Patrick Buat-Ménard

Particulate content of savanna fire emissions

As part of the FOS-DECAFE experiment at Lamto (Ivory Coast) in January 1991, various aerosol samples were collected at ground level near prescribed fires or under local background conditions, to characterize the emissions of particulate matter from the burning of savanna vegetation. This paper deals with total aerosol (TPM) and carbon measurements. Detailed trace element and polycyclic hydrocarbon data are discussed in other papers presented in this issue.Near the fire plumes, the aerosols from biomass burning are primarily of a carbonaceous nature (C%∼70% of the aerosol mass) and consist predominantly of submicron particles (more than 90% in mass.) They are characterized by their organic nature (black to total carbon ratio Cb/Ct in the range 3–20%) and their high potassium content (K/Cb∼0.6). These aerosols undergo aging during their first minutes in the atmosphere causing slight alterations in their size distribution and chemical composition. However, they remain enriched in potassium (K/Cb=0.21) and pyrene, a polycyclic aromatic hydrocarbon, such that both of these species may be used as tracers of savanna burning aerosols. We show that during this period of the year, the background atmosphere experiences severe pollution from both terrigenous sources and regional biomass burning (44% of the aerosol). Daynight variations of the background carbon concentrations suggest that fire ignition and spreading occur primarily during the day. Simultaneous TPM and CO2 real-time measurements point to a temporal and spatial heterogeneity of the burning so that the ratio of the above background concentrations (ΔTPM/ΔCO2) varies from 2 to 400 g/kg C. Smoldering processes are intense sources of particles but particulate emissions may also be important during the rapidly spreading heading fires in connection with the generation of heavy brown smoke. We propose emission factor values (EF) for aerosols from the savanna biomass burning aerosols: EF (TPM)=11.4±4.6 and 69±25 g/kg Cdry plant and EF(Ct)=7.4±3.4 and 56±16 g C/kg Cdry plant for flaming and smoldering processes respectively. In these estimates, the range of uncertainty is mostly due to the intra-fire variability. These values are significantly lower than those reported in the literature for the combustion of other types of vegetation. But due to the large amounts of vegetation biomass being burnt in African savannas, the annual flux of particulate carbon into the atmosphere is estimated to be of the order of 8 Tg C, which rivals particulate carbon emissions from anthropogenic activities in temperate regions.

Trace elements in tropical African savanna biomass burning aerosols

As a part of the FOS/DECAFE experiment, aerosol particles emitted during prescribed savanna fires were collected in January 1991 at Lamto (Ivory Coast), either close to the emission or in ambient air. Analytical transmission electron microscopy pointed out the presence of sub-micrometer soots, salt condensates, vegetation relicts and soil derived particles. The samples were also analyzed for their total particulate matter (TPM) content and elemental composition by PIXE or XRF. At the emission, high concentrations of soil derived elements (Fe and Al) pointed out an intense remobilization process during the fires. Biomass burning emissions contributed to more than 90% of the measured concentrations, of P, Cl, S, K, Cu and Zn, which were found primarily in the fine fraction with the exception of P. Near the emission, K was mainly present as KCl, evolving to K2SO4 in the ambient samples. Trace elements emission factors were obtained for the first time for the African savanna burning and their annual emissions were estimated: our median K emission factor (0.78 g/kg of C) is higher than estimates for other ecosystems (0.2–0.58 g/kg of C); Zn emissions (0.008 Tg/year) account for 4 to 11% of the global anthropogenic emissions.

SOURCE, TRANSPORT AND DEPOSITION OF ATMOSPHERIC PHOSPHORUS OVER THE NORTHWESTERN MEDITERRANEAN

Atmospheric concentrations and total deposition (wet+dry) of phosphorus were measured over the northwestern Mediterranean between april 1985 and march 1988. A seasonal cycle of both atmospheric concentrations and total deposition is observed, the higher values being recorded during the dry season. Air-mass trajectory analyses allow an identification of the major sources of atmospheric phosphorus: soil-derived dust from desert areas of north Africa and anthropogenic emissions from european countries. The impact of the atmospheric input as a source of phosphorus for surface Mediterrancan waters is tentatively assessed on both annual and seasonal time scales. The results suggest that the atmospheric input of phosphorus could be significant to Mediterranean oligotrophic zones, especially during summer when phosphorus input from deeper waters into the photic layer is minimum.

Assessment of the African airborne dust mass over the western Mediterranean Sea using Meteosat data

The mass of African dust present over the western Mediterranean during a transport episode from northwestern Africa, which occurred in early July 1985, is estimated using a desert aerosol model, an Earth-atmosphere radiative transfer model and Meteosat visible channel data from 4 days running. Dust pixels are selected from Meteosat images, and their aerosol optical thickness is retrieved. A proportionality factor between aerosol optical thickness and atmospheric columnar aerosol loading is computed and applied to the dust pixels. The total mass of atmospheric particles over the basin is obtained by interpolation and spatial integration. The maximum aerosol optical thickness is 1.8. The maximum aerosol columnar loading is evaluated to be 2.3 g m−2. The integrated mass of particles present at a given time is estimated to raise up to about 0.6 × 1012 g at the maximum and the total mass of dust exported from Africa to be of the order of 1012 g. The method is carefully evaluated and uncertainties are discussed, with particular emphasis on the relationship between atmospheric dust mass and aerosol optical depth. The overall uncertainty on the total mass is roughly a factor ±3. In the absence of clouds it appears that the major uncertainty results from the lack of knowledge of the actual mass-size distribution of suspended dust particles, pointing out the lack of relevant data on particles larger than 10 μm in diameter. A simple calculation based on results from both computations and simultaneous field measurements yields a net transfer velocity of particles from the dust layer of approximately 1 cm s−1.

Source terms and source strengths of the carbonaceous aerosol in the tropics

Atmospheric aerosol samples were collected in the Ivory Coast, primarily at Lamto (6°N, 5°W) between 1979 and 1981. The samples were analysed for total particulate carbon concentration and isotopic composition (13C/12C) by mass spectrometry. Observed concentrations were found high compared to values reported for temperate regions. Fine particulate carbon in the submicrometersize range accounted for 50 to 80% of the reported concentrations. At Lamto, both particulate carbon concentrations and isotopic ratios exhibit a large temporal variability which is shown to reflect the diversity of sources and their seasonal evolution. Natural emissions from the equatorial forest during the wet season, and biomass burning during the dry season, appear to be the major sources. The latter, though active during only a third of the year, is, on an annual basis, the most important source. Based on the data obtained at Lamto, an attempt has been made to estimate the flux of fine particulate carbon emitted from the tropical regions into the global troposphere. This flux, which is of the order of 20×1012 g C/yr, appears to be equivalent to the flux of fine particulate carbon emitted from industrial sources. These results suggest that the tropospheric burden of fine particulate carbon in lowlatitude regions is dominated by the long-range transport of carbonaceous aerosols originating from the Tropics.

Present Transport and Deposition Patterns of African Dusts to the North-Western Mediterranean

The elements associated with mineral aerosol particles exhibit, in the Western Mediterranean, sporadic but intense concentration peaks. Twenty dust events were recorded during a one-year sampling period with their frequency being maximum in spring and summer. Three-dimensional air-mass trajectories as well as satellite imagery (Meteosat II) show that all these events are associated with transport of soil dust from Africa. Three principal source-regions have been distinguished by using air-mass trajectories. Each of them seems to be characterized also by the chemical composition of the dust collected in Corsica. Moreover, the emissions and transport of dust particles from these various source-regions were found to occur during different times of the year. This has been explained by the seasonal atmospheric circulation patterns over North Africa and the Western Mediterranean.

Transient Pb isotopic signatures in the Western European atmosphere

The progressive phasing-out of lead from gasoline has resulted in a significant decrease in the global atmospheric lead burden over the last 20 years. Here we show that in Europe this change in lead concentration - determined by the analysis of aerosols - is accompanied by a systematic change in lead isotopic compositions and that there was a significant reversal in this trend during the 1970s. Such changes in isotopic signatures over time have been accurately documented in North America. We observe that European and North American records display opposite trends in isotopic composition, thus providing a powerful tool for assessing the response time of the North Atlantic environment and its surrounding continents to transient lead inputs

Size distribution of dimethylsulfoniopropionate (DMSP) in areas of the tropical northeastern Atlantic Ocean and the Mediterranean Sea

Abstract Size fractionation of dimethylsulfoniopropionate (DMSP) was carried out in 2 areas of the Mediterranean Sea and 2 areas of the tropical northeastern Atlantic Ocean. In the Mediterranean Sea, particles in the size range 10–200 μm collected 15 m deep accounted for 16.0 ± 4.6% and 39% of total particulate DMSP off Villefranche/Mer and off Banyuls/Mer, respectively. In the Atlantic Ocean, the study of the vertical size distribution of particulate DMSP revealed that DMSP containing particles in the size range 10–200 μm tend to accumulate at the pycnocline and accounted for 26.0 ± 7.2% and 36.3 ± 11.6% of total particulate DMSP at sites 18°30′N, 21°W and 21°N, 31°W, respectively. Although particulate DMSP in mixed layer waters of the tropical northeastern Atlantic Ocean was carried mainly by particles in the size range 0.7–10 μm, no significant correlation was found between concentration of dissolved compounds (dissolved DMSP + DMS in samples filtered through GF/F filters) and this DMSP fraction. Dissolved DMSP + DMS levels were significantly correlated ( r 2 = 0.33, n = 24, P = 0.002) only with levels of particulate DMSP in the size range 10–200 μm. Since particulate DMSP in the size range 10–200 μm appeared not to be associated with microphytoplanktonic populations (diatoms and dinoflagellates), it is suggested that some heterotrophic organisms (microzooplankton) and/or detrital microscopic material (aggregates, fecal pellets) could play a key role in controlling the concentrations of dissolved DMSP and DMS in these waters. Depth profiles of DMSP levels in the size range 0.7–10 μm revealed that DMSP covaried with diadinoxanthin and zeaxanthin, two nonphotosynthetic carotenoids with photoprotective properties. This result provides the first in-situ indication of light-dependent DMSP accumulation in nannophytoplankton (most likely prymnesiophytes) and prochlorophytes.

Thermal Separation of Soot Carbon

We present here a simple and versatile variant of the thermal analysis of soot carbon, and a discussion on the encountered analytical artifacts. The method is based on a two-step combustion procedure; the removal of the organic material that does not absorb visible light is optimized during a precombustion step at 340°C during 2 hours under a pure oxygen flow, and the remaining carbon is then determined by coulometric titration of the CO2 evolved from the combustion of the samples. These analytical conditions minimize the crossover between the different components of the aerosol, but better to a clear-cut division between organics and soot carbon, the quantitative evaluation of their thermal evolution is obtained. Artifacts have been tested thoroughly with various standards and replicates of ambient air samples collected. The method gives reliable soot carbon determination at the microgram level in samples from a wide variety of environments. Combined H / C atomic ratio measurements and investigations of ...

Particle Geochemistry in the Atmosphere and Oceans

The purpose of this chapter is to review our present knowledge of the geochemistry of atmospheric and oceanic particles. I also want to show that these two cycles of particulate matter are coupled by the physical, chemical, and biological processes that take place at the air-sea interface and in the surface waters of the ocean. This geochemical coupling is particularly strong for trace metals. Within a few years at most, atmospheric inputs of trace metals are transferred to the deep ocean by the removal of these elements from the surface layers of the ocean by settling particles, primarily of biogenic origin. On the other hand, atmospheric sea-salt particles, enriched in organic matter and trace metals, are continuously recycled between the oceans and the atmosphere and the fraction transported to land will ultimately find its way back to the oceans, transported by rivers. As a consequence, air-to-sea and sea-to-air particulate transfers are important for the understanding of many bio-geochemical cycles and their alterations by man.