Abstract
We describe a new 4D-Var inversion framework for nitrous oxide (N2O)
based on the GEOS-Chem chemical transport model and its adjoint, and
apply it in a series of observing system simulation experiments to
assess how well N2O sources and sinks can be constrained by the current
global observing network. The employed measurement ensemble includes
approximately weekly and quasicontinuous N2O measurements (hourly
averages used) from several long-term monitoring networks, N2O
measurements collected from discrete air samples onboard a commercial
aircraft (Civil Aircraft for the Regular Investigation of the atmosphere
Based on an Instrument Container; CARIBIC), and quasi-continuous
measurements from the airborne HIAPER Pole-to-Pole Observations (HIPPO)
campaigns. For a 2-year inversion, we find that the surface and HIPPO
observations can accurately resolve a uniform bias in emissions during
the first year; CARIBIC data provide a somewhat weaker constraint.
Variable emission errors are much more difficult to resolve given the
long lifetime of N2O, and major parts of the world lack significant
constraints on the seasonal cycle of fluxes. Current observations can
largely correct a global bias in the stratospheric sink of N2O if
emissions are known, but do not provide information on the temporal and
spatial distribution of the sink. However, for the more realistic
scenario where source and sink are both uncertain, we find that
simultaneously optimizing both would require unrealistically small
errors in model transport. Regardless, a bias in the magnitude of the
N2O sink would not affect the a posteriori N2O emissions for the 2-year
timescale used here, given realistic initial conditions, due to the
timescale required for stratosphere-troposphere exchange (STE). The same
does not apply to model errors in the rate of STE itself, which we show
exerts a larger influence on the tropospheric burden of N2O than does
the chemical loss rate over short (< 3 year) timescales. We use a
stochastic estimate of the inverse Hessian for the inversion to evaluate
the spatial resolution of emission constraints provided by the
observations, and find that significant, spatially explicit constraints
can be achieved in locations near and immediately upwind of surface
measurements and the HIPPO flight tracks; however, these are mostly
confined to North America, Europe, and Australia. None of the current
observing networks are able to provide significant spatial information
on tropical N2O emissions. There, averaging kernels (describing the
sensitivity of the inversion to emissions in each grid square) are
highly smeared spatially and extend even to the midlatitudes, so that
tropical emissions risk being conflated with those elsewhere. For global
inversions, therefore, the current lack of constraints on the tropics
also places an important limit on our ability to understand
extratropical emissions. Based on the error reduction statistics from
the inverse Hessian, we characterize the atmospheric distribution of
unconstrained N2O, and identify regions in and downwind of South
America, central Africa, and Southeast Asia where new surface or profile
measurements would have the most value for reducing present uncertainty
in the global N2O budget.
