Overview
GEOS-Chem chemistry is implemented within CESM as an alternative chemistry option to CAM-chem, with an independent development history from the MOZART series of chemical mechanisms. CESM3 includes chemistry from GEOS-Chem version 14.5 (citation/DOI here) using Cloud-J photolysis (citation/DOI here) and the HETP (or ISORROPIA?) thermodynamic balance module.
The implementation of GEOS-Chem within CESM is described in Fritz et al., 2021 GMD with updates from Lin et al., 2024 ACP. A brief overview of physical processes and differences from CAM-chem are below. A discussion of the science implications of these choices can be found in the original references.
- Dry deposition is handled similarly to CAM-chem but with GEOS-Chem computing dry deposition velocities over land. Wet deposition uses the Neu wet deposition scheme also similarly to CAM-chem.
- File-based emissions are handled exclusively by HEMCO; the CAM-chem approach of using srf_emis_specifier and ext_frc_specifier are not supported.
- State-based emissions: Lightning NOx uses the CAM-chem parameterization. Biogenic emissions via MEGAN are handled via the land model and are the same as CAM-chem but mapped to GEOS-Chem species. Sea salt and dust emissions are via the aerosol and land model and handled the same as CAM-chem. GEOS-Chem implementations of other emissions such as seaflux (cite) and inorganic iodine (cite) are via HEMCO.
- Aerosol processes are modeled by MAM4 with GEOS-Chem only modifying the bulk masses of each aerosol species proportionally to their existing masses per mode. Aerosol nucleation uses the SO2 + OH reaction in GEOS-Chem to provide the gas-phase H2SO4 production rate.
Source Code
The GEOS-Chem interface to CESM is in directory src/chemistry/geoschem, primarily in file chemistry.F90. In the GEOS-Chem compsets, it overloads subroutines chem_gasphase_chemdr and chemistry_timestep_tend used in CAM-chem. The externally maintained GEOS-Chem source code is in src/chemistry/geoschem/geoschem_src.