Treatment of non-ideality in the SPACCIM multiphase model – Part 2: Impacts on the multiphase chemical processing in deliquesced aerosol particles
Authors: Rusumdar Ahmad Jhony , Tilgner Andreas , Wolke Ralf , Herrmann Hartmut 
Published: September 2020

Abstract. Tropospheric deliquesced particles are characterised by concentrated
non-ideal solutions (“aerosol liquid water” or ALW) that can affect the
occurring multiphase chemistry. However, such non-ideal solution effects
have generally not yet been considered in and investigated by current
complex multiphase chemistry models in an adequate way. Therefore, the
present study aims at accessing the impact of non-ideality on multiphase
chemical processing in concentrated aqueous aerosols. Simulations with the
multiphase chemistry model (SPACCIM-SpactMod) are performed under different
environmental and microphysical conditions with and without a treatment of
non-ideal solutions in order to assess its impact on aqueous-phase chemical
processing. The present study shows that activity coefficients of inorganic ions are
often below unity under 90 % RH-deliquesced aerosol conditions and that
most uncharged organic compounds exhibit activity coefficient values of
around or even above unity. Due to this behaviour, model studies have
revealed that the inclusion of non-ideality considerably affects the
multiphase chemical processing of transition metal ions (TMIs), oxidants,
and related chemical subsystems such as organic chemistry. In detail, both
the chemical formation and oxidation rates of Fe(II) are substantially
lowered by a factor of 2.8 in the non-ideal base case compared to the ideal
case. The reduced Fe(II) processing in the non-ideal base case, including
lowered chemical rates of the Fenton reaction (−70 %), leads to a reduced
processing of HOx∕HOy under deliquesced aerosol conditions.
Consequently, higher multiphase H2O2 concentrations (larger by a
factor of 3.1) and lower aqueous-phase OH concentrations (lower by a factor
of ≈4) are modelled during non-cloud periods. For H2O2,
a comparison of the chemical reaction rates reveals that the most important
sink, the reaction with HSO3-, contributes with a 40 % higher
rate in the non-ideal base case than in the ideal case, leading to more
efficient sulfate formation. On the other hand, the chemical formation rates
of the OH radical are about 50 % lower in the non-ideal base case than in
the ideal case, leading to lower degradation rates of organic aerosol
components. Thus, considering non-ideality influences the chemical
processing and the concentrations of organic compounds under deliquesced
particle conditions in a compound-specific manner. For example, the reduced
oxidation budget under deliquesced particle conditions leads to both
increased and decreased concentration levels, e.g. of important
C2∕C3 carboxylic acids. For oxalic acid, the present study
demonstrates that the non-ideality treatment enables more realistic
predictions of high oxalate concentrations than observed under ambient
highly polluted conditions. Furthermore, the simulations imply that
lower humidity conditions, i.e. more concentrated solutions, might promote
higher oxalic acid concentration levels in aqueous aerosols due to
differently affected formation and degradation processes.