Environ. Sci. Technol. 1995, 29, 486-493
Biumms h m n ig in the Amazon: Ck of the h i s
Savannah ANDREW G. A L L E N t A N D ANTONIO H. MIGUEL* Department of Fundamental Chemistry, Chemistry Institute, University of Sdo Paulo, Sdo Paul0 01498-970, Brazil ~~
Samples (170) of biomass combustion smoke were obtained in Brazilian tropical rainforest and savannah during August-September 1992. Speciation of the ionic fraction of fine ( d < 2 p m ) and coarse (15 p m > d > 2 p m ) mode particles was achieved using ion chromatography, and the data were interpreted according to geographical region, vegetation type, and phase of combustion. The species measured were CH&OO-, HCOO-, CI-, N03-, so$-, c20h2-, Po43-, Na+, NH4+, K+, Mg2+, and Ca2+. Mean total ionic contents for the fine mode were 5.6% (Brasilia) and 2.2% (Rondonia) and for the coarse mode were 8.3% (Brasilia) and 13.7% (Rondonia). The fine mode ionic fraction was dominated by potassium, chloride, and sulfate, while calcium and acetate were important in the coarse mode. Highest ionic contributions were always observed during flaming combustion. Sources of aerosols were condensationlcoagulation, release of partially combusted plant material, and suspension of soil particles.
Introduction Biomass burning is a major global source of atmospheric particles (1-4). Currently over 80% of emissions from biomass burning occurs in the tropics (5, 6'), while during the last decade the rate of biomass burning has increased largely due to accelerating deforestation in Amazonia (7) and increased frequency of burning of African savannah (5). The global annual biomass consumption is -8680 Tg (1Tg = 1OI2 g) of dry material (8). Of this total, -3690 Tg is attributed to the burning of savannah grasslands, -2020 Tg to agricultural wastes, -1540 Tg to forests, and -1430 Tg to fuel wood. The largest single contribution to the forest component occurs in tropical South America. The particles derived from biomass burning are largely composed of condensed hydrocarbons, together with an admixture of elemental (graphic)carbon plus ionic species. Such particles are likely to act as efficient cloud condensation nuclei (CCN),dependent on their content of hygroscopic, water-soluble material. Such material may either constitute most of the mass of the individualparticles, may be deposited on the surface of insoluble material (such as graphite, soil dust, or biological fragments), or may be internally mixed with the carbonaceous components. The latter two processes predominate in primary biomass burning aerosols. The modified clouds produced by the presence of increased number concentrations of CCN may increase reflectance to space of incident solar radiation (9, IO),or, where the aerosols possess a significant graphitic carbon content, increase absorption of solar radiation (11). Between 80 and 100% of all particles present in biomass burning smoke are likely to be active as CCN at typical atmospheric supersaturations (121,although only a limited number of field measurements of the water-soluble components of biomass burning aerosols are currentlyavailable to support this estimate (12-15). The present study aims to significantly extend the existent data set with a detailed characterizationof the ionic compositionof smoke particles sampled close to the point of emission according to combustion phase, vegetation type, and geographical region.
Methodology Field Sites. Two genographically distinct locations were visited between August 11 and September 21, 1992. The first site, located at the Instituto Brasileiro de Geografia e Estatistica (IBGE),-35 km south of Brasilia,was characterized by savannahvegetation (cerrado)typical of the central Brazilian plateau and a dry season extending from April to September. Savannah burning activity was concentrated in the months ofAugust and September. Cerrado comprises five differentvegetation categories (16'): campo limpo,open tropical grassland < 1 m height (Cl);campo sujo, grassland with other low vegetation < 3 m (C2);campo cerrado (also cerrado aberto), grassland with small trees 10m (C5). For the present work,firesinthefirstthreecategories(Cl-C3) werestudied. Fuels in these biomasses were predominantlydry litter and grasses together with smaller tree branches and bark. A second set of samples was obtained during fires at locations in the western Amazon state of Rondonia. Vegetation comprised primary and secondary rainforest (PF and SF) as well as rough grassland (G) resultant from previously converted forest. Fuel material in the rainforest was composed of fallen tree debris and foliage,forest floor litter, small plants and seedlings, and trees remaining standing. Sampling and Analysis. Aerosolsampleswereobtained within smoke plumes at a distance of a few meters from burning vegetation. Two size fractions were obtained by placing two filters in series behind a cyclone designed to exclude particles larger than -15.0 pm diameter; particles of diameter 2.0-15 pm were collected using 47-mm, 8.0pm Nucleopore polycarbonatemembranes,while particles
