Inverse modeling estimates of the global nitrous oxide surface flux from 1998-2001

[1] Measurements of nitrous oxide in air samples from 48 sites in the Cooperative Global Air Sampling Network made by NOAA/ESRL GMD CCGG ( the Carbon Cycle Greenhouse Gases group in the Global Monitoring Division at the NOAA Earth System Research Laboratory in Boulder, Colorado) and the three-dimensional chemical transport model TM3 were used to infer global nitrous oxide fluxes and their uncertainties from 1998 - 2001. Results are presented for four semihemispherical regions (90 degrees S - 30 degrees S, 30 degrees S to equator, equator to 30 degrees N, 30 degrees N - 90 degrees N) and six broad super regions'' ( Southern Land, Southern Oceans, Tropical Land, Tropical Oceans, Northern Land, and Northern Oceans). We found that compared to our a priori estimate ( from the International Geosphere-Biosphere Programme's Global Emissions Inventory Activity), the a posteriori flux was much lower from 90 degrees S - 30 degrees S and substantially higher from equator to 30 degrees N. Consistent with these results, the a posteriori flux from the Southern Oceans region was lower than the a priori estimate, while Tropical Land and Tropical Ocean estimates were higher. The ratio of Northern Hemisphere to Southern Hemisphere fluxes was found to range from 1.9 to 5.2 ( depending on the model setup), which is higher than the a priori ratio (1.5) and at the high end of previous estimates. Globally, ocean emissions contributed 26 - 36% of the total flux ( again depending on the model setup), consistent with the a priori estimate (29%), though somewhat higher than some other previous estimates.