Robert J. Delmas

The Ice Record of Greenhouse Gases

Gases trapped in polar ice proved our most direct record of the changes in greenhouse gas levels during the past 150,000 years. The best documented trace-gas records are for CO[sub 2] and CH[sub 4]. The measurements corresponding to the industrial period document the recent changes in growth rate. The variability observed over the last 1000 years constrains the possible feedbacks of a climate change on the trace gases under similar conditions as exist today. Changes in the levels of greenhouse gases during the glacial-interglacial cycle overall paralleled, at least at high southern latitudes, changes in temperature; this relation suggests that greenhouse gases play an important role as an amplifier of the initial orbital forcing of Earths climated and also helps to assess the feedbacks on the biogeochemical cycles in a climate system in which the components are changing at different rates.

Carbon dioxide and methane emissions and the carbon budget of a 10-year old tropical reservoir (Petit Saut, French Guiana)

emissions, 0.07 ± 0.01) the first 3 years after impounding (1994–1996) and then decreased to 0.12 ± 0.01 Mt yr 1 C( CO2, 0.10 ± 0.01; CH4, 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO2 emissions occurred by diffusion from the reservoir surface, 31% from the estuary, 7% by degassing at the outlet of the dam, and a negligible fraction by bubbling. CH4 diffusion and bubbling from the reservoir surface were predominant (40% and 44%, respectively) only the first year after impounding. Since 1995, degassing at an aerating weir downstream of the turbines has become the major pathway for CH4 emissions, reaching 70% of the total CH4 flux. In 2003, river carbon inputs were balanced by carbon outputs to the ocean and were about 3 times lower than the atmospheric flux, which suggests that 10 years after impounding, the flooded terrestrial carbon is still the predominant contributor to the gaseous emissions. In 10 years, about 22% of the 10 Mt C flooded was lost to the atmosphere. Our results confirm the significance of greenhouse gas emissions from tropical reservoir but stress the importance of: (1) considering all the gas pathways upstream and downstream of the dams and (2) taking into account the reservoir age when upscaling emissions rates at the global scale.

Methane and carbon dioxide emissions from tropical reservoirs : Significance of downstream rivers

[1] Methane (CH4) andcarbon dioxide (CO2) concentrations and water-air fluxes were measured in three tropical reservoirs and their respective rivers downstream of the dams. From reservoirs, CH4 and CO2 flux were in the range of 3 ± 2 and 254± 392mmol.m � 2 .d � 1 ,respectively.Riversdownstreamof dams were significantly enriched in CH4 and CO2 originating from reservoir hypolimnions. From rivers, CH4 and CO2 flux were in the range of 60 ± 38 and 859 ± 400 mmol.m � 2 .d � 1 , respectively. Despite their relatively small surfaces, rivers downstream of dams accounted for a significant fraction (9– 33% for CH4 and 7–25% for CO2) of the emissions across the reservoir surfaces classically taken into account for reservoirs. A significant fraction of CH4 appeared to degas at the vicinity of the dam (turbines and spillways), although it could not be

Gaseous emissions and oxygen consumption in hydroelectric dams: A case study in French Guyana

Methane, carbon dioxide, and hydrogen sulfide emissions from the hydroelectric dam of Petit Saut on the Sinnamary River in French Guyana have been measured over a 2 year period. Since the beginning of the reservoir filling (January 1994), 300 km 2 of tropical forest have been submerged. Emissions of CH 4 by diffusion and by bubbling into the atmosphere or by degassing of the water released into the river, as well as the stock of dissolved gases in the lake, and their temporal evolutions were determined. Maximum emissions of 800 t CH per day were reached in February 1995, corresponding to dissolved CH 4 concentrations of 14 mg 4 L -1 in the water column. The biological oxidation of methane results in a strong oxygen consumption in lake and river waters. Total emissions of CH 4 and CO 2 from January 1994 to December 1995 were calculated from the whole data set, which also allows us to calculate the total carbon loss since reservoir filling. About 10% of the carbon stored in soil and vegetation was released in gaseous form within 2 years.

Global inventory of NOx sources

Nitrogen oxides are key compounds for the oxidation capacity of the troposphere. NOx concentrations depend on the proximity of sources because of their short atmospheric lifetime. An accurate knowledge of the distribution of their sources and sinks is therefore crucial. At global scale, the dominant sources of nitrogen oxides - combustions of fossil fuel (∼50%) and biomass burning (∼20%) - are basically anthropogenic. Natural sources, including lightning and microbial activity in soils, represent therefore less than 30% of total emissions. Fertilizer use in agriculture constitutes an anthropogenic perturbation to the microbial source. The methods to estimate the magnitude and distribution of these dominant sources of nitrogen oxides are discussed. Some minor sources which may play a specific role in tropospheric chemistry such as NOx emission from aircraft in the upper troposphere or input from production in the stratosphere from N2O photodissociation are also considered.

Loss of volatile acid species from upper firn layers at Vostok, Antarctica

The adhesion between carbon fibers and unsaturated matrix resins used in the preparation of composite structures is improved by the use of an unsaturated epoxide bifunctional coupling agent. The coupling agent can be applied to the carbon fibers or incorporated into the matrix resin system.

Long‐term greenhouse gas emissions from hydroelectric reservoirs in tropical forest regions

The objective of this work is to quantify long-term emissions of two major greenhouse gases, CO2 and CH4, produced by the decomposition of the flooded organic matter in tropical artificial reservoirs. In a previous paper [Galy-Lacaux et al., 1997], gas emissions from the tropical reservoir of Petit Saut (French Guiana) were quantified over the first two years after impounding. This work presents emission fluxes and distributions of dissolved methane and carbon dioxide measured in the reservoir of Petit Saut over three and a half years, since the beginning of impounding (1994) and during operation (1995–1997). To assess long term emissions, an experimental campaign was conducted on four hydroelectric reservoirs (Taabo, Buyo, and Ayame I and II) built between 1960 and 1980 in the Ivory Coast. Average dissolved CH4 concentration in the water column of the Petit Saut reservoir first increased, up to a maximum of 14 mg L−1, in May 1995. Then the time course of dissolved CH4 over the three and a half year period, showed periodical variations. These changes were related to changes in the inlet water flow and the residence time of water in the reservoir. In the older African reservoirs, average dissolved methane concentrations were lower and ranged between 0.20 and 0.32 mg L−1. The whole data set allows us to propose an analytical algorithm in order to predict the time course of dissolved CH4 concentration in the Petit Saut reservoir. Temporal variations of total CH4 and CO2 emissions from the reservoir over three and a half years were extrapolated with this algorithm to calculate long term carbon losses. Over a 20-year period the estimated carbon losses in the form of CO2 and CH4 were dominated by the outlet fluxes of dissolved gases (2160 ± 400 Gg (C)), and they correspond to a total net carbon loss of 3.2 Tg (C). The contribution of the Petit Saut reservoir to greenhouse gas emission, over 20 years, is estimated to be 66 ± 20 Tg of CO2 equivalent (56 Tg as CH4 and 9.7 Tg as CO2).

Sulfur‐containing species (methanesulfonate and SO4) over the last climatic cycle in the Greenland Ice Core Project (central Greenland) ice core

A high-resolution profile covering the last two centuries and a discontinuous study spanning the complete last glacial-interglacial cycle of methanesulfonate (MSA) (CH3SO3−) and sulfate were obtained along Summit (central Greenland) ice cores. MSA concentrations were close to 4±1.4 ng g−1 from 1770 to 1870 A.D. and 3 ng g−1 in 1900, and exhibited a well-marked decreasing trend from 1945 to the present. These changes of Summit snow MSA concentrations between 1770 and 1945 are discussed in terms of possible modulation of dimethylsulfide (DMS) marine emissions influencing the Greenland Ice Sheet by past climatic fluctuations in these regions. The decrease of MSA levels in Summit snow layers deposited since 1945 suggests either a decline in marine biota at high northern latitudes or a changing yield of MSA from DMS oxidation driven by modification of the oxidative capacity of the atmosphere in response to increasing anthropogenic NOx, and hydrocarbon emissions. While interglacial ice concentrations of MSA and sulfate are close to 2.9±1.9 ng g−1 and 27±10 ng g−1, respectively, reduced MSA (1.2±0.7 ng g−1) and enhanced sulfate (55±19 ng g−1) levels characterized the early Holocene stage (9000 to 11,000 years B.P.). MSA concentrations in glacial ice remain similar to the ones observed during interglacial stages. In contrast, sulfate levels are strongly enhanced (243±84 ng g−1) during the last glacial maximum (14,400 to 15,700 B.P.) compared with the interglacial ones. These variations of sulfur-containing species in response to past climatic conditions are similar to those found in other Greenland cores. In contrast, they are different from those revealed in the Antarctic Vostok ice core, where colder climates were associated with an increase by a factor of 5 and 2 in MSA and sulfate concentrations, respectively. These glacial-interglacial changes are discussed in terms of present and past contributions of marine DMS emissions versus other sulfate sources such as volcanic emissions and continental dust to the Greenland precipitation.

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.

Biomass burning in the tropical savannas of Ivory Coast: An overview of the field experiment Fire of Savannas (FOS/DECAFE 91)

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 (ΔX/ΔCO2) 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 (ΔCO/ΔCO2 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.