Inverting for Emissions of Ozone Precursors Using the Adjoint of a CTM

In order to optimise the emissions of ozone precursors (CO, NOx, hydrocarbons) in the IMAGES global chemical transport model, we apply the adjoint technique. Misfits between modelled and measured concentrations are quantified by introducing the cost function and looking for a solution that corresponds to its minimum. The minimum of the cost function is calculated via an iterative procedure that makes use of the adjoint model operator, that is, the gradient of the cost function with respect to a set of control parameters to be optimised. The advantage of the adjoint model technique compared to other inversion methods is that no linear response of the calculated concentrations to changes in the emissions is assumed. Furthermore, the emissions of several chemical compounds can be varied and optimised simultaneously and the chemical feedbacks existing between different chemical compounds can be explicitly taken into account. These features are very important for compounds like CO and NOx which have common emission sources (like biomass burning), and are strongly inter-related through the chemistry of the OH radical. In the present study, the control parameters to be optimised are the annual emissions of CO, NOx and a few NMVOCs (ethane, propane and acetone) over large regions and for different broad categories. Making use of an emission inventory based on EDGAR 3 and GEIA, we present the results for emission optimisations performed using different combinations of the following observational datasets considered for the same year: ground-based measurements of CO and NMVOCs, MOPITT-derived CO columns, GOME-derived NO2 tropospheric columns, and aircraft measurements of several NMVOCs. Finally, the a posteriori concentrations are compared to independent observations provided by aircraft campaigns.