Multi-phase chemical mechanism development
A. Tilgner and H. Herrmann
The Chemical Aqueous Phase Radical Mechanism (CAPRAM) for modeling tropospheric multiphase chemistry in its version 2.3 was submitted in December 1998 to J. Atmos. Chem. Its development in the framework of CMD (Chemical Mechanism Development) started at the
and was continued at the Institute for Tropospheric Research in
. The mechanism at that stage contained 157 gas phase reactions, 256 aqueous phase reactions and 34 heterogeneous processes. The mechanism was coupled to the gas phase mechanism RADM2 (Regional Acid Deposition Model 2; Stockwell et al., 1990). Phase transfer was treated by means of the resistance model of Schwartz (1986). In addition to Henry’s law constants also mass accommodation coefficients and gas phase diffusion coefficients were implemented. CAPRAM 2.3 contains (1) a detailed treatment of the oxidation of organic compounds with one and two carbon atoms, (2) an explicit description of S(IV)-oxidation by radicals and iron(III), as well as by peroxides and ozone, (3) the reactions of OH, NO3, Cl2-, Br2-, and CO3- radicals, as well as reactions of the transition metal ions (TMI’s) iron, manganese and copper.
The later version, CAPRAM 2.4 (MODAC mechanism) is a detailed and extended chemical mechanism describing tropospheric aqueous phase chemistry. It has been developed in the course of the EU-funded project MODAC (MOdel Development for tropospheric Aerosol and cloud Chemistry) and in the CMD subproject of EUROTRAC-2. Its description has been submitted in February 2002 to Journal Geophysical Research and was published in July 2003 (Ervens et al., 2003). The mechanism in this version contains 237 gas phase reactions, 438 aqueous phase processes and 34 heterogeneous processes. The mechanism, based on the former version 2.3 (Herrmann et al., 2000), contains extended organic and transition metal chemistry and is formulated more explicitly based on a critical review of the literature. The gas phase scheme was replaced by the newer gas phase mechanism RACM (Regional Atmospheric Chemistry Mechanism; Stockwell et al., 1997). Phase transfer is treated using the resistance model of Schwartz (1986).
A condensed version (183 reactions) has also been developed to allow the use of CAPRAM 2.4 (MODAC-mechanism) in larger scale models. Here the ability of a simpler mechanism to reproduce concentration levels of selected target species (i.e. NOx, S(IV), H2O2, NO3, OH, O3 and H+) within the limits of ± 5 % was used to eliminate unimportant reactions from the complete CAPRAM 2.4 (MODAC mechanism) scheme. Testing of the condensed mechanism was completed using a range of initial conditions chosen to represent different atmospheric scenarios, in order to produce a robust yet concise set of reactions. The most interesting results were obtained using atmospheric conditions typical for an urban scenario and the effects introduced by updating the aqueous phase chemistry were highlighted, in particular, with regards to radicals, redox-cycling of transition metal ions and organic compounds. Finally, the reduced scheme was incorporated into a 1-D marine cloud model to demonstrate the applicability of this mechanism.
In the course of a focused study in another EU project, interactions of aromatic compounds with tropospheric aqueous particles (CAPRAM aromatics module) were considered. Many interesting features for the formation and fate of substituted phenols were studied during the UNARO (Uptake and Nitration of AROmatics) EU-funded project. However, the current knowledge of the oxidation of aromatics in the gas phase is limited and in parts very controversially discussed. In addition, a lack of uptake parameters and aqueous phase rate constants is existing. For a better treatment of aromatics oxidation in the multiphase system first a proper groundwork of more detailed studies of elementary process steps needs to be done.
For the simulation of halogen activation in tropospheric clouds in different environments a further extension, the CAPRAM halogen module, was developed in the course of CMD and published in Chemosphere (Herrmann et al., 2003). This mechanism considers additional reactions of halogen containing species both in the gas and aqueous phases. In contrast to other mechanisms explaining halogen activation, the main focus in the CAPRAM halogen module was to better elucidate the role of radical processes in the particle phase. Another aspect is that this mechanism is applied not only for the study of aqueous marine aerosols but also for studying possible effects of clouds in coupling to tropospheric halogen chemistry. Model tests demonstrate that for marine and urban clouds direct phase transfer of halogen atoms, formed in the particle phase by radical conversions involving NO3, SO4- and OH, is the most important source of halogen atoms in the gas phase. If sea salt aerosol is present, formation of Cl2 and Br2 in the particle phase followed by their photolysis in the gas phase will be responsible for halogen atom formation in the gas phase. The CAPRAM halogen module contains 21 gas phase, 47 aqueous phase and 8 phase transfer processes.
The latest box model development was to include size resolved particle systems considering different numbers of size bins (1,2,3,4,5,10,20,30,50) for the study of chemical processes in tropospheric clouds and deliquescent aerosol particles. In comparison to a monodisperse system, preliminary tests showed important concentration changes when a size segregated system was considered. The largest changes occurred in the marine scenario, due to the more important contribution of phase transfer processes.
CAPRAM 3.0 (Herrmann et al., 2005) is the latest development of CAPRAM series which incorporates the former version CAPRAM 2.4 (Ervens et al., 2003) and a new extended reaction mechanism for atmospherically important hydrocarbons containing more than two and up to six carbon atoms. The mechanism has been developed within the MODMEP (“MOdeling of MultiphasE Processes: Tools and chemical mechanisms”) project of the AFO2000 program of the BMBF and the EU MOST (Multiphase chemistry of Oxygenated Species in the Troposphere) project, (http://most.univ-lyon1.fr/). The description was accepted for Atmospheric Environment and published in August 2005 in a special issue of this journal containing the results of the project cluster FEBUKO and MODMEP. The chemistry of several organic compounds containing three and four carbon atoms is now described in detail. Almost 400 new reactions are now implemented considering the chemistry of organic compounds containing different functional groups, i.e. alcohols, carbonyl compounds, mono- and dicarboxylic acids, polyfunctional compounds as well as some esters and heterocyclic compounds. CAPRAM 3.0 represents the most explicit aqueous phase mechanism of the CAPRAM series. At the current state of mechanism development the aqueous phase chemistry mechanism contains oxidation pathways of 34 chemical species (5 alcohols, 10 carbonyl compounds, 13 mono- and dicarboxylic acids, 1 ester, 4 polyfunctional compounds and 1 heterocyclic compound). In CAPRAM 3.0, the oxidation of these compounds is initiated by an H-abstraction at the weakest carbon-hydrogen bond due to the reaction with OH and NO3 radicals as well as the subsequent formation and decay of the peroxyl radical. The aqueous chemistry has been coupled to the gas phase mechanism RACM (Stockwell et al. 1997), and phase exchange is treated using the resistance model of Schwartz (1986), as well. Additionally to compounds already treated in the former version 2.4, in CAPRAM 3.0 the phase transfer of the following compounds was implemented: 1-propanol; 2-propanol; 1-butanol; 2-butanol; propionaldehyde; butyraldehyde; propanoic acid; butyric acid; methylglyoxal; acetone; methyl ethyl ketone (MEK); hydroxyacetone; 1,4-butenedial; methyl isobutyl ketone (MIBK); ethyl formate; N-methyl-pyrrolidin-2-one and ethylene glycol.
As can be seen from the abovementioned species list CAPRAM 3.0 contains also a description of the chemistry of oxygenated organic compounds which are used as solvents and fuel additives. Compounds such as 2,3-butadione, ethyl formate, methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK) might be released to the troposphere following their industrial application. These compound themselves or their respective oxidation products might play a role in the tropospheric aqueous phase in a close interplay with their respective gas phase chemistry. For this part of the mechanism development kinetic data from the recent review of Herrmann (2003) and very recent data originating from the EU project MOST were adopted throughout the mechanism.
CAPRAM in its different versions has been applied in various case studies including a treatment of cloud evolution by B. Vogel and the initialization by different scenarios as developed by W. Seidl (deceased). More recently, CAPRAM 3.0 was used in the fully coupled microphysical and multiphase chemistry model SPACCIM (SPectral Aerosol Cloud Chemistry Interaction Model; Wolke et al., 2005) to interpret hill cap cloud passage field experiments. This model studies are based on the work of the joint research projects FEBUKO (field investigation of budgets and conversions of particle phase organics in tropospheric cloud processes) and MODMEP and were focused on the physico-chemical modification of the multiphase system (Herrmann et al., 2005b). The air parcel model SPACCIM (Wolke et al., 2005) was developed for the description of cloud processes combining a complex multiphase chemistry with detailed microphysics. The description of both separate processes is performed for a highly size-resolved particle and droplet spectrum. The model allows a detailed description of the processing of gases and deliquescent particles before the cloud formation, under cloud conditions and after cloud evaporation. The adiabatic air parcel model contains a detailed description of microphysical processes of deliquescent aerosol particles and droplets (Simmel et al., 2005). The simulation results were compared to experimental cloud water composition data at Schmücke summit site as well as gas and aerosol measurements at downwind site in order to interpret the experimental data and to evaluate the model results (Tilgner et al., 2005).