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. Tropospheric deliquesced particles are characterised by concentratednon-ideal solutions (“aerosol liquid water” or ALW) that can affect theoccurring multiphase chemistry. However, such non-ideal solution effectshave generally not yet been considered in and investigated by currentcomplex multiphase chemistry models in an adequate way. Therefore, thepresent study aims at accessing the impact of non-ideality on multiphasechemical processing in concentrated aqueous aerosols. Simulations with themultiphase chemistry model (SPACCIM-SpactMod) are performed under differentenvironmental and microphysical conditions with and without a treatment ofnon-ideal solutions in order to assess its impact on aqueous-phase chemicalprocessing. The present study shows that activity coefficients of inorganic ions areoften below unity under 90 % RH-deliquesced aerosol conditions and thatmost uncharged organic compounds exhibit activity coefficient values ofaround or even above unity. Due to this behaviour, model studies haverevealed that the inclusion of non-ideality considerably affects themultiphase chemical processing of transition metal ions (TMIs), oxidants,and related chemical subsystems such as organic chemistry. In detail, boththe chemical formation and oxidation rates of Fe(II) are substantiallylowered by a factor of 2.8 in the non-ideal base case compared to the idealcase. The reduced Fe(II) processing in the non-ideal base case, includinglowered chemical rates of the Fenton reaction ( − 70 %), leads to a reducedprocessing of HOx∕HOy under deliquesced aerosol conditions.Consequently, higher multiphase H2O2 concentrations (larger by afactor of 3.1) and lower aqueous-phase OH concentrations (lower by a factorof ≈4 ) are modelled during non-cloud periods. For H2O2 ,a comparison of the chemical reaction rates reveals that the most importantsink, the reaction with HSO 3 - , contributes with a 40 % higherrate in the non-ideal base case than in the ideal case, leading to moreefficient sulfate formation. On the other hand, the chemical formation ratesof the OH radical are about 50 % lower in the non-ideal base case than inthe ideal case, leading to lower degradation rates of organic aerosolcomponents. Thus, considering non-ideality influences the chemicalprocessing and the concentrations of organic compounds under deliquescedparticle conditions in a compound-specific manner. For example, the reducedoxidation budget under deliquesced particle conditions leads to bothincreased and decreased concentration levels, e.g. of important C2∕C3 carboxylic acids. For oxalic acid, the present studydemonstrates that the non-ideality treatment enables more realisticpredictions of high oxalate concentrations than observed under ambienthighly polluted conditions. Furthermore, the simulations imply thatlower humidity conditions, i.e. more concentrated solutions, might promotehigher oxalic acid concentration levels in aqueous aerosols due todifferently affected formation and degradation processes.

Treatment of non-ideality in the SPACCIM multiphase model – Part 2: Impacts on the multiphase chemical processing in deliquesced aerosol particles