Latitudinal gradient of nitrous oxide: inferring source distribution from global measurements and model

Nitrous oxide (N2O) plays major role in the earth’s climate system through global warming and stratospheric ozone depletion. Recent observations from the HIPPO (Hiaper Pole to Pole Observations) campaign suggest enhanced N2O concentrations in lower and middle troposphere over tropical latitudes. However, the Atmospheric general circulation model-based Chemistry Transport model (ACTM) failed to simulate such features as in the measured N2O. We confirmed no systematic differences in ACTM and HIPPO latitudinal gradients exist for other long-lived species in the troposphere, e.g., sulfur hexafluoride (SF6), methane (CH4) and carbon dioxide (CO2). Further, we use measurements of all species from discrete samples collected at Earth's surface from NOAA/ESRL's global cooperative air sampling network to identify potential deficiencies in N2O simulations alone, which is unlikely to be arising from model transport error. We find that ACTM simulation is successfully capturing the increase in N2O by ~2 ppb from 30S to 30N, but always overestimate for the latitudes north of 30N. The latitudinal distributions of N2O emissions from all-anthropogenic, natural soil and ocean show the largest anthropogenic emission at 45-60N, which is based on the emission database developed in the 1990s. A net decrease in N2O emission in the mid-/high latitude region might have occurred in the past couple of years or earlier emission inventories overestimated the northern high latitude N2O emission.