A kinetic chemistry tagging technique and its application to modelling the stable isotopic composition of atmospheric trace gases

摘要

Isotope composition, in many cases, holds unique information on the sources,chemical modification and sinks of atmospheric trace gases. Vital to theinterpretation and use of an increasing number of isotope analyses isappropriate modelling. However, the exact implementation of isotopicinformation in chemistry-climate models is a challenge, and often studies usesimplifications which limit their applicability. Here we implement a thoroughisotopic extension in MECCA, a comprehensive kinetic chemistry sub-model. Tothis end, we devise a generic tagging technique for the kinetic chemistrymechanisms implemented as the sub-submodel MECCA-TAG. The technique isdiagnostic and numerically efficient and supports the investigation ofvarious aspects of kinetic chemistry schemes. We focus specifically on theapplication to the modelling of stable isotopic composition. The results ofMECCA-TAG are evaluated against the reference sub-submodelMECCA-DBL, which is implicitly full-detailed, but computationallyexpensive and thus sub-optimal in practical applications. Furthermore, weevaluate the elaborate carbon and oxygen isotopic mechanism by simulating themulti-isotope composition of CO and other trace gases in the CAABA/MECCAbox-model. The mechanism realistically simulates the oxygen isotopecomposition of key species, as well as the carbon isotope signature transfer.The model adequately reproduces the isotope chemistry features for CO, takinginto account the limits of the modelling domain. In particular, themass-independently fractionated (MIF) composition of CO due to reactions ofozone with unsaturated hydrocarbons (a source effect) versus its intrinsicMIF enrichment induced in the removal reaction via oxidation by OH isassessed. The simulated ozone source effect was up to +1‰ inΔ17O(CO). The versatile modelling framework we employ (the ModularEarth Submodel System, MESSy) opens the way for implementation of the noveldetailed isotopic chemistry treatment in the three-dimensionalatmospheric-chemistry general circulation model EMAC. We therefore alsopresent estimates of the computational gain obtained by the developedoptimisations.