of Congo.We provide more refined age constraints for these three localities based on invertebrate and vertebrate faunas, foraminiferal and dinoflagellate cyst assemblages, and carbon isotope records. We find an almost complete absence of Danian-aged rocks in the Landana section, contrary to prevailing interpretations over the last half a century(only the layer 1, at the base of the section, seems to be Danian). Refining the age of these Paleocene layers is crucial for analyzing fish evolution in a global framework, with implications for the early appearance of Scombridae(tunas and mackerels) and Tetraodontiformes(puffer fishes). The combination of vertebrate fossil records from Manzadi and Landana sections suggests important environmental changes around the K/Pg transition characterized by an important modification of the ichthyofauna. A small faunal shift may have occurred during the Selandian. More dramatic is the distinct decrease in overall richness that lasts from the Selandian to the Ypresian. The Lutetian of West Central Africa is characterized by the first appearance of numerous cartilaginous and bony fishes. Our analysis of the ichthyofauna moreover indicates two periods of faunal exchanges: one during the Paleocene, where Central Africa appears to have been a source for the European marine fauna, and another during the Eocene when Europe was the source of the Central Africa fauna. These data indicate that Central Africa has had connections with the Tethyian realm.  相似文献   
6.
We measured CO2, CO, CH4, H2, and NO2 in air masses polluted by savanna fires over Côte d'Ivoire, western Africa. Elevated concentrations of these trace gases were found in fire plumes and also in extensive haze layers. Trace gas mixing ratios ranged as high as 605 ppmv for CO2, 14.8 ppmv for CO, 2.7 ppmv for CH4, 4.2 ppmv for H2, and 25 ppbv for NO2. We compare our emission ratios to those obtained in previous field and laboratory studies. The emission ratios, expressed as an average and as a range or as an average only, were: dCO/dCO2 5.3×10–2 (3–18×10–2); dCH4/dCO 5.3×10–2; dH2/dCO 2.4×10–1 and dNO2/dCO2 1.8×10–4 (1.5–2.2×10–4). The values found match those found during similar measurements, though our results point to rather vigorous burning in the savanna of western Africa.  相似文献   
7.
The dust haze conditions, typical of the African atmosphere south of the Sahara, are a result of wind-generated dust from arid lands. The magnitude of the dust haze is evaluated for the 30-year period beginning in 1957 by calculating the number of occurrences where the observed visibility was reduced below threshold values of 10 km and 5 km. The frequency of low visibility was several times greater for the 1977–1986 period than for the 1957–1966 period. Large decreases in visibility are observed after the severe droughts of 1972–1973 and 1982-1984. Contrasting regional differences of the dustiness evolution are noticed. These differences are closely related to the differences in the regional rainfall evolution. The increase in dustiness is believed to arise from dust produced in new desertic areas which result from rainfall shortages along the southern border of the Sahara.  相似文献   
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