Modeling the effect of non-ideality, dynamic mass transfer and viscosity on SOA formation in a 3-D air quality model
Authors: Kim Youngseob , Sartelet Karine , Couvidat Florian 
Published: January 2019

Abstract. In this study, assumptions (ideality and thermodynamic
equilibrium) commonly made in three-dimensional (3-D) air quality models were
reconsidered to evaluate their impacts on secondary organic aerosol (SOA)
formation over Europe. To investigate the effects of non-ideality, dynamic mass transfer and aerosol
viscosity on the SOA formation, the Secondary Organic Aerosol Processor
(SOAP) model was implemented in the 3-D air quality model Polyphemus. This
study presents the first 3-D modeling simulation which describes the impact
of aerosol viscosity on the SOA formation. The model uses either the
equilibrium approach or the dynamic approach with a method specially designed
for 3-D air quality models to efficiently solve particle-phase diffusion when
particles are viscous. Sensitivity simulations using two organic aerosol models implemented in
Polyphemus to represent mass transfer between gas and particle phases show
that the computation of the absorbing aerosol mass strongly influences the
SOA formation. In particular, taking into account the concentrations of
inorganic aerosols and hydrophilic organic aerosols in the absorbing mass of
the aqueous phase increases the average SOA concentration by 5 % and 6 %,
respectively. However, inorganic aerosols influence the SOA formation not
only because they constitute an absorbing mass for hydrophilic SOA, but also
because they interact with organic compounds. Non-ideality (short-, medium- and
long-range interactions) was found to influence SOA concentrations by about
30 %. Concerning the dynamic mass transfer for the SOA formation, if the viscosity
of SOA is not taken into account and if ideality of aerosols is assumed, the
dynamic approach is found to give generally similar results to the
equilibrium approach (indicating that equilibrium is an efficient hypothesis
for inviscid and ideal aerosols). However, when a non-ideal aerosol is
assumed, taking into account the dynamic mass transfer leads to a decrease of
concentrations of the hydrophilic compounds (compared to equilibrium). This
decrease is due to differences in the values of activity coefficients, which
are different between values computed for bulk aerosols and those for each
size section. This result indicates the importance of non-ideality on the
dynamic evolution of SOA. For viscous aerosols, assuming a highly viscous organic phase leads to an
increase in SOA concentrations during daytime (by preventing the evaporation
of the most volatile organic compounds). The partitioning of nonvolatile
compounds is not affected by viscosity, but the aging of more volatile
compounds (that leads to the formation of the less volatile compounds) slows
down as the evaporation of those compounds is stopped due to the viscosity of
the particle. These results imply that aerosol concentrations may deviate
significantly from equilibrium as the gas–particle partitioning could be
higher than predicted by equilibrium. Furthermore, although a compound
evaporates in the simulation using the equilibrium approach, the same
compound can condense in the simulation using the dynamic approach if the
particles are viscous. The results of this study emphasize the need for 3-D air quality models to
take into account the effect of non-ideality on SOA formation and the effect
of aerosol viscosity for the more volatile fraction of semi-volatile organic
compounds.