Fluxos de óxido nitroso na interface ar-mar na Baía de Guanabara

Abstract

Recebido em 23/5/07; aceito em 11/4/08; publicado na web em 24/9/08 NITROUS OXIDE FLUXES AT THE AIR-SEA INTERFACE IN GUANABARA BAY. In Surface water concentrations of N 2 O were measured at 37 stations in Guanabara Bay and fluxes estimated across the air-sea interface. Concentrations averaged 8.2 ± 2.2 nmol L -1 and 90% of the stations showed supersaturation averaging 33%. N 2 O fluxes were estimated using a two-film model which is given by the product of the concentration difference across the film and the gas transfer coefficient (k w ). Two parametrizations of k w were used which provided average fluxes of 0.3 and 3.0 µg N m -2 h -1. Flux measurements using floating chambers (not reported here) seem to agree with the upper limit of these estimates.Keywords: nitrous oxide; air-sea exchange; Guanabara Bay. A desnitrificação pode tanto produzir quanto consumir N 2 O. Nesse último caso ele é utilizado como receptor de elétrons e reduzido a N 2 :A eutrofização de sistemas aquáticos provoca aumento na produção de N 2 O.7 Seitzinger e Kroeze 8 estimaram para 1990 uma emissão global de N 2 O de rios, estuários e plataformas continentais da ordem de 1,9 (0,9-9,0) Tg N ano -1 , dos quais cerca de 90% foram atribuídos às atividades antrópicas, destacando-se o uso de fertilizantes, mas também o aporte atmosférico e a descarga de esgotos. A contribuição desses sistemas representa aproximada mente 25% da emissão global de N 2 O atribuída às atividades antrópicas.2 Seitzinger e Kroeze 8 prevêem para meados do século XXI um incremento na emissão global de N 2 O de rios, estuários e plataformas continentais, em relação a 1990, que atingiria aproximadamente 4,9 (1,3-13,0) Tg N ano -1 . Deve-se ressaltar que a maior parte dos estudos relacionados às emissões de N 2 O em sistemas aquáticos costeiros e marinhos expressivamente influenciados por atividades humanas ocorrem no Hemisfério Norte.No Brasil, se desconhece qualquer estudo relacionado à emissão de gases de N (NH 3 e N 2 O) em sistemas aquáticos costeiros eutrofizados excetuando-se o recentemente publicado por Guimarães e de Mello, 9 que apresenta uma estimativa das emissões de NH 3 das águas da Baía de Guanabara. Face à grande carga de N orgânico lançada diariamente na Baía de Guanabara 10 e aos elevados níveis de nitrogênio amoniacal (NH x ) verificados nestas águas 9,11 é de se

Spatio‐temporal variability of methane (CH4) concentrations and diffusive fluxes from a tropical coastal embayment surrounded by a large urban area (Guanabara Bay, Rio de Janeiro, Brazil)

“…The eutrophication in Guanabara Bay tends to promote CO 2 uptake (Cotovicz et al 2015), whereas for CH 4 the strong organic pollution tends to favour emissions to the atmosphere. Comparing the fluxes of CO 2 (Cotovicz et al 2015) with the CO 2 -eq emissions for CH 4 (this study) and N 2 O (Guimarães and de Mello 2008), the sink character for CO 2 is counterweighted by the emissions character for CH 4 and N 2 O. It entails that the negative radiative balance of Guanabara Bay due to CO 2 uptake is lowered by the CH 4 and N 2 O emissions at about 20%.…”

“…We used the concepts of global warming potential (GWP) and CO 2 ‐equivalent emissions (CO 2 ‐eq) (IPCC ) to compare the fluxes of CH 4 with those of CO 2 (Cotovicz et al ) and also those of N 2 O published by Guimarães and de Mello () at air‐water interface in Guanabara Bay. These concepts allow us to account for the fluxes of different greenhouse gases on a common scale.…”

“…The CH 4 fluxes calculated with each equation were expressed in μmol m −2 d −1 in each sector. To compare the air‐water fluxes of CH 4 with those of CO 2 (Cotovicz et al ) and N 2 O (Guimarães and de Mello ) we used the concepts of global warning potential (GWP), by considering that 1 g of CH 4 and N 2 O have a GWP equivalent of 28 g and 268 g of CO 2 , respectively, on a 100 yr basis (IPCC ).…”

“…The eutrophication process in Guanabara Bay tends to promote net CO 2 uptake (Cotovicz et al 2015), whereas for CH 4 the strong pollution in S2 seems to reinforce the emission. Guanabara Bay is also a source of atmospheric N 2 O, another important greenhouse gas, which showed highest concentrations in the polluted sectors of the bay (S2 and S5), and an average flux between 0.5 lmol m 22 d 21 and 5.1 lmol m 22 d 21 (Guimarães and de Mello 2008).…”

Nitrous oxide emissions from an intermittent aeration activated sludge system of an urban wastewater treatment plant

“…Second, the syringe was manually shaken 200 times (~1 min) and the headspace carefully transferred to a dry 20-mL syringe. 5,11,23 Finally, after analysis, N 2 O concentration (C; in nmol L -1 ) in the liquid was calculated as follows:…”