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.