A quasi chemistry-transport model mode (QCTM)\udis presented for the numerical chemistry-climate simulation\udsystem ECHAM/MESSy Atmospheric Chemistry (EMAC).\udIt allows for a quantification of chemical signals through sup-\udpression of any feedback between chemistry and dynamics.\udNoise would otherwise interfere too strongly. The signal is\udcalculated from the difference of two QCTM simulations, a\udreference simulation and a sensitivity simulation. In order to\udavoid the feedbacks, the simulations adopt the following of-\udfline chemical fields: (a) offline mixing ratios of radiatively\udactive substances enter the radiation scheme, (b) offline mix-\uding ratios of nitric acid enter the scheme for re-partitioning\udand sedimentation from polar stratospheric clouds, (c) and\udoffline methane oxidation is the exclusive source of chemi-\udcal water-vapor tendencies. Any set of offline fields suffices\udto suppress the feedbacks, though may be inconsistent with\udthe simulation setup. An adequate set of offline climatolo-\udgies can be produced from a non-QCTM simulation using\udthe setup of the reference simulation. Test simulations reveal\udthe particular importance of adequate offline fields associated\udwith (a). Inconsistencies from (b) are negligible when using\udadequate fields of nitric acid. Acceptably small inconsisten-\udcies come from (c), but should vanish for an adequate pre-\udscription of chemical water vapor tendencies. Toggling be-\udtween QCTM and non-QCTM is done via namelist switches\udand does not require a source code re-compilation.
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