Inversion of CO and NOx emissions using the adjoint of the IMAGES model J.-F. Müller and T. Stavrakou Belgian Institute for Space Aeronomy, Brussels, Belgium
Abstract. We use ground-based observations of CO mixing ratios and vertical column
abundances together with tropospheric NO2 columns from the GOME satellite
instrument as constraints for improving the global annual emission estimates of CO
and NOx for the year 1997. The agreement between concentrations calculated by the
global 3-dimensional CTM IMAGES and the observations is optimized using the adjoint
modelling technique, which allows to invert for CO and NOx fluxes simultaneously,
taking their chemical interactions into account. Our analysis quantifies a total of 39 flux
parameters, comprising anthropogenic and biomass burning sources over large continental regions,
soil and lightning emissions of NOx, biogenic emissions of CO and non-methane
hydrocarbons, as well as the deposition velocities of both CO and NOx.
Comparison between observed, prior and optimized CO mixing ratios at NOAA/CMDL sites
shows that the inversion performs well at the northern mid- and high latitudes, and that it
is less efficient in the Southern Hemisphere, as expected due to the scarsity of measurements over this part of the globe. The inversion,
moreover, brings the model much closer to the measured NO2 columns over all regions.
Sensitivity tests show that anthropogenic sources exhibit weak sensitivity to changes of the
a priori errors associated to the bottom-up inventory, whereas biomass burning sources are
subject to a strong variability. Our best estimate for the 1997 global top-down CO
source amounts to 2760 Tg CO. Anthropogenic emissions increase by 28%, in agreement with
previous inverse modelling studies, suggesting that the present bottom-up inventories
underestimate the anthropogenic CO emissions in the Northern Hemisphere.
The magnitude of the optimized NOx global source decreases by 14% with respect
to the prior, and amounts to 42.1 Tg N, out of which 22.8 Tg N are due to anthropogenic
sources. The NOx emissions increase over Tropical regions, whereas they decrease
over Europe and Asia. Our inversion results have been evaluated against independent
observations from aircraft campaigns. This comparison shows that the optimization of CO
emissions constrained by both CO and NO2 observations leads to a better
agreement between modelled and observed values, especially in the Tropics and
the Southern Hemisphere, compared to the case where only CO observations are used.
A posteriori estimation of errors on the control parameters shows that a significant
error reduction is achieved for the majority of the anthropogenic source parameters, whereas
biomass burning emissions are still subject to large errors after optimization.
Nonetheless, the constraints provided by the GOME measurements allow to reduce the uncertainties
on savanna burning emissions of both CO and NOx, suggesting thus that the
incorporation of these data in the inversion yields more robust results for carbon monoxide.
Citation: Müller, J.-F. and Stavrakou, T.: Inversion of CO and NOx emissions using the adjoint of the IMAGES model, Atmos. Chem. Phys., 5, 1157-1186, doi:10.5194/acp-5-1157-2005, 2005.