Resumo

SANHUEZA, Eugenio et al. Química atmosférica en la Gran Sabana I: Composición y fotoquímica de gases inorgánicos y orgánicos. INCI [online]. 2005, vol.30, n.10, pp.602-610. ISSN 0378-1844.

Trace gases and photochemical processes were characterized in the atmosphere of La Gran Sabana, Venezuela. Compounds from anthropogenic sources (eg. CO, SO2, benzene, toluene) are in very low concentrations, indicating that the region is pristine. Ozone levels frequently showed a decrease of ~0.75ppbv·h-1 starting mid morning. The estimated O3 consumption of 0.15-0.27ppbv·h-1 suggests an additional photochemical consumption of O3 in the boundary layer, promoted by very low NO concentrations, in agreement with the measured low levels of HNO3. A linear relationship between [O3] and [CO] was observed with a DO3/DCO slope of 0.12v/v, significantly lower than recorded elsewhere, likely due to lower efficiency of O3 production during the oxidation of reactive hydrocarbons. HNO3 and NH3 levels are lower than the ones found in other tropical sites, in agreement with the low concentration of NH4+ and NO3- aerosols. HCl levels agree with its production from chlorocarbons oxidation, mainly far away. Isoprene (emitted by the vegetation) levels are significantly lower that those observed at other tropical sites, showing a relatively low biogenic activity. The diurnal variations of various VOC (eg. methanol, isoprene, isoprene oxidation products) indicate an important transport from the Guyana Esequiba forests. Radical production from HCHO photolysis (estimated as ~0.17ppbv·h-1) is higher than the one due to O3 photolysis (£0,1ppbv·h-1), showing the importance of formaldehyde in the atmospheric photochemistry of the region. The changes in O3 levels observed during or after a rain event indicate that it promotes a rapid transport of air masses from the residual boundary layer to the nocturnal mixing layer.

Palavras-chave : Atmósfera; Compuestos Orgánicos Volátiles; Gases Atmosféricos; Gran Sabana; Monóxido de Carbono; Ozono; Venezuela.