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Keywords: Oceanografia;  Emissão de gases;  Amônia(NH3);  Óxido Nitroso (N2O);  Baía de Guanabara, RJ
Issue Date: 24-Feb-2005
Abstract: In the marine environment, the chemistry of nitrogen is controlled by redox reactions, mediated by phytoplankton and bacteria, that produce gases exchanged with the atmosphere due to the desequilibrium between the concentrations of air and sea. The two main gases of nitrogen involved in this process are ammonia (NH3) and nitrous oxide (N2O). NH3 is the dominant base in the atmosphere and has great importance as neutralizer of the acids in the air. N2O is a trace gas that absorbs infrared radiation contributing to global warming and to ozone depletion in the stratosphere. Around Guanabara Bay about 7.8 million of habitants release domestic sewage directly in to the water elevating the concentrations of inorganic nitrogen, which leads us to believe that this is an important source of NH3 and N2O to atmosphere. A preliminary estimate of the fluxes of NH3 and N2O in the air-sea interface of Guanabara Bay were made using two approachs: spatial and temporal. For the spatial approach 37 samples were collected from 5 to 7 April 2004. For the temporal approach weekly samples were collected at 10 beaches of the city of Rio de Janeiro, during the period of July 2003 and February 2004 (macro temporal, n = 267), and samples in intervals of 3 hours in a central point from 9 to 12 February 2004 (micro temporal, n = 25). The temperature, salinity and pH were measured in situ and the ammoniacal nitrogen analyzed by the indophenol method. Only in the spatial approach were analyzed the nitrite, nitrate (spectrophotometer methods) and N2O by gaseous chromatography with ECD. The fluxes were calculated based on the two-lawyer model adopting atmospheric concentrations of 1 and 5 μg m-3 for NH3 and 325 ppb for N2O. The fluxes of NH3 in the air-sea interface varied from 31 to 3486 μg N m-2 h-1. Considering the greatest concentrations adopted in air (5 μg NH3 m-3), 92% of the fluxes in the spatial, 68% of macro temporal and 20% of micro temporal approaches indicated emission of NH3 to the atmosphere. The fluxes of N2O varied between 6 and 2123 ng N m-2 h-1, of which 89% indicated emission to the air. A high correlation of N2O with nitrite (r=0.84) was verified indicating that probably the dominant process of production of N2O in the surface waters of Guanabara Bay is nitrification. All data indicate a greater organic contribution in the West sector of Guanabara Bay, contributing with high fluxes of NH3 and N2O to the atmosphere
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