Inorganic and carbonaceous aerosols during the Southern African Regional Science Initiative (SAFARI 2000) experiment: Chemical characteristics, physical properties, and emission data for smoke from African biomass burning

Abstract

We collected filter samples of the atmospheric aerosol during the Southern African Regional Science Initiative (SAFARI 2000) experiment onboard the UK Met Office C‐130 aircraft. The main operational area was the Atlantic Ocean offshore of Namibia and Angola, where biomass‐smoke haze at least 1–2 days old was widespread. The size‐fractionated aerosol samples were analyzed for the major inorganic ions, carbonaceous material (elemental and organic carbon), and elements with atomic numbers between 11 (Na) and 82 (Pb). The regional haze aerosol was composed mostly of carbonaceous aerosols (on the average, 81% of the submicron mass), with secondary inorganic aerosols (sulfate, ammonium, and nitrate) accounting for another 14%. K + and Cl − , typical pyrogenic species, constituted only 2% of the mass. The aerosol chemical data were used to estimate mass emission fluxes for various aerosol components. For African savanna/grassland burning, the estimated emission flux of carbonaceous particles (particulate organic matter plus elemental carbon) is 14 ± 1 Tg yr −1 , and that of the nitrogen species (nitrate and ammonium) is 2 ± 2 Tg yr −1 . For the flight segments in regional haze, the mean particle scattering coefficient at 550 nm was σ s = 101 ± 56 Mm −1 and the mean particle absorption coefficient σ a at 565 nm averaged 8 ± 5 Mm −1 (mean single scattering albedo of 0.93 ± 0.06 at 550 nm). The dry mass scattering efficiency α s , calculated from the linear regression of the mean scattering versus the estimated submicron mass, is estimated to be between 4.2 ± and 4.6 ± 0.6 m 2 g −1 , depending on the assumptions made in calculating the aerosol mass. The dependence of the scattering enhancement ratios Δσ s /ΔCO on the distance from the burning regions suggests that the evolution of particle size with time influences the light scattering efficiency. Fresh smoke was sampled during a dedicated flight in the proximity and within the plume of an active biomass burning fire. Here the enhancement ratio with respect to CO of particles in the Aitken‐size range (5–100 nm diameter) was ΔN Aitken /ΔCO ∼25 cm −3 (STP) ppb −1 . These particles were removed rapidly after emission, and they were not detectable in the regional haze. The enhancement ratio for accumulation mode particles (0.1–1 μm diameter) ΔN Acc /ΔCO was ∼26–30 cm −3 (STP) ppb −1 in young smoke, and 16 ± 3 cm −3 (STP) ppb −1 in aged haze, suggesting that the number concentration of accumulation mode particles was reduced by about 41% during aging.