The GMI CTM uses the Flux Form Semi-Lagrangian dynamical core of Lin and Rood . It can be integrated with any vertical resolution and coordinate supplied by the input meteorological fields but all recent simulations use a horizontal resolution of 1° x 1.25° with 72 vertical levels (i.e., the MERRA2 native grid). Simulations use a hybrid sigma-pressure coordinate with terrain-following levels near the surface, transitioning to constant pressure levels near 150 hPa. Over the years, the GMI CTM has been integrated with many versions of NASA Goddard DAS fields, from the original ‘GEOS-Strat’ to MERRA [Strahan et al., 2013] and MERRA-2 (Strahan et al. 2019). The CTM has parameterized tropospheric physical processes including convection, boundary layer turbulent transport, wet scavenging in convective updrafts, wet and dry deposition, and anthropogenic, natural and biogenic emissions.
GMI CTM simulations can be integrated with different chemical mechanisms. The stratosphere-troposphere mechanism (simply referred to as ‘GMI’ in the GEOS models) contains roughly 120 gas phase species, more than 300 chemical kinetic reactions, more than 80 photolytic reactions, and includes heterogeneous chemical reactions involved in polar ozone depletion. There is a coupled chemistry-aerosol mechanism, which allows coupling between the strat-trop mechanism and an aerosol model similar to GOCART [Bian et al., 2016]. For diagnosis of transport processes on a wide range of temporal and spatial scales, there is a tracer suite currently containing 35 tracers. These mechanisms are described below.
The Stratospheric-Tropospheric (‘Combo’ or ‘GMI’) Mechanism
The GMI combined tropospheric-stratospheric chemical mechanism was first described in Duncan et al. . The stratospheric portion of the mechanism came from the Lawrence Livermore National Lab ‘Impact’ model [Rotman et al., 2004, and references therein], while the tropospheric component was originally developed for the Harvard University GEOS-Chem tropospheric CTM [Horowitz et al., 1998]. The mechanism contains approximately 120 species, 322 thermal reactions, and 82 photolytic decompositions, including both stratospheric halogen chemistry and tropospheric non-methane hydrocarbon chemistry. The chemical mechanism is integrated using the SMVGEAR-II solver of Jacobson . Photolytic decomposition uses the Fast-JX photolysis scheme, which is an outgrowth of the Fast-j scheme of Wild et al.  for tropospheric photolytic reactions, and the Fast-j2 scheme of Bian and Prather , which treats stratospheric photolytic reactions. Heterogeneous chemical reactions on stratospheric sulfate aerosols as well as Type 1 and Type 2 PSCs are treated as described in Considine et al.  and Considine et al. . There are updates to the mechanism (e.g., to reaction rates, cross sections, and emissions input) every few years. Some details can be found on the Simulations page.
The GMI aerosol mechanism (Liu et al., 2007; Bian et al., 2009) simulates five major atmospheric aerosol components: black carbon, organic carbon, sulfate, dust, and sea salt. The aerosol module includes primary emissions, chemical production of sulfate in clear air and in-cloud aqueous phase, gravitational sedimentation, dry deposition, wet scavenging in and below clouds, and hygroscopic growth. Model outputs include SO2 (fossil fuel and natural), DMS, H2O2, sulfate (fossil fuel and natural, 3 size bins), black carbon (biomass burning and natural), organic matter (fossil fuel, biomass burning, and natural), mineral dust (5 size bins), and sea salt (4 size bins).
Coupled Chemistry-Aerosol Mechanism
This mechanism couples the strat-trop chemistry and aerosol mechanisms described above. This mechanism was developed and tested by Dr. Huisheng Bian and Mr. Steve Steenrod [Bian et al., 2016]. It was used for ATom simulations. This version improves previous aerosol simulations by including two more atmospheric aerosol components: nitrate and ammonium. This chemistry module simulates 154 gas and aerosol species with 337 thermal reactions and 81 photolytic decompositions. The sulfate gas phase chemistry is now treated along with all other species in the strat-trop (‘GMI’) chemical mechanism. Aerosol nitrate and ammonium are calculated by a thermodynamic equilibrium model RPMARES, adopted from GEOS-CHEM, for a system of sulfate-nitrate-ammonium-water. An irreversible absorption of HNO3 on mineral dust and sea salt is also included.
The GMI tracer suite is designed to diagnose a wide range of atmospheric processes. The suite includes stratospheric ozone tracers for diagnosing the stratosphere’s influence on tropospheric ozone and a fixed-lifetime tracer (‘E90’) for diagnosing the chemical tropopause following Prather et al. . Microsoft Word - gmi_tracersuite (nasa.gov) is a list of the most commonly used tracers. Optionally there are specialized age-of-air tracers and fixed lifetime tracers for diagnosing transport from various regions on specific timescales, and tracers such as radon and lead-210 for diagnosing processes such as land and marine convection and deposition. More details can be found on the Simulations page and in Orbe et al. (2017). Tracer simulation outputs are found here: Index of /datashare/dirac/gmidata2/users/steenrod/tracers (nasa.gov)