Nitrous oxide emissions 1999 to 2009 from a global atmospheric inversion
Abstract. N2O surface fluxes were estimated for 1999 to 2009 using a time-dependent Bayesian inversion technique. Observations were drawn from 5 different networks, incorporating 59 surface sites and a number of ship-based measurement series. To avoid biases in the inverted fluxes, the data were adjusted to a common scale and scale offsets were included in the optimization problem. The fluxes were calculated at the same resolution as the transport model (3.75° longitude × 2.5° latitude) and at monthly time resolution. Over the 11-year period, the global total N2O source varied from 17.5 to 20.1 Tg a−1 N. Tropical and subtropical land regions were found to consistently have the highest N2O emissions, in particular in South Asia (20 ± 3% of global total), South America (13 ± 4%) and Africa (19 ± 3%), while emissions from temperate regions were smaller: Europe (6 ± 1%) and North America (7 ± 2%). A significant multi-annual trend in N2O emissions (0.045 Tg a−2 N) from South Asia was found and confirms inventory estimates of this trend. Considerable interannual variability in the global N2O source was observed (0.8 Tg a−1 N, 1 standard deviation, SD) and was largely driven by variability in tropical and subtropical soil fluxes, in particular in South America (0.3 Tg a−1 N, 1 SD) and Africa (0.3 Tg a−1 N, 1 SD). Notable variability was also found for N2O fluxes in the tropical and southern oceans (0.15 and 0.2 Tg a−1 N, 1 SD, respectively). Interannual variability in the N2O source shows some correlation with the El Niño–Southern Oscillation (ENSO), where El Niño conditions are associated with lower N2O fluxes from soils and from the ocean and vice versa for La Niña conditions.