Aerosols from Biomass Burning


Biomass burning is a globally important source of aerosols. Although burning of vegetation occurs in temperate and boreal regions, emissions from these regions are negligible in comparison to the tropics. The intensity of burning is greatest in Africa, where savanna burning alone accounts for more than 40% of the total biomass burned worldwide. Burning of forest during shifting agriculture and during permanent land clearance produces emissions of a similar order of magnitude. In addition, in the tropical regions the burning of fuel wood and agricultural waste is the main energy source (as opposed to fossil fuels, etc.). A recent analysis suggests that up to 3000 Tg of biofuel may be burned worldwide; it is mainly used in developing regions experiencing rapid population growth. Recently it was shown that vast amounts of a strongly light-absorbing carbonaceous aerosol are emitted from the Indian subcontinent. Biomass burning produces both primary aerosol particles and precursor gases for secondary aerosol formation. About 70% of the mass of particulates released is fine carbon-containing particles. In this particulate matter, most of the carbon is in organic compounds, but up to about 20% occurs as black carbon (elemental carbon, or "soot"). Globally, these emissions are important; according to the IPCC, black carbon emitted worldwide from biomass burning is similar in magnitude to that from fossil fuel burning (both ca. 6-9 Tg/yr). Because they absorb light, black carbon aerosols are treated differently than scattering aersols in studies of climate forcing . Biomass burning may account for close to half 40% of the global emission of anthropogenic VOC (volative organic carbon). The gaseous organic compounds released during biomass burning condense easily (at low saturation values) and react with each other to form aerosol particles. Gaseous hydrocarbons released during combustion include ethane, propane, ethene and propene. Acetylene, methyl chloride. and COS are also released from burning vegetation. Although COS is not a hydrocarbon, it has a long atmospheric residence time, can be oxidixed to sulfate and contribute to sulfate aerosol formation. The presence of polar functional groups, particularly carboxylic and dicarboxylic acids, makes many of the organic compounds in aerosols water-soluble and allows them to participate in cloud droplet nucleation.