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

Abstract

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.

Chemical apportionment of southern African aerosol mass and optical depth

“…2), after applying corrections described in Sect. 2, agree well with other studies of African aerosols dominated by biomass burning (Ruellan et al, 1999;Formenti et al, 2003). Using aerosol mass concentrations together with measurements of aerosol optical properties (Magi et al, 2008), mass scattering cross sections (MSC) and mass absorption cross sections (MAC) for the aerosol species were derived.…”

“…vegetation, combustion completeness). Ruellan et al (1999) reported mean OC and BC mass concentrations of about 22±11 and 5±2 µg/m 3 , respectively, between the surface and about 4 km. This is very similar to median OC and BC mass concentrations of 17 µg/m 3 (12-23 µg/m 3 interquartile range) and 3.4 µg/m 3 (1.8-4.5 µg/m 3 interquartile range), respectively, reported in Table 5 for tropical southern Africa.…”

“…The higher SO 4 concentration is expected since many of samples in this study were affected by proximity to concentrated burning (tropics) or proximity to industrial sources (extratropics). Ruellan et al (1999) collected aircraft-based in situ aerosol samples during the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) field campaign in November and December 1996 over Northern Hemisphere equatorial Africa (3-5 • N, 17-19 • E). Fires in this region in November and December are as numerous as those in tropical southern Africa during August and September, so there should be some similarities in the aerosol regardless of the differences in burning characteristics (i.e.…”

Sources of nitrate in stratocumulus cloud water: Airborne measurements during the 2011 E-PEACE and 2013 NiCE studies

“…Nitric acid can partition to particles, such as those in the coarse size range (Fig. 2), and be transported over long distances (Ruellan et al, 1999). The concentration of super-100 nm particles in the free troposphere was enhanced (maximum concentration ¼ 4608 cm À3 ) due to wildfire smoke (Fig.…”

Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

“…Primary soot particles, 10-50 nm spherules, do not exist by themselves in ambient air-instead they cluster together immediately after their formation in a flame to form aggregates, which are their most stable form (Wentzel et al, 2003). In fresh smoke, these clusters tend to form open structures, which are then transformed by aging processes, including the uptake of water, into more closely packed particle types (Colbeck et al, 1990;Weingartner et al, 1997;Ruellan et al, 1999;Abel et al, 2003;Onischuk et al, 2003). However, soot particles associated with the smoldering stage of biomass combustion usually are present as much larger spherical and compacted aggregates that seem to be more resistant to atmospheric aging processes .…”