We give here a narrative description of the current standard version 12.1.0 of the GEOS-Chem model, with two purposes:

We strongly encourage you to be generous in citations—this not only recognizes the developer's work but also increases the traceability of your paper. Offering co-authorship is encouraged for new version 12.1 and v11-02 developments, as flagged in this narrative and if they are important for your work; see the New GEOS-Chem Developments page for more specific information on the developer(s) to be credited.

The narrative below is reviewed and updated by the GEOS-Chem Steering Committee at every new model version release. Questions and comments regarding GEOS-Chem literature should be directed to the relevant Working Group Chair or Model Scientist.

Original reference

The original reference for GEOS-Chem is Bey et al. [2001]. The acronym stands for Goddard Earth Observing System (GEOS)–Chemistry but we don't recommend spelling it out because the GEOS Earth System Model can use other chemical modules besides GEOS-Chem, and GEOS-Chem can use other meteorological drivers besides GEOS. As is often the case, the acronym has outlived the full name.

Configurations

The core of GEOS-Chem is a chemical module that computes the local changes in atmospheric concentrations due to emissions, chemistry, and deposition. The computation is done in individual vertical columns for user-specified horizontal locations, vertical grids, and time steps. This chemical module can be implemented in three different configurations, which all share the same GEOS-Chem chemical module:

Meteorological fields and grid resolution

GEOS-Chem in off-line mode is driven by assimilated meteorological data from the Goddard Earth Observation System (GEOS) of the NASA Global Modeling and Assimilation Office (GMAO). The two main data archives used by GEOS-Chem are:

These archives have 3-hour temporal resolution for 3-D fields and 1-hour resolution for 2-D fields. GEOS-Chem simulations can be conducted at the native resolution of the GEOS fields or at coarser resolution. They can be conducted at native resolution over a nested domain with dynamic 1-way or 2-way boundary conditions from a coarser global simulation (see Nesting below). The operator splitting time step in GEOS-Chem is optimized to achieve high accuracy and this is described by Philip et al. [2016] (new development in v11-01).

The GEOS-Chem chemical module can be used in on-line applications on any grid. It is compatible with the Earth System Modeling Framework (ESMF). On-line coupling with the GEOS-5 ESM is mature and has been applied to a c720 (≈12 km) simulation [Hu et al. 2018]. Couplings with the Beijing Climate Center (BCC) ESM and with the NCAR CESM2 are presently underway.

Nesting

The nested version of GEOS-Chem, originally described by Y. X. Wang et al. [2004], allows continental-scale simulations at the native-grid horizontal resolution of the GEOS data with dynamic boundary conditions from a coarser global simulation. The nesting can either be 1-way, with no influence from the nested domain on the global domain, or 2-way where the two domains interact with each other. The 2-way nesting capability with multiple nests is described by Yan et al. [2014] and on this wiki page.

The current nested version of GEOS-Chem uses GEOS-FP data with 0.25° x 0.3125° resolution or MERRA-2 data with 0.5° x 0.625° resolution within the nested domain. The capability to operate at that resolution with full aerosol-oxidant chemistry was developed by Zhang et al. [2015] for East Asia and Kim et al. [2015] for North America. A software tool developed by the GEOS-Chem Support Team is available to process the original GEOS meteorological data to any user-selected window for application of the nested model.

Transport and deposition

GEOS-Chem Classic uses the TPCORE advection algorithm of Lin and Rood [1996] on the rectilinear grid. GCHP uses the FV3 advection algorithm of Putnam and Lin [2007] on the cubed sphere. Convective transport in GEOS-Chem is computed from the convective mass fluxes in the meteorological archive as described by Wu et al. [2007]. Boundary layer mixing in GEOS-Chem uses either the non-local scheme implemented by Lin and McElroy [2010] or full mixing up to the GEOS-diagnosed mixing depth.

The wet deposition scheme in GEOS-Chem is described by Liu et al. [2001] for water-soluble aerosols and by Amos et al. [2012] for gases. Scavenging of aerosol by snow and cold/mixed precipitation is described by Wang et al. [2011, 2014].

Dry deposition is based on the resistance-in-series scheme of Wesely [1989] as implemented by Wang et al. [1998a]. Aerosol deposition is from Zhang et al. [2001]. Aerosol deposition to snow/ice is described by Fisher et al. [2011]. Gravitational settling is from Fairlie et al. [2007] for dust and Alexander et al. [2005] for coarse sea salt. Sea-salt deposition is from Jaegle et al. [2011]. See the mercury section for description of air-sea-land exchange of mercury.

Radiation

GEOS-Chem can calculate the radiative flux effects from atmospheric composition using the optional RRTMG module. Implementation of RRTMG in GEOS-Chem is described in Heald et al. [2014].

Photolysis frequencies for stratospheric and tropospheric chemistry are calculated with the Fast-JX code of Bian and Prather [2002] as implemented in GEOS-Chem by Mao et al. [2010] and Eastham et al. [2014].

Absorption of UV by brown carbon can be implemented with the optional Hammer et al. [2016] scheme. Absorption properties of black carbon are from X. Wang et al. [2014] (new development in v11-02).

Emissions

All GEOS-Chem emissions are configured at run-time using the HEMCO module described by Keller et al. [2014]. HEMCO allows users to mix and match inventories from the GEOS-Chem library or add their own, apply scaling factors, overlay and mask inventories, etc. without having to edit or compile the code. HEMCO also has extensions to compute emissions with meteorological dependencies and to process other input/output data in GEOS-Chem.

Anthropogenic. Anthropogenic emissions of CO, NOx, and SO2 in GEOS-Chem use as default the CEDS global inventory (new development in v11-02). EDGARv4.3.1 emissions are also available as an option (new development in v11-02). Anthropogenic emissions of NMVOCs use as default the RETRO monthly global inventory for 2000 implemented as described by Hu et al. [2015a]. Ethane emissions are from Tzompa-Sosa et al. [2016] (new development in v11-02). Trash burning emissions are from Wiedinmyer et al. [2014]. Global anthropogenic emissions for carbonaceous aerosols (BC/OC) are from Bond et al. [2007], as implemented into GEOS-Chem by Leibensperger et al. [2011].

All these default inventories are scaled for individual years on the basis of economic data, and are superseded by improved inventories in regions where we have better information. The basic structure of the anthropogenic emissions inventory is described by van Donkelaar et al. [2008] including diurnal profiles and algorithms to update to individual years. Regional emission estimates (base years in parentheses) are used in particular for

Future anthropogenic emissions following the RCP scenarios have been implemented into GEOS-Chem by Holmes et al. [2013].

Aircraft. Aircraft emissions are from the AEIC inventory [Stettler et al., 2011].

Ships. Global shipping emissions are from CEDS. Shipping emissions of NOx are processed by the PARANOX module of Vinken et al. [2012] to account for ozone and HNO3 production in the plume. The PARANOX module was updated by Holmes et al. [2014].

Open Fires. Emissions from open fires for individual years are from the GFED4.1 inventory with options to use instead the FINNv1.5 inventory [Wiedinmyer et al., 2011], the QFED inventory, or the GFAS inventory (new development in version 12.2).

Lightning. Lightning NOx emissions are as described by Murray et al. [2012] to match OTD/LIS climatological observations of lightning flashes, with continual updates documented on the lightning wiki page.

Biogenic VOCs. Biogenic VOC emissions in GEOS-Chem are from the MEGAN v2.1 inventory of Guenther et al. [2012] as implemented by Hu et al. [2015b]. Dependence on CO2 was added by Tai et al. [2013]. Acetaldehyde emissions are from Millet et al. (2010) (new development in v11-02). Biogenic non-agricultural ammonia sources are from GEIA (new development in v11-02).
Soils. Biogenic soil NOx emissions are from Hudman et al. [2012].

Ocean. Marine emissions of DMS are from the Lana et al. dataset as implemented in GEOS-Chem by Breider et al. [2017]. Air-sea exchange of acetone assumes fixed ocean concentrations as described by Fischer et al. [2012]. Ocean acetaldehyde emissions are from Millet et al. (2010). Ammonia emissions from Arctic seabirds are from Croft et al. (2016) (new development in v11-02) . Ocean ammonia emissions are from GEIA (new development in v11-02).

Volcanoes. Eruptive and non-eruptive volcanic SO2 emissions for individual years are from the AEROCOM data base originally developed by Thomas Diehl and implemented into GEOS-Chem by Fisher et al. [2011].

Other. See the carbon gases section for GEOS-Chem references on emissions of CO2 and methane. See the aerosols section for GEOS-Chem references on primary aerosol emissions. See the mercury section for GEOS-Chem references on emissions of mercury. See the POPs section for GEOS-Chem references on emissions of persistent organic pollutants (POPs).

Chemistry

The standard simulation in GEOS-Chem includes coupled aerosol-oxidant chemistry in the troposphere and stratosphere. Specifics are given below. The chemical solver is KPP [Damian et al., 2002] as implemented in GEOS-Chem with the FlexChem interface.

Gas-phase tropospheric chemistry

GEOS-Chem includes detailed HOx-NOx-VOC-ozone-halogen-aerosol tropospheric chemistry. The chemical mechanism in GEOS-Chem v11-2 was updated to the most recent JPL/IUPAC recommendations. PAN chemistry is as described by Fischer et al. [2014] (new development in v11-02). Isoprene oxidation is based on Travis et al. [2016] and Fisher et al. [2016] (new development in v11-02). Detailed Cl-Br-I halogen chemistry is as described by Sherwen et al. [2016] (new development in v11-02), with addition of HOBr + S(IV) (Chen et al., 2017) (new development in v11-02). Criegee chemistry was updated by Millet et al. [2015]. See the radiation section for the calculation of photolysis frequencies. Methane is prescribed as a surface boundary condition from monthly mean maps of spatially-interpolated NOAA flask data, and subsequently allowed to advect and react in all photochemical mechanisms as implemented by Murray [2016] (new development in v11-02).

Aerosols interact with gas-phase chemistry in GEOS-Chem through the effect of aerosol extinction on photolysis rates [Martin et al., 2003], heterogeneous chemistry [Jacob, 2000], and gas-aerosol partitioning of semi-volatile compounds (see the aerosols section). N2O5 uptake by aerosols is from Evans and Jacob [2005]. HO2 uptake is from Mao et al. [2013] with a reactive uptake coefficient of 0.2 for conversion to H2O. Acid uptake by dust particles from Fairlie et al. [2010] is provided as an option.

Tropospheric ozone can also be simulated in GEOS-Chem as a linearized odd oxygen tracer with archived sources and loss rate constants from the full-chemistry simulation. This method was originally described by Wang et al. [1998c] and its most recent implementation in GEOS-Chem is described by Zhang et al. [2008].

Stratospheric chemistry

GEOS-Chem includes detailed stratospheric chemistry fully coupled with tropospheric chemistry through the Unified tropospheric-stratospheric Chemistry eXtension (UCX) as described in Eastham et al. [2014]. UCX is the standard version of the model, but an option is also available to use parameterized linear chemistry in the stratosphere (“troposphere-only simulation”) including the Linoz algorithm of McLinden et al. [2000] for ozone and monthly mean sources and loss rate constants for other gases [Murray et al., 2012].

Aerosols

Sulfate-nitrate-ammonium aerosol. The original SNA aerosol simulation in GEOS-Chem coupled to gas-phase chemistry was developed by Park et al. [2004]. SNA thermodynamics are computed with the ISORROPIA II thermodynamic module [Fontoukis and Nenes, 2007], as implemented in GEOS-Chem by Pye et al. [2009]. Cloudwater pH for in-cloud sulfate formation is as given by Alexander et al. [2012]. HOBr has been added by Chen et al. (2017) as a S(IV) oxidant (new development in v11-02). In-cloud SO2 oxidation by transition metals is as described by Alexander et al. (2009) (new development in v11-02) .

Carbonaceous aerosol. Q. Wang et al. [2014] describes the current BC simulation in GEOS-Chem. Organic aerosol in the default model follows a simple, irreversible, direct yield scheme similar to Kim et al. [2015] (new development in v11-02) . Complex SOA can be used as an option following the simplified Volatility Basis Set (VBS) scheme of Pye et al. [2010] and the aqueous-phase isoprene SOA scheme of Marais et al. [2016] (new development in v11-02) coupled to the isoprene gas-phase chemistry mechanism.

Dust aerosol. The dust simulation in GEOS-Chem is described by Fairlie et al. [2007]. Dust size distributions are from Li Zhang et al. [2013]. Fine anthropogenic dust from combustion and industrial sources is from the AFCID inventory of Philip et al. [2017] ((new development in version 12.1)

Sea salt. The sea salt aerosol simulation in GEOS-Chem is described by Jaegle et al. [2011].

Marine POA. Marine POA is emitted following the optional Gantt et al. [2015] scheme.

Aerosol microphysics. Two alternate simulations of aerosol microphysics are implemented in GEOS-Chem: the TOMAS simulation of Trivitiyanurak et al. [2008] updated by Kodros and Pierce [2017] and the APM simulation of Yu and Luo [2009].

Aerosol optical depth. Aerosol optical depth is calculated in GEOS-Chem using RH-dependent aerosol optical properties from Martin et al. [2003]. Dust optics are from Ridley et al. [2012]. These calculations can be performed at user-specified wavelengths from 230 nm to 56 μm when using RRTMG (see the radiation section).

Aerosol-only simulation. In addition to the fully coupled gas-aerosol simulation described in the Tropospheric Chemistry section, there is an option to conduct aerosol-only simulations using fixed 3-D monthly oxidant concentrations (from a GEOS-Chem simulation of old vintage) and simple SOA. This is described by Leibensperger et al. [2012].

Carbon gases

CO2. The current simulation is described by Nassar et al. [2010]. Anthropogenic emissions are updated from Nassar et al. [2013] (new development in v11-01) .

Methane. The current simulation is described by Maasakkers et al. [2018] (new development in v11-02).

CO. Simulation of CO in GEOS-Chem can be conducted either as part of the standard full-chemistry simulation or as a separate tagged-tracer simulation that resolves CO sources from individual regions or processes, and uses archived OH fields from a full-chemistry simulation to compute the CO sink. The most recent version is described by Fisher et al. [2017] (new development in v11-02).

Mercury

The original GEOS-Chem coupled atmosphere-ocean simulation of mercury was described by Selin et al. [2007] for the atmosphere and by Strode et al. [2007] for the ocean. Extension to a coupled atmosphere-ocean-land model was described by Selin et al. [2008]. The current version of the atmospheric simulation is as described by Horowitz et al. [2017] (new development in v11-02), and the current version of the ocean simulation is as described by Soerensen et al. [2010], with updated ocean rate coefficients from Song et al. [2015]. Treatment of Arctic sea ice and rivers is as described by Fisher et al. [2012, 2013]. Gas-aerosol partitioning of Hg(II) is from Amos et al. [2012].There is an option to couple GEOS-Chem with the terrestrial mercury module developed by Smith-Downey et al. [2010].

Anthropogenic emissions are from Y. Zhang et al. [2016]. Future SRES emission scenarios have been implemented by Corbitt et al. [2011].

Persistent Organic Pollutants (POPs)

The model includes a simulation of PAHs as described by Friedman et al. [2014].

Other species

Specific simulations of other species were included in the standard GEOS-Chem model at various stages in the model history but have not been revisited or maintained since. These include H2/HD from Price et al. [2007], HCN/CH3CN from Li et al. [2003], CH3I from Bell et al. [2002], and acetylene from Xiao et al. [2007]. They should be considered obsolete but can provide a foundation for future development.

Model Adjoint

See the GEOS-Chem adjoint wiki page for description and references.

References