Methane and nitrous oxide dissolved concentration and emission flux of plain river network in winter

Abstract

Methane (CH4) and nitrous oxide (N2O) concentration, saturation and fluxes in river network of Shanghai city (Yangtze Delta) and Tianjin city (Haihe watershed) were investigated in winter. Results showed that CH4 and N2O concentration were high in river network and supersaturated. The average of CH4 concentration was 0.86 μmol/L (saturation: 758%), and ranged from 0.043±0.001 to 25.3±9.32 μmol/L. The average of N2O concentration was 86.8 nmol/L (saturation: 488%), and ranged from 9.71±0.41 to 691±35.2 nmol/L. CH4 and N2O concentration in two sewage draining rivers of Tianjin city were clearly higher than other rivers (Average CH4 and N2O concentration were 38.4 μmol/L and 88.9 nmol/L, respectively). There is significant spatial variation of CH4 and N2O concentration and saturation, data of concentration and saturation in Shanghai river network was significant higher than in Tianjin city. Air-water interface fluxes of CH4 and N2O widely varied from -1.35±0.22 to 665±246 μmol/m2 h (Average was 24.1 μmol/m2 h), and from -0.19±0.02 to 22.6±5.05 μmol/m2 h (Average was 2.28 μmol/m2 h), respectively. CH4 concentration had significant negative correlation with dissolved oxygen (DO), but positive with ammonium (NH4+), and N2O concentration had significant positive correlation with NH4+ and nitrate and nitrite (NO3-+NO2-). Air-water interface fluxes of CH4 and N2O in urban area were evidently higher than in suburban and rural area, implicating that seriously polluted river is a potential source of atmospheric CH4 and N2O.