The burning of living and dead vegetation (biomass burning) is widespread over Earth, and this is a globally significant source for aerosols and for a variety of radiatively active and chemically reactive trace gases. Most of the biomass burned is caused by human activities as opposed to natural fires, and this mainly occurs in the tropics, involving savannas more than forests (Hao and Liu, 1994). Some burning sources are persistent, but emissions from savanna burning vary biennially because the growth of the savanna vegetation occurs during the wet season, and afterwards as biomass dries, it is burned.
Aerosols produced by biomass burning are composed of some black carbon (soot) but mainly organic carbon with hydrogenated and oxygenated functional groups. The amount of black carbon produced by fires is highly variable, and this is determined by the type of fuel consumed and whether the fire is in the ignition phase, flaming, or smoldering. Moreover, the composition of the aerosols can change quite rapidly—over time scales of seconds to minutes—as the particles are advected away from the fire. The transformation of particles in a smoke plume can lead to internal mixtures, i.e., particles with cores of black carbon and low-volatility organic compounds become coated with outer layers of more volatile organics (Mazurek et al., 1996).
The amount of particulate matter put into the atmosphere by biomass burning is estimated to be —90 Tg per year (37 Tg/yr of this is from savanna burning), and this amounts to more than 20% of the total suspended particulates from all anthropogenic sources (Andreae et al., 1996). The black carbon from biomass burning (60 Tg/yr) accounts for an even larger fraction, i.e., two-thirds of the global black carbon emissions from all anthropogenic sources. These authors estimate that the number of cloud condensation nuclei generated globally by biomass burning activities is 35 x 1027, over 10% of which is from savanna fires.
The organic carbon composition of biomass burning aerosols is not fully characterized owing at least in part to the diversity of fuels burned in different geographical regions. However, studies of tropical biomass burning have shown the major organic components are straight-chain, aliphatic, and oxygenated compounds, triterpenoids from plant waxes, resins/gums, and biopolymers (Simoneit et al., 1996). The fatty acid composition of aerosols produced in laboratory and field burns (Ballentine et al., 1996) was found to be dominated by saturated even-chain compounds, reflecting the importance of terrigenous plant waxes.
The polycyclic aromatic hydrocarbons (PAHs) in aerosols from biomass burning include biphenyl, trimethylnapthalenes, phenanthrene, anthracene, methylphenan-threnes, fluoranthene, pyrene, methylpyrenes, chrysene, benzanthracene, benzofluor-anthenes, benzo ([e] and [a]) pyrenes, indenopyrene, benzo(ghi)perylene, and coronene (Simoneit et al., 1996; Ballentine et al., 1996). Simoneit et al. (1996)
also reported the occurrence of oxy-PAHs in burning products, including fluorenone, anthra-9,10-quinone, cyclopenta(def)phenanthrene-4-one, benzo[a]fluorene-l 1-one, benzanthrone, and napthanthrone. Both the PAHs and the oxy-PAHs are produced by incomplete combustion, and the production and transport of PAHs is of particular concern owing to the carcinogenicity of these compounds. Several organic compounds (amyrones, friedeline, aromatic A-noroleananes, syringaldehyde, vanillin, syringic acid, and vanillic acid) have been proposed as tracers of aerosols from biomass burning (Simoneit et al., 1996).
Other proposed tracers of biomass burning include the trace elements potassium and zinc (Andreae, 1983). These elements are enriched in biomass burning aerosols, and the ratios of K and Zn to black carbon (Cb) in aerosols from burning have been found to be quite constant (K/Cb -1.3; Zn/Cb ~5.4%o, Cachier et al., 1996). Elemental analyses of coarse aerosols from prescribed savanna burns by Maenhaut et al. (1996) showed the fires were a major source for black carbon, P, K, Ca, Mn, Zn, Sr, and I. For fine particles (r < 1 |im), the flaming and smoldering phases of the fires were a major source for black carbon, CI, Br, I, K, Cu, Zn, Rb, Sb, Cs, and Pb; and under flaming conditions also important for Na and S. These authors also found that fires could mobilize significant amounts of mineral dust, presumably through the convection associated with the fires.
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