Cloud-active particles from African savanna combustion experiments

Abstract

In order to study the behaviour of the combustion aerosols and to quantify the emission of particles, especially of CCN, various burning experiments on Guinean savanna species as commonly growing in Ivory Coast and throughout Africa were carried out in a large test chamber (160 m 3) or in a combustion chamber capable of separating the flaming phase from the smoldering one. The total particulate matter, as dominated by the smoldering process, ranges from 0.005 to 0.019 g per g of dry matter (d.m.), while the number of Aitken nuclei is of the same order of magnitude for both processes, averaging ≈ 2.5 × 10 12 g −1 d.m. A large part of the particles are hygroscopic even at relative humidity (RH) quite below water saturation. Such a behaviour may be explained by the chemical composition of resolved-size particle samples. Ion chromatography reveals that most particles with diameter ≤0.5 μm contain the larger mass fraction of chloride, nitrate and sulfate, so that the combustion particles can deliquesce as salt particles or as salt-coated particles at critical RH related to NH 4NO 3, NaNO 3, NH 4Cl, (NH 4) 2SO 4, etc. The CCN emission factor (EF) is 2 to 6 times higher for the smoldering than for the flaming, probably due to the larger particle size range. For all experiments, the EF averages ≈ 2 × 10 11 g −1 d.m. at 0.1% supersaturation (SS), leading to a flux of 300 to 600 CCN s −1 cm −2 and thus to an overall production rate of 3 × 10 19 to 6 × 10 19 CCN s −1 at 0.1% SS for the African savannas, that is a source of the same order of magnitude as the other ones, natural or anthropogenic.