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Abstract. 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/.tg 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/.tg m -3, n = 6), the savanna boundary layer (9.8 +_ 3.9/.tg m -3, n = 6), or in the Harmattan layer (8.7 +_ 1.6/.tg 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.

[1] An investigation of aerosol chemistry was carried out at Sevettijarvi in Finnish Lapland between September 1997 and June 1999. Aerosol particles were collected on a 2-day basis using two-stage virtual impactors and were analyzed with ion chromatography for major inorganic cations and anions and for a suite of organic acids. Aerosols were also sampled in parallel on a 4-day basis for the analysis of organic carbon (OC) and black carbon (BC). The average total mass is about 3 mg m À3 and does not significantly vary according to the season or the type of air mass. The major chemical components are sulfate, sea salts, and organic carbon, which account together for more than 80% of the total aerosol mass. BC, ammonium, nitrate, methanesulfonic acid, and the estimated crustal fraction each accounts for a few percent at most in any situation. Non-sea-salt (nss) sulfate concentrations are maximum during late winter and spring, related to the Arctic haze, associated with increased concentrations in BC, ammonium, and nss K + . The organic fraction is at its lowest in winter, as are the concentrations of most organic acids. OC and short-chain organic acid concentrations increase during springtime, which may be due to enhanced photochemistry at polar sunrise. The chemical profile is rather different during summer, with a strong decrease of the anthropogenic fraction and a larger occurrence of episodic marine events. However, the main characteristic is the very large increase in OC concentrations, which is the main component of the aerosol at that time and may be linked with local and regional enhanced biogenic activity. The aerosol at Sevettijarvi presents some specificity compared with other Arctic sites, with a much smaller impact of Arctic haze and marine events in winter and a much larger impact of biogenic sources in summer. The low contribution of the crustal fraction indicates low occurrences of transport of desert dust from Eurasia. The time series of concentrations indicate a large variability in the chemical profiles on short timescales, linked with changes in the origin of the air masses. It shows that even purely marine aerosol still comprises about 10% of nss sulfate associated with BC and OC. The profile in the continental case is largely dominated by nss sulfate, with strong increases in the ammonium and BC fractions.

Abstract. This paper presents an overview of the Experiment for Regional Sources and Sinks ofOxidents (EXPRESSO) includingAverage dynamic and turbulence characteristics over savanna and forest ecosystems were retrieved from aircraft measurements. They illustrate the complex atmospheric circulation occurring in this region in the vicinity of the Intertropical Convergence Zone. Satellite receivers were operated three times a day to produce maps of fire distribution. Statistics and mapping of burned surfaces t?om NOAA-AVHRR and ERS-Along Track Scanning Radiometer space systems have been developed. The influence ofbiogenic and biomass burning sources on the chemical composition of the lower atmosphere was studied through both aircraft and tower measurements. The EXPRESSO field campaign was followed by modeling efforts (regional and global scales) in which model components are evaluated using the experimental data.