Pierre Masclet

Airborne aerosols over central Africa during the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO)

As part of the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) conducted over central Africa in November 1996, 24 airborne aerosol samples were obtained and further analyzed for black and organic carbon (BC and OC), water-soluble organic carbon (WSOC), polycyclic aromatic hydrocarbons (PAHs), soluble ions, elemental composition, and morphology. Particles were collected in the different atmospheric layers either above the tropical forest or the savanna of central Africa near the Intertropical Convergence Zone. Particle number concentrations (10–14,000 nm diameter) were found to be high in all atmospheric layers (3100±2060 cm−3, n = 24). Soil-derived particles were less abundant than expected (20±18 μg m−3, n = 21), and their presence was assessed mainly to reentrainment by fires. On the other hand, pyrogenic particles were abundant, and high levels of black carbon (BC) concentrations were found either in the forest boundary layer (3.8±2.3 μg m−3, n = 6), the savanna boundary layer (9.8±3.9 μg m−3, n = 6), or in the Harmattan layer (8.7±1.6 μg m−3, n = 3). Other fire tracers (such as K, oxalate, or PAHs) confirmed the overwhelming impact of savanna fires in the regional troposphere. Another result is the possible occurrence of vertical and horizontal exchanges between the different layers and through the ITCZ. WSOC was measured in our samples representing on average 46±9% (n = 11) of the total particulate organic carbon. High values were found in the Harmattan layer, where on average WSOC accounts for 85±18%, (n = 3) of the total particulate organic carbon, pointing out the potential of biomass burning particles to act as cloud condensation nuclei (CCN). Different chemical indicators were used, which produce convergent information on the aging of biomass burning particles. Among these indicators, the ratio WSOC/OC was found to increase by a factor of 2 to 3 from the ground to the Harmattan layer. A product of this work is also the presence of high concentrations of organic acids (formate, acetate, and also oxalate) in the forest boundary layer suggesting a strong biogenic source for these compounds. Finally, during the EXPRESSO experiment, which took place at the beginning of the dry season, savanna fires were prevailing at a regional scale, whereas dust inputs by Harmattan airflow were still low. Our results suggest that in these conditions nitrate primarily remains in the gaseous phase, and thus the translocation of nitrogen nutrients is confined to the region.

Emissions of Polycyclic aromatic hydrocarbons by savanna fires

Although Polycyclic aromatic hydrocarbons (PAH) are known as anthropogenic compounds arising from the combustion or the pyrolysis of fossil fuels, they may be also emitted by the combustion of vegetation. A field study was carried out in January 1991 at Lamto (Ivory Coast) as part of the FOS DECAFE experiment (Fire Of Savanna). Some ground samplings were devoted to the qualitative and quantitative characterization of atmospheric emissions by savanna fires during prescribed burns and under background conditions. Specific collections for gaseous and particulate PAHs have shown that the African practice of burning the savanna biomass during the winter months is an important source of PAHs. These compounds are emitted mainly in gaseous form but a significant fraction, essentially heavy PAHs, is associated with fine carbonaceous particles and can therefore represent a hazard for human health, since some of these compounds are mutagenic and carcinogenic. Twelve compounds were identified during the fire episodes and in the atmospheric background. The total concentration in the fires is of the order of 10 ng m−3 for the gas phase and from 0.1 to 1 ng m−3 in the aerosols. In the atmospheric background the mean concentrations are regular, 0.15 ng m−3 and 2 pg m−3, respectively. These concentrations are comparable with what is observed in European rural zones. The particulate emissions of PAHs by the savanna fires are distinguished by the abundance of some compounds which can be considered as tracers, although they are also slightly emitted by fossil fuel sources. These compounds are essentially pyrene, chrysene and coronene. In the gas phase, although no individual PAH may be considered as specific of the biomass combustion emissions, the relative abundances of the main PAHs are characteristic of the biomass burning. The concentrations of pyrene and fluorene are always predominant; these compounds could be considered as characteristic emission products of smoldering and flaming episodes, respectively. In the background the PAH composition shows that in a tropical region the air consists of a mixture coming from the various sources, but the biomass combustion is by far the most important source.The fluxes of total PAH emitted by savanna biomass burning in Africa were estimated to be of the order of 17 and 600 ton yr−1, respectively, for the particulate PAHs and the gaseous PAHs, respectively.

Transfer function of polycyclic aromatic hydrocarbons from the atmosphere to the polar ice. I: Determination of atmospheric concentrations at dye 3, Greenland

Preliminary measurements of the gaseous and particulate fractions of atmospheric PAH have been performed in winter and spring during the DGASP program on the Greenland Ice Sheet. The results indicate the absence of gaseous fraction (<0.5 pg m−3) associated with very lw concentrations restricted to heavy compounds only (18–200 pg m−3 for total PAH) in the particulate phase; the repartition of the various PAH is very stable from one sample to another, indicative of near-complete reaction allowed by long transport time. Nevertheless, discrimination of the specific signal from the different sources involved may have been impaired by the long sampling time used.

Use of capillary electrophoresis to monitor concentrations of organic acids in snow and rain water

Abstract The concentrations of six organic acids present in snow and rain water were determined during the spring of 1992, in order to test the possibilities of capillary electrophoresis for measurements in the environment. The sampling sites were located in the French Alps. Fourteen samples were taken. This method allows routine analysis. The following acids, normally found in snow and rain waters, were satisfactorily separated after 5 min: formic, acetic, propionic, butyric, oxalic and benzoic, with concentrations of the order of 10 −6 M. The sample size was very small since a 100-μl aliquot is sufficient to perform an analysis without previous treatment or preliminary concentration. For these three reasons — time, sensitivity and sample amount — capillary electrophoresis is suitable for measurements in the environment. Short sampling times can be used, for example, in order to follow rain events. Monitoring organic acids in hydrometeors collected at high altitude show that their concentrations are fairly constant for the various individual rain or snow samples but are different for rain and snow samples. The total concentrations observed for the five acids detected (all organic acids except oxalic) ranged from 5 to 10 μM for rain water and 0.5 to 5 μM for snow. The pofile established from the relative abundance of the acids diffeers from snow to rain and depends on the season, which confirms that formic acid is a biogenic tracer and acetic acid is an anthropic tracer. Furthermore, we show, for the first time, that butyric acid is a good tracer of bacterial activity.

Study of heterogenous reactions of polycyclic aromatic hydrocarbons I: Weakening of PAH-support bonds under photonic irradiation

The various parameters determining the reactions of PAH in the particulate phase are studied. An alternating pulsed-flow reactor allowing continuous mixing of the particles and evacuation of the reactant gases, has been constructed. A preliminary study of the photolysis of particulate PAH by sunlight and in the absence of oxidizing gases shows that photolysis is negligible for short irradiation times (<2 h) for most of the 6 PAH studied: anthracene, fluoranthene, pyrene, benzo[a]pyrene, benzo[ghi]perylene a d benzo[b]fluoranthene, and for the two substrates used: alumina and fly ash. On the other hand, this study shows that the particle surface is deactivated and that the PAH-support bonds are weakened; there is partial desorption of PAHs under photonic impact. Two types of PAH are present in the particulate phase; reversibly physisorbed and irreversibly chemisorbed PAH. This phenomenon has not been considered in kinetic studies. It may even cast some doubt upon the determination of atmospheric concentrations which are probably underestimated by classical analytical methods.

Contamination of lake water by pesticides via atmospheric transport.

High altitude lakes cannot be contaminated by a direct introduction of xenobiotic chemical compounds such as pesticides. Nevertheless they can be contaminated by these compounds after meso scale atmospheric transport. Laboratory experiments were carried out to determine the life time of pesticides in the atmosphere. Simulation of atmospheric degradation by UV and/or atmospheric oxidants of pesticides absorbed on terrigenic aerosols (kaolin) or anthropic aerosols (fly ash) shows that some pesticides such as atrazine, simazine or trifluraline have sufficient life times to be transported over several hundred miles from the source areas to the receptor sites and then contaminate the water after wet or dry deposition, while other pesticides such as diuron, carbaryl or fenitrothion are rapidly destroyed during atmospheric transport and cannot reach remote areas. Simultaneously, field measurements were carried out in the Savoy region in France to see if these pesticides were present in the troposphere and in the Alpine lake water. Significant amounts of atrazine, simazine, pendimethalin, and trifluralin were found both in the air and in the water, while no trace of isoproturon, diuron and fenitrothion were detected in the atmosphere in accordance with their expected life times. The presence of carbaryl, however, in lake water is strange since this compound has a short life time. These preliminary results will be used to keep track of the water in parallel with ecotoxicological tests and biochemical assays.