SOAP models the partitioning of semi-volatil organic
between gas and particle phases.
SOAP determines the partitioning of organic compounds between gas and
particle phases. It is designed to be modular with different user options
depending on the computation time and the complexity required by the
user. This model is based on the molecular surrogate approach, in which each
surrogate compound is associated with a molecular structure to estimate some
properties and parameters (hygroscopicity, absorption into the aqueous phase
of particles, activity coefficients and phase separation).
Each surrogate can be hydrophilic (condenses only into the aqueous phase of particles), hydrophobic (condenses only into the organic phases of particles) or both (condenses into both the aqueous and the organic phases of particles). Activity coefficients are computed with the UNIFAC thermodynamic model for short-range interactions and with the AIOMFAC parameterization for medium and long-range interactions between electrolytes and organic compounds. Phase separation is determined by Gibbs energy minimization.
The user can choose between an equilibrium representation and a
dynamic representation of the organic aerosol (OA). In the equilibrium
representation, compounds in the particle phase are assumed to be at
equilibrium with the gas phase. In the implicit dynamic representation, particles can
be viscous, with OA divided into layers, the first layer being at the
center of the particle (slowly reaches equilibrium) and the final layer
being near the interface with the gas phase (quickly reaches
equilibrium). The condensation/evaporation of organic compounds could then be
limited by the diffusion in the organic phases due to the high viscosity. This dynamic implicit representation is a simplified approach
to model condensation/evaporation designed to work with a low number of
layers and short CPU time.
The development of the first release of the code was done with support from
the french ministry of ecology, by CNRS-INSU (GMES-MDD program) and by ADEME,
the French Agency for the Environment and Energy Management.