Influence of non-ideality on condensation to aerosol S. Compernolle, K. Ceulemans, and J.-F. Müller Belgian Institute for Space-aeronomy, Ringlaan 3, 1180 Brussels, Belgium
Abstract. Secondary organic aerosol (SOA) is a complex mixture of water and
organic molecules. Its composition is determined by the presence of
semi-volatile or non-volatile compounds, their saturation vapor pressure and
activity coefficient. The activity coefficient is a non-ideality effect
and is a complex function of SOA composition. In a previous publication,
the detailed chemical mechanism (DCM) for α-pinene oxidation
and subsequent aerosol formation BOREAM was presented. In this work,
we investigate with this DCM the impact of non-ideality by simulating
smog chamber experiments for α-pinene degradation and aerosol
and taking the activity coefficient into account of all molecules in the aerosol phase. Several versions of the UNIFAC method are tested for this
purpose, and missing parameters for e.g. hydroperoxides and nitrates
are inferred from fittings to activity coefficient data generated
using the SPARC model.
Alternative approaches to deal with these missing parameters
are also tested, as well as an activity coefficient calculation method
based on Hansen solubility parameters (HSP).
It turns out that for most experiments, non-ideality
has only a limited impact on the interaction between the organic molecules,
and therefore on SOA yields and composition, when water uptake is
The reason is that often, the activity coefficient is on average close to 1 and, specifically
for high-VOC experiments, partitioning is not very sensitive on the activity coefficient
because the equilibrium is shifted strongly towards condensation.
Still, for ozonolysis experiments with low amounts of volatile
organic carbon (low-VOC), the UNIFAC parameterization of Raatikainen
et al. leads to significantly higher SOA yields (by up to a factor
1.6) compared to the ideal case and to other parameterizations. Water
uptake is model dependent, in the order: ideal > UNIFAC-Raatikainen
> UNIFAC-Peng > UNIFAC-Hansen ≈ UNIFAC-Magnussen ≈
UNIFAC-Ming. In the absence of salt dissolution, phase splitting
from pure SOA is unlikely.
Citation: Compernolle, S., Ceulemans, K., and Müller, J.-F.: Influence of non-ideality on condensation to aerosol, Atmos. Chem. Phys., 9, 1325-1337, doi:10.5194/acp-9-1325-2009, 2009.