Abiotic release of nitrous acid (HONO) in equilibrium with soil nitrite (NO2(-)) was suggested as an important contributor to the missing source of atmospheric HONO and hydroxyl radicals (OH). The role of total soil-derived HONO in the biogeochemical and atmospheric nitrogen cycles, however, has remained unknown. In laboratory experiments, we found that for nonacidic soils from arid and arable areas, reactive nitrogen emitted as HONO is comparable with emissions of nitric oxide (NO). We show that ammonia-oxidizing bacteria can directly release HONO in quantities larger than expected from the acid-base and Henry's law equilibria of the aqueous phase in soil. This component of the nitrogen cycle constitutes an additional loss term for fixed nitrogen in soils and a source for reactive nitrogen in the atmosphere.

No abstract

The article presents new results on the structure and the above-ground biomass of the various population types of mangroves in French Guiana. Nine mangrove stands were studied, each composed of three to ten adjoining plots with areas that varied depending on the density of the populations. Structural parameters and indices were calculated. Individuals representative of the three groups of taxa present were felled:Avicennia germinans (L) Stearn, Rhizophora spp., and Laguncularia racemosa (L) Gaertn. The trunks, branches and leaves were sorted and weighed separately. The biomass was obtained by determining the allometric relationships, the general equation selected being of the type y = a x, where the diameter (x) is the predictive variable. The total above-ground biomass varied from 31 t ha for the pioneer stages to 315 t ha for mature coastal mangroves, but with large variations depending on the structural characteristics at each site. The results place the Guianese mangroves among those with high biomass, although lower than those reported for Asia. Based on the relationships between structural parameters and standing biomass, in particular with the use of the "self-thinning rule", population dynamics models are proposed.

A multimodel comparison of the performance of land surface parameterization schemes increases understanding of the land-atmosphere feedback mechanisms over West Africa.

Abstract. In this paper we present the long term monitoring of ambient gaseous concentrations within the framework of the IDAF (IGAC-DEBITS-AFRICA) program. This study proposes for the first time an analysis of long-term inorganic gas concentrations (1998 to 2007) of SO 2 , NO 2 , HNO 3 , NH 3 and O 3 , determined using passive samplers at seven remote sites in West and Central Africa. Sites are representative of several African ecosystems and are located along a transect from dry savannas-wet savannas-forests with sites at Banizoumbou (Niger), Katibougou and Agoufou (Mali), Djougou (Benin), Lamto (Cote d'Ivoire), Zoetele (Cameroon) and Bomassa (Congo). The strict control of measurement techniques as well as the validation and inter-comparison studies conducted with the IDAF passive samplers assure the quality and accuracy of the measurements. For each type of African ecosystem, the long term data series have been studied to document the levels of surface gaseous concentrations. The seasonal and interannual variability have also been analyzed as a function of emission source variations. We compared the measured West and Central African gas Correspondence to: M. Adon (adonatma@yahoo.fr) concentrations to results obtained in other parts of the world. Results show that the annual mean concentrations of NO 2 , NH 3 , HNO 3 measured in dry savannas are higher than those measured in wet savannas and forests that have quite similar concentrations. Annual mean NO 2 concentrations vary from 0.9±0.2 in forests to 2.4±0.4 ppb in the dry savannas, NH 3 from 3.9±1.4 to 7.4±0.8 ppb and HNO 3 from 0.2±0.1 to 0.5±0.2 ppb. Annual mean O 3 and SO 2 concentrations are lower for all ecosystems and range from 4.0±0.4 to 14.0±2.8 and from 0.3±0.1 to 1.0±0.2 ppb, respectively. A focus on the processes involved in gas emissions from dry savannas is presented in this work, providing explanations for the high concentrations of all gases measured at the three dry savannas sites. At these sites, seasonal concentrations of all gases are higher in the wet season. Conversely, concentrations are higher in the dry season in the wet savannas. In forested regions, we measure no significant difference between wet and dry seasons. This unique database of long term gases concentrations monitoring is available at:

No abstract

No abstract

The main objectives of this paper are: (i) to present the rationale for maintaining a long term hydrometeorological observing system in West Africa ; (ii) to review the state of the art regarding our knowledge on the land-atmosphere interactions in this region and the main scientific questions to address in order to improve our understanding of these interactions; (iii) to present the observational strategy of this long term observing system; and finally iv) the paper describes the scope and content of the special issue. As present, 18 papers are accepted for publication in the special issue, 6 are in final form, 12 others were accepted pending minor of moderate revision (written in green in the scope and content section and in the list of references); one is still undergoing revision of the re-submitted version (written in red in the scope and content section). Given the time needed for reviewing a paper and in order not to delay the final publication of the special issue, this overview paper is submitted now, since it is considered that the papers accepted pending minor of moderate revision, will indeed be included in the special issue. We thank you very much for considering this manuscript for the special issue.