Inversion of CO and NOx emissions using the adjoint of the IMAGES model

[摘要] We use ground-based observations of CO mixing ratios and vertical columnabundances together with tropospheric NO₂ columns from the GOME satelliteinstrument as constraints for improving the global annual emission estimates of COand NO_x for the year 1997. The agreement between concentrations calculated by theglobal 3-dimensional CTM IMAGES and the observations is optimized using the adjointmodelling technique, which allows to invert for CO and NO_x fluxes simultaneously,taking their chemical interactions into account. Our analysis quantifies a total of 39 fluxparameters, comprising anthropogenic and biomass burning sources over large continental regions,soil and lightning emissions of NO_x, biogenic emissions of CO and non-methanehydrocarbons, as well as the deposition velocities of both CO and NO_x.Comparison between observed, prior and optimized CO mixing ratios at NOAA/CMDL sitesshows that the inversion performs well at the northern mid- and high latitudes, and that itis 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 thea priori errors associated to the bottom-up inventory, whereas biomass burning sources aresubject to a strong variability. Our best estimate for the 1997 global top-down COsource amounts to 2760 Tg CO. Anthropogenic emissions increase by 28%, in agreement withprevious inverse modelling studies, suggesting that the present bottom-up inventoriesunderestimate the anthropogenic CO emissions in the Northern Hemisphere.The magnitude of the optimized NO_x global source decreases by 14% with respectto the prior, and amounts to 42.1 Tg N, out of which 22.8 Tg N are due to anthropogenicsources. The NO_x emissions increase over Tropical regions, whereas they decreaseover Europe and Asia. Our inversion results have been evaluated against independentobservations from aircraft campaigns. This comparison shows that the optimization of COemissions constrained by both CO and NO₂ observations leads to a betteragreement between modelled and observed values, especially in the Tropics andthe Southern Hemisphere, compared to the case where only CO observations are used.A posteriori estimation of errors on the control parameters shows that a significanterror reduction is achieved for the majority of the anthropogenic source parameters, whereasbiomass burning emissions are still subject to large errors after optimization.Nonetheless, the constraints provided by the GOME measurements allow to reduce the uncertaintieson savanna burning emissions of both CO and NO_x, suggesting thus that theincorporation of these data in the inversion yields more robust results for carbon monoxide.

[学科分类] 大气科学