Isotopomeric analysis of N 2 O dissolved in a river in the Tokyo metropolitan area

River water has been suggested as a potential source of nitrous oxide (N 2 O), which is a greenhouse gas that is accumulating rapidly in the troposphere and which is a precursor to stratospheric NO x that depletes ozone. Previous studies on freshwater N 2 O sources have specifically examined estuaries where sedimentary N 2 O production might be important and a few points near anthropogenic nitrogen sources such as agricultural or municipal wastewater areas. Here we present the first observation of a temporal and horizontal distribution of N 2 O and its isotopomers between the midstream and estuary of an urban river. Surface water was supersaturated (100–6800%) with N 2 O at all stations during the study period. The average or maximum saturation value was greater than described in most previous reports. High N 2 O concentrations were observed near sewage plants and the unique signature of isotopomer ratios implied direct N 2 O addition from the plants. The isotopomer ratios also suggested N 2 O production/consumption at the sediment‐water interface. Fluxes and isotopomer ratios of N 2 O, from the river to the atmosphere, estimated from our observations, indicated that the urban river is indeed a source of atmospheric N 2 O and that its production could be distinguished from other natural or anthropogenic sources using isotopomer ratios. Copyright © 2009 John Wiley & Sons, Ltd.