Multiphase Modelling and CAPRAM

In the research area of multiphase chemistry modelling, detailed aqueous-phase mechanisms are developed and applied within numerical models studying the physico-chemical multiphase processes occurring in cloud droplets and deliquescent aerosol particles. The primary focus of these model applications is the analysis of aqueous-phase oxidation processes and their impact on the tropospheric oxidation capacity, as well as their contribution to the formation of secondary particulate matter. 

To simulate the complex tropospheric multiphase system, the spectral air parcel model SPACCIM (SPectral Aerosol Cloud Chemistry Interaction Model; Wolke et al., 2005) is employed, which has been developed within the modelling department. The currently applied multiphase mechanism combines the gas-phase mechanism MCMv3.3.1 (Master Chemical Mechanism; mcm.york.ac.uk) and the in-house aqueous-phase mechanism CAPRAM 4.0 (Chemical Aqueous Phase Radical Mechanism, Bräuer et al., 2019). The phase transfer of soluble species is treated in the model following the approach of Schwartz (1986). Within the aqueous-phase chemistry mechanism, particular emphasis is given to chemical processes of organic compounds and key atmospheric radicals such as NO3, OH, SO4-, Cl2-, Br2-, CO3- and peroxy radicals. In contrast to many existing aqueous-phase mechanisms, CAPRAM 4.0 includes reactions of organic compounds with up to four carbon atoms (alcohols, carbonyls, carboxylic acids), extending beyond the commonly represented C1-C2 chemistry. The mechanism also contains a comprehensive reaction scheme for sulfur(IV) oxidation involving radicals, transition metal ions (iron, copper and manganese), peroxides and ozone.

In addition to the core CAPRAM mechanism, several specialized modules have been developed and implemented. These include reduced chemical mechanisms for application in regional and large-scale chemistry transport models (Deguillaume et al., 2009, Hoffmann et al., 2020), with ongoing efforts focused on further reduction and optimization. The CAPRAM team has extensively developed and continuously updated halogen chemistry modules v1.0/2.0/3.0 (Herrmann et al., 2003; Bräuer et al., 2013, Hoffmann et al., 2019), which include radical reactions of halogen-containing species in both gas and aqueous phases and are particularly relevant for modelling multiphase chemistry in the marine boundary layer. Further developments include modules describing the oxidation of dimethyl sulfide (DMS) in tropospheric multiphase chemistry (CAPRAM-DM1.0, Hoffmann et al., 2016), substituted aromatic compounds in the aqueous phase (CAPRAM-AM1.0, Hoffmann et al., 2018), and amine chemistry. Additionally, the mechanism generator GECKO-A (Aumont et al., 2005) has been extended to aqueous-phase processes, enabling automated generation of organic chemistry mechanisms in CAPRAM based on predefined protocols (Bräuer et al., 2019). This extension is supported by an aqueous-phase kinetic database comprising experimentally determined rate constants, structure-activity relationships, and other correlations for mechanism construction. 

More recent developments include the introduction of a heterogeneous dust chemistry module, CAPRAM-HET1.0 (Aiyuk et al., 2024), and the implementation of a bulk-interfacial partitioning chemistry mechanism (CAPRAM-HET2.0; Aiyuk et al., 2025). In addition, the tropospheric multiphase chemistry of biomass burning emissions module (CAPRAM-BBM1.0) is under development, aligned with CAPRAM-AM1.0. Recent work also focuses on integrating neural network and machine learning techniques to enhance model performance, improve air quality predictions, and support mechanism reduction strategies. 

Further information, references, and access to available mechanisms and reaction modules can be found on the CAPRAM - website.

References:

Aiyuk, M. B., Hoffmann, E. H., Tilgner, A., Wolke, R., & Herrmann, H. (2024). A CAPRAM Modeling Study on the Role of Heterogeneous Reactions on Dust in Tropospheric Chemistry. ACS Earth and Space Chemistry, 8(10), 2052-2066.

Aiyuk, M. B., Hoffmann, E. H., Tilgner, A., Wolke, R., & Herrmann, H. (2025). Modeling Tropospheric Aqueous Interfacial Chemistry and Bulk Interaction with CAPRAM-HET2. 0. ACS Earth and Space Chemistry, 9(5), 1204-1216.

Hoffmann, E. H., Tilgner, A., Felber, T., Aiyuk, M. B., Schaefer, T., & Herrmann, H. (2025). Modeling the Tropospheric Aqueous-Phase Chemistry of Photosensitizers under Wildfire-Plume and Urban Conditions with CAPRAM-PS1. 0. ACS Earth and Space Chemistry, 9(6), 1593-1606.

Bräuer, P., A. Tilgner, R. Wolke, and H. Herrmann (2013), Mechanism development and modelling of tropospheric multiphase halogen chemistry: The CAPRAM Halogen Module 2.0 (HM2), J Atmos Chem, 19-25. dx.doi.org/10.1007/s10874-013-9249-6

Bräuer, P., Mouchel-Vallon, C., Tilgner, A., Mutzel, A., Böge, O., Rodigast, M., Poulain, L., van Pinxteren, D., Wolke, R., Aumont, B., and Herrmann, H. (2019): Development of a protocol for the auto-generation of explicit aqueous-phase oxidation schemes of organic compounds, Atmos Chem Phys, 19, 9209–9239, doi.org/10.5194/acp-19-9209-2019, 2019.

Deguillaume, L., A. Tilgner, R. Schrödner, R. Wolke, N. Chaumerliac, and H. Herrmann (2009), Towards an operational aqueous phase chemistry mechanism for regional chemistry-transport models: CAPRAM-RED and its application to the COSMO-MUSCAT model, J Atmos Chem, 64(1), 1-35. http://dx.doi.org/10.1007/s10874-010-9168-8

Ervens, B., C. George, J. E. Williams, G. V. Buxton, G. A. Salmon, M. Bydder, F. Wilkinson, F. Dentener, P. Mirabel, R. Wolke, H. Herrmann (2003), CAPRAM 2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application, Journal of Geophys Res-Atmos, 108(D14), -. http://www.dx.doi.org/10.1029/2002JD002202

Herrmann, H., A. Tilgner, P. Barzaghi, Z. Majdik, S. Gligorovski, L. Poulain, and A. Monod (2005), Towards a more detailed description of tropospheric aqueous phase organic chemistry: CAPRAM 3.0, Atmos Environ, 39(23-24), 4351-4363. dx.doi.org/10.1016/j.atmosenv.2005.02.016

Hoffmann, E. H.; Tilgner, A.; Wolke, R.; Böge, O.; Walter, A. and Herrmann, H. (2018), Oxidation of substituted aromatic hydrocarbons in the tropospheric aqueous phase: kinetic mechanism development and modelling, Phys Chem Chem Phys 20, 10960. doi.org/10.1039/C7CP08576A

Hoffmann, E. H.; Tilgner, A.; Schrödner, R.; Bräuer, P.; Wolke, R. and Herrmann, H. (2016), An advanced modeling study on the impacts and atmospheric implications of multiphase dimethyl sulfide chemistry, Proc Natl Acad Sci USA 113, 11776-11781. https://doi.org/10.1073/pnas.1606320113

Hoffmann, E. H., Tilgner, A., Wolke, R., Herrmann, H. (2019) Enhanced chlorine and bromine atom activation by hydrolysis of halogen nitrates from marine aerosols at polluted coastal areas, Environ Sci Technol, 53, 771-778, https://doi.org/10.1021/acs.est.8b05165.

Hoffmann, E. H., Schrödner, R., Tilgner, A., Wolke, R., Herrmann, H. (2020) CAPRAM reduction towards an operational multiphase halogen and dimethyl sulfide chemistry treatment in the chemistry transport model COSMO-MUSCAT(5.04e), Geosci Model Develop, 13, 2587-2609, doi.org/10.5194/gmd-13-2587-2020.

Schwartz, S. E. (1986), Mass-transport considerations pertinent to aqueous phase reactions of gases in liquid-water clouds Rep. NATO ASI Series Vol. G6, NATO. 

Tilgner, A., P. Bräuer, R. Wolke, and H. Herrmann (2013), Modelling multiphase chemistry in deliquescent aerosols and clouds using CAPRAM3.0i, J Atmos Chem, 70(3), 221-256. 

Tilgner, A., and H. Herrmann (2010), Radical-driven carbonyl-to-acid conversion and acid degradation in tropospheric aqueous systems studied by CAPRAM, Atmos Environ, 44, 5415-5422. http://dx.doi.org/10.1016/j.atmosenv.2010.07.050