Resumo

PACHECO, Milexi; SANHUEZA, Eugenio  e  ROMERO, Johnny. Química atmosférica en la Gran Sabana II: Distribución de tamaño y composición de los aerosoles solubles en agua. INCI [online]. 2005, vol.30, n.10, pp.611-617. ISSN 0378-1844.

At three sites from La Gran Sabana, Venezuela, total and size fractionated suspended particles were collected (1997-2003). Using ion chromatography inorganic anions (NO₃^-, SO₄⁼, Cl- and PO₄⁼), organic anions (HCOO-, CH₃COO-, CH₃COCOO- and C₂O₄⁼) and cations (NH₄⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺) were analyzed. Soluble organic N was also determined. No significant seasonal or spatial variation was registered, suggesting homogeneous sources and that biomass burning, more frequent during less rainy periods, does not produce a direct impact in the chemical composition of the regional atmosphere. Excess concentrations of Na⁺, K⁺ and Ca⁺⁺ were observed, indicating additional sources. These cations are in fine and coarse particles, suggesting a source of fine particles, like re-suspension of ashes; HCO₃^- should be the companion anion. NH₄⁺ levels are low and would neutralize H₂SO₄. 72% of SO₄⁼ is of non-marine origin, it is in fine particles and its concentration (~0.55µg/m³) is similar to those in polar regions, suggesting that biogenic activities in the region do not produce significant amounts of SO4= precursor compounds. NO₃^- is present in coarse particles and its concentration (~0.3µg/m³) is similar to those found in tropical forests. PO₄⁼ levels are low, evenly distributed between fine and coarse particles; its possible formation from gaseous phosphine is discussed. Levels of HCOO-, CH₃COO-, CH₃COCOO- and C₂O₄⁼ are similar to those reported in rural regions of the world. HCOO- and CH3COO- are in coarse particles, whereas CH₃COCOO^- and C₂O₄⁼ are in fine particles. The source of these anions would be the oxidation of natural VOCs. Their possible role as cloud condensation nuclei, especially C₂O₄⁼, indicates the importance of characterizing organic aerosols in the tropical atmosphere. Soluble N₂ is in high percentage (~75%) in the organic fraction of particles, indicating a biogeochemical cycle of N dominated by the soluble organic N. Finally, acid-basic characteristics of aerosols are discussed, considering the reaction of NH₃ with strong acids, the solubilization of CaCO₃ and the release of HCl from marine aerosols.

Palavras-chave : Aerosoles Inorgánicos y Orgánicos; Atmósfera; Características Ácido-Básicas; Distribución de Tamaño; Gran Sabana; Venezuela.