Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 5 2006 10.5194/acpd-6-10591-2006 http://www.atmos-chem-phys-discuss.net/6/10591/2006/ http://www.atmos-chem-phys-discuss.net/6/10591/2006/acpd-6-10591-2006.html http://www.atmos-chem-phys-discuss.net/6/10591/2006/acpd-6-10591-2006.pdf 10591 10648 2006-10-19 Development of the adjoint of GEOS-Chem D. K. Henze J. H. Seinfeld California Institute of Technology, Pasadena, CA, USA We present the adjoint of the global chemical transport model GEOS-Chem, focusing on the chemical and thermodynamic relationships between sulfate &ndash; ammonium &ndash; nitrate aerosols and their gas-phase precursors. The adjoint model is constructed from a combination of manually and automatically derived discrete adjoint algorithms and numerical solutions to continuous adjoint equations. Explicit inclusion of the processes that govern secondary formation of inorganic aerosol is shown to afford efficient calculation of model sensitivities such as the dependence of sulfate and nitrate aerosol concentrations on emissions of SO_x, NO_x, and NH₃. The adjoint model is extensively validated by comparing adjoint to finite difference sensitivities, which are shown to agree within acceptable tolerances; most sets of comparisons have a nearly 1:1 correlation and <i>R</i>²&gt;0.9. We explore the robustness of these results, noting how insufficient observations or nonlinearities in the advection routine can degrade the adjoint model performance. The potential for inverse modeling using the adjoint of GEOS-Chem is assessed in a data assimilation framework through a series of tests using simulated observations, demonstrating the feasibility of exploiting gas- and aerosol-phase measurements for optimizing emission inventories of aerosol precursors.