Development of a protocol for the auto-generation of explicit aqueous-phase oxidation schemes of organic compounds

This paper presents a new CAPRAM–GECKO-A protocol for mechanism auto-generation of aqueous-phase organic processes. For the development, kinetic data in the literature were reviewed and a database with 464 aqueous-phase reactions of the hydroxyl radical with organic compounds and 130 nitrate radical reactions with organic compounds has been compiled and evaluated. Five different methods to predict aqueous-phase rate constants have been evaluated with the help of the kinetics database: gas–aqueous phase correlations, homologous series of various compound classes, radical reactivity comparisons, Evans–Polanyi-type correlations, and structure–activity relationships (SARs). The quality of these prediction methods was tested as well as their suitability for automated mechanism construction. Based on this evaluation, SARs form the basis of the new CAPRAM–GECKO-A protocol. Evans–Polanyi-type correlations have been advanced to consider all available H atoms in a molecule besides the H atoms with only the weakest bond dissociation enthalpies (BDEs). The improved Evans–Polanyi-type correlations are used to predict rate constants for aqueous-phase NO3 and organic compounds reactions. Extensive tests have been performed on essential parameters and on highly uncertain parameters with limited experimental data. These sensitivity studies led to further improvements in the new CAPRAM–GECKO-A protocol but also showed current limitations. Biggest uncertainties were observed in uptake processes and the estimation of Henry's law coefficients as well as radical chemistry, in particular the degradation of alkoxy radicals. Previous estimation methods showed several deficits, which impacted particle growth. For further evaluation, a 1,3,5-trimethylbenzene oxidation experiment has been performed in the aerosol chamber "Leipziger Aerosolkammer" (LEAK) at high relative humidity conditions and compared to a multiphase mechanism using the Master Chemical Mechanism (MCMv3.2) in the gas phase and using a methylglyoxal oxidation scheme of about 600 reactions generated with the new CAPRAM–GECKO-A protocol in the aqueous phase. While it was difficult to evaluate single particle constituents due to concentrations close to the detection limits of the instruments applied, the model studies showed the importance of aqueous-phase chemistry in respect to secondary organic aerosol (SOA) formation and particle growth. The new protocol forms the basis for further CAPRAM mechanism development towards a new version 4.0. Moreover, it can be used as a supplementary tool for aerosol chambers to design and analyse experiments of chemical complexity and help to understand them on a molecular level.