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We describe a new 4D-Var inversion framework for nitrous oxide (N2O) based on theGEOS-Chem chemical transport model and its adjoint, and apply it in a seriesof observing system simulation experiments to assess how well N2Osources and sinks can be constrained by the current global observingnetwork. The employed measurement ensemble includes approximately weekly andquasi-continuous N2O measurements (hourly averages used) from severallong-term monitoring networks, N2O measurements collected from discreteair samples onboard a commercial aircraft (Civil Aircraft for the Regular Investigation of theatmosphere Based on an Instrument Container; CARIBIC), and quasi-continuousmeasurements from the airborne HIAPER Pole-to-Pole Observations (HIPPO) campaigns. For a2-year inversion, we find that the surface and HIPPO observations canaccurately resolve a uniform bias in emissions during the first year;CARIBIC data provide a somewhat weaker constraint. Variable emission errorsare much more difficult to resolve given the long lifetime of N2O, andmajor parts of the world lack significant constraints on the seasonal cycleof fluxes. Current observations can largely correct a global bias in thestratospheric sink of N2O if emissions are known, but do not provideinformation on the temporal and spatial distribution of the sink. However,for the more realistic scenario where source and sink are both uncertain, wefind that simultaneously optimizing both would require unrealistically smallerrors in model transport. Regardless, a bias in the magnitude of theN2O sink would not affect the a posteriori N2O emissions for the2-year timescale used here, given realistic initial conditions, due to thetimescale required for stratosphere–troposphere exchange (STE). The samedoes not apply to model errors in the rate of STE itself, which we showexerts a larger influence on the tropospheric burden of N2O than doesthe chemical loss rate over short (< 3 year) timescales. We use astochastic estimate of the inverse Hessian for the inversion to evaluate thespatial resolution of emission constraints provided by the observations, andfind that significant, spatially explicit constraints can be achieved inlocations near and immediately upwind of surface measurements and the HIPPOflight tracks; however, these are mostly confined to North America, Europe,and Australia. None of the current observing networks are able to providesignificant spatial information on tropical N2O emissions. There,averaging kernels (describing the sensitivity of the inversion to emissionsin each grid square) are highly smeared spatially and extend even to themidlatitudes, so that tropical emissions risk being conflated with thoseelsewhere. For global inversions, therefore, the current lack of constraintson the tropics also places an important limit on our ability to understandextratropical emissions. Based on the error reduction statistics from theinverse Hessian, we characterize the atmospheric distribution ofunconstrained N2O, and identify regions in and downwind of SouthAmerica, central Africa, and Southeast Asia where new surface or profilemeasurements would have the most value for reducing present uncertainty inthe global N2O budget

Simulation of atmospheric N&lt;sub&gt;2&lt;/sub&gt;O with GEOS-Chem and its adjoint: evaluation of observational constraints

Simulation of atmospheric N<sub>2</sub>O with GEOS-Chem and its adjoint: evaluation of observational constraints