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The chemistry of the OsloCTM3 covers both tropospheric and stratospheric chemistry, treated by separate modules. The tropospheric code is stand-alone, but the stratospheric code needs the tropospheric chemistry module to work. The historical reason for two schemes is that limitations in computational power previously meant one could only afford to study either the troposphere or the stratosphere. Nevertheless, such a division is possible since species with negligible chemical conversions in the stratosphere (troposphere) do not have to be treated in the stratospheric (tropospheric) chemistry scheme. Only compounds important for both tropospheric and stratospheric chemistry are treated in both schemes. To select which chemical scheme to apply, we use the tropopause height, based on the potential vorticity and potential temperature of the meteorological data.

The kinetics is based on JPL 2006, while the photodissociation coefficients are calculated on-line using the FastJX. The numerical integration of chemical kinetics is done applying the Quasi Steady State Approximation (QSSA) (Hesstvedt et al., 1978), using three different integration methods depending on the chemical lifetime of the species.

The tropospheric chemistry scheme is run with a numerical time step of 15 minutes (5 minutes for OH/HO2/RO2-reactions), contains 46 species and also some intermediate short-lived species treated within chemistry only. The scheme accounts for 86 thermal reactions, 17 photolytic reactions and 2 heterogeneous reactions (which are independent of the stratospheric heterogeneous chemistry). It includes detailed hydrocarbon chemistry and has been thoroughly tested (Berntsen and Isaksen, 1997; Kraabøl et al., 1999; Berglen et al., 2004; Brunner et al., 2003). The main reference for the tropospheric chemistry is Berntsen and Isaksen (1997).

The stratospheric chemistry module treats 55 species and 7 families, and a total of 159 reactions (104 thermal, 47 photolytic and 8 heterogeneous), which are integrated with a numerical time step of 5 minutes. Of these species, 17 are also treated in the tropospheric scheme. The heterogeneous chemistry scheme is a part of the stratospheric chemistry. The microphysics and heterogeneous chemistry scheme (Smyshlyaev et al., 1998; de Zafra and Smyshlyaev, 2001), represents formation and evolution of PSCs, including denitrification and dehydration through sedimentation. The main reference for the stratospheric and tropospheric chemistry is Søvde et al. (2008).

OsloCTM3 comprises several aerosol parametrizations:

Black Carbon / (primary) Organic Carbon

A stand-alone part of the model and a simple set-up with emissions, transport, conversion from hydrophobic to hydrophilic aerosols, wet and dry deposition. The parameterization is a bulk scheme, i.e. particles are represented only by mass with no size distribution. The aerosols are represented by a hydrophobic and a hydrophilic mode. After emission, the insoluble species are transformed to soluble species after a given aging time, to simulate the chemical conversions taking place. After the given aging time, the insoluble particles appear in the soluble modes, and are washed out. Aging times depend on season and latitude (Skeie et al., 2011; Lund and Berntsen, 2011) For details, see Lund et al. 2018.

Secondary organic aerosol

A separate module for secondary organic aerosols (SOA) is also available. It is based on Chung and Seinfeld (2002). Briefly, hydrocarbons undergo gas-phase oxidation via reaction with either O3, OH or NO3 to form condensable species using a two-product model (Hoffmann et al., 1997). The partitioning between the gas and aerosol phases is calculated assuming equilibrium and using specified partitioning coefficients. Precursor hydrocarbons include both biogenic species and species primarily emitted from anthropogenic activities, and the resulting SOA are separated into natural and anthropogenic. Primary organic carbon serves as surfaces for condensation of SOA. The main reference for the SOA module is Hoyle et al. 2007.

 

Nitrate and ammonium

The gas–aerosol partitioning of semi-volatile inorganic aerosols is treated with a thermodynamic model (Myhre et al., 2006). The chemical equilibrium among inorganic species (ammonium, sodium, sulfate, nitrate, and chlorine) is simulated with the Equilibrium Simplified Aerosol Model (EQSAM) (Metzger et al., 2002). The aerosols are assumed to be metastable, internally mixed, and obey thermodynamic gas–aerosol equilibrium. Nitrate and ammonium aerosols are represented by a fine mode, associated with sulfur, and a coarse mode associated with sea salt. The fine mode nitrate is linked to the sulfate module, and hence requires tropospheric chemistry to run. The main reference is Myhre et al. (2006).

Sulfate

Sulfate is calculated using the sulfur chemistry scheme and currently applies for the troposphere only. It is primarily a chemical scheme, but sulfate is assumed to be in aerosol phase, i.e. comprising H2SO4 and the ions SO4-- and HSO4-. The main reference is Berglen et al. (2004).

Mineral dust

OsloCTM3 uses the DEAD model (Zender et al., 2003) to calculate mineral dust, with emissions are driven by the model winds. It is a stand-alone application which can be run alone with two or more dust tracers. Main reference is Grini et al. (2005).

Sea salt

Sea salt is also a stand-alone module, with emissions driven by the model winds. The sea salt module was originally introduced by Grini et al. (2002) and has later been updated with a new production parameterization following recommendations by Witek et al. (2016). Using satellite retrievals, Witek et al. (2016) evaluated different sea spray aerosol emission parametrizations and found the best agreement with the emission function from Sofiev et al. (2011) including the sea surface temperature adjustment from Jaeglé et al. (2011).

 

References

Berglen, T. F., T. K. Berntsen, I. S. A. Isaksen, and J. K. Sundet: A global model of the coupled sulfur/oxidant chemistry in the troposphere: The sulfur cycle, J. Geophys. Res., 109(D19310), doi:10.1029/2003JD003948, 2004.

Berntsen, T., and I. S. A. Isaksen: A global 3-D chemical transport model for the troposphere, 1, Model description and CO and Ozone results, J. Geophys. Res., 102(D17), 21239-21280, doi:10.1029/97JD01140, 1997.

Brunner, D.; J. Staehelin, H. L. Rogers, M. O. Köhler, J. A. Pyle, D. Hauglustaine, L. Jourdain, T. K. Berntsen, M. Gauss, I. S. A. Isaksen, E. Meijer, P. van Velthoven, G. Pitari, E. Mancini, V. Grewe, and R. Sausen: An evaluation of the performance of chemistry transport models by comparison with research aircraft observations. Part 1: Concepts and overall model performance. Atmos. Chem. Phys., 3, 1609-1631, doi:10.5194/acp-3-1609-2003, 2003.

Chung, S. H. and J. H. Seinfeld: Global distribution and climate forcing of carbonaceous aerosols, J. Geophys. Res., 107(D19), doi:10.1029/2001JD001397, 2002.

de Zafra, R., and S. Smyshlyaev:On the formation of HNO3 in the Antarctic mid-to-upper stratosphere in winter, J. Geophys. Res., 106(D19), 23115-23125, doi:10.1029/2000JD000314, 2001.

Grini, Alf, Gunnar Myhre, Jostein K. Sundet, and Ivar S. A. Isaksen: Modeling the Annual Cycle of Sea Salt in the Global 3D Model Oslo CTM2: Concentrations, Fluxes, and Radiative Impact, Journal of Climate, Vol. 15, No. 13, pp 1717-1730, doi:10.1175/1520-0442(2002)015<1717:MTACOS>2.0.CO;2, 2002.

Hesstvedt, E., O. Hov, and I. S. A. Isaksen: Quasi steady-state approximation in air pollution modelling: Comparison of two numerical schemes for oxidant prediction. Int. Journal of Chem. Kinetics, X, 971-994, doi:10.1002/kin.550100907, 1978.

Hoffmann T., Odum J. R., Bowman F., Collins D., Klockow D., Flagan R. C. & Seinfeld J. H. Formation of Organic Aerosols from the Oxidation of Biogenic Hydrocarbons, Journal of Atmospheric Chemistry. 26(2), 189-222, 10.1023/a:1005734301837, 1997.

Hoyle, C.R., Terje Berntsen, Gunnar Myhre and Ivar S. A. Isaksen: Secondary organic aerosol in the global aerosol - chemical transport model Oslo CTM2, Atmos. Chem. Phys., Vol. 7, pp 5675-5694, doi:10.5194/acp-7-5675-2007, 2007.

Jaeglé, L., Quinn, P. K., Bates, T. S., Alexander, B., and Lin, J.-T.: Global distribution of sea salt aerosols: new constraints from in situ and remote sensing observations, Atmos. Chem. Phys., 11, 3137–3157, https://doi.org/10.5194/acp-11-3137-2011, 2011.

Kraabøl, A. G., F. Stordal, P. Konopka, and S. Knudsen: The NILU aircraft plume model: A technical description. Technical Report TR 4/99, Norwegian Institute for Air Research, 1999.

Lund, M. T., Myhre, G., Haslerud, A. S., Skeie, R. B., Griesfeller, J., Platt, S. M., Kumar, R., Myhre, C. L., and Schulz, M.: Concentrations and radiative forcing of anthropogenic aerosols from 1750 to 2014 simulated with the Oslo CTM3 and CEDS emission inventory, Geosci. Model Dev., 11, 4909–4931, https://doi.org/10.5194/gmd-11-4909-2018, 2018.

Lund, M. T. and T. Berntsen: Parameterization of black carbon aging in the OsloCTM2 and implications for regional transport to the ArcticParameterization of black carbon aging in the OsloCTM2 and implications for regional transport to the Arctic, Atmos. Chem. Phys.., 12, 6999-7014, doi:10.5194/acp-12-6999-2012, 2012.

Metzger, S., Dentener, F., Pandis, S., and Lelieveld, J.:Gas/aerosol partitioning: 1. A computationally efficient model, J. Geophys. Res.-Atmos., 107, ACH 16-1–ACH 16-24, https://doi.org/10.1029/2001jd001102, 2002b.

Myhre, G., A. Grini and S. Metzger: Modelling of nitrate and ammonium-containing aerosols in presence of sea salt, Atmos. Chem. Phys., Vol. 6, pp 4809-4821, doi:10.5194/acp-6-4809-2006, 2006.

Skeie, R. B.; T. Berntsen, G. Myhre, C. A. Pedersen, J. Ström, S. Gerland, and J. A. Ogren: Black carbon in the atmosphere and snow, from pre-industrial times until present, Atmos. Chem. Phys., 11, 6809-6836, doi:10.5194/acp-11-6809-2011, 2011.

Sofiev, M., Soares, J., Prank, M., de Leeuw, G., and Kukkonen, J.: A regional-to-global model of emission and transport of sea salt particles in the atmosphere, J. Geophys. Res.-Atmos., 116, D21302, https://doi.org/10.1029/2010JD014713, 2011.

Smyshlyaev, S. P., V. L. Dvortsov,M. A. Geller, and V. Yudin:A two-dimensionalmodel with input parameters from a GCM: Ozone sensitivity to different formulations for the longitudinal temperature variation. J. Geophys. Res., 103(D21), 28373-28387, doi:10.1029/98JD02354, 1998.

Søvde, O. A., M, Gauss, S. Smyshlyaev, and I. S. A. Isaksen: Evaluation of the chemical transport model Oslo CTM2 with focus on Arctic winter ozone depletion. J. Geophys. Res., vol. 113, D09304. doi:10.1029/2007JD009240, 2008.

Witek, M. L., Diner, D. J., and Garay, M. J.: Satellite assessment of sea spray aerosol productivity: Southern Ocean case study, J. Geophys. Res.-Atmos., 121, 872–894, https://doi.org/10.1002/2015JD023726, 2016.