Abstract
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 formation 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 ignored. 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.
Highlights
Oxidation of complex VOC molecules leads to a myriad of compounds, many of which having sufficiently low saturation vapor pressures pi0 in order to condense and form Secondary organic aerosol (SOA)
Mass yields calculated with the Hansen solubility parameters (HSP) method are systematically lower than with UNIFAC-Raatikainen (Fig. 4) or without activity coefficients
Since in our system all organic molecules are about the same size, the activity coefficient is mainly determined by the interaction part
Summary
Oxidation of complex VOC molecules (e.g. terpenes, aromatics) leads to a myriad of compounds, many of which having sufficiently low saturation vapor pressures pi0 in order to condense and form SOA. Water vapor will partition appreciably to the SOA phase, notwithstanding its high saturation vapor pressure. By its very nature, SOA is a complex mixture of water and organic molecules. The partitioning of a gas to a mixture is determined by the product γipi0 rather than by its saturation vapor pressure pi0, where γi is the composition dependent activity coefficient. Including the effect of non-ideality means calculating the activity coefficient explicitly instead of setting each γi=1. UNIFAC (UNIversal Functional group Activity Coefficient) (Fredenslund et al, 1975) is arguably the most popular method to calculate activity coefficients
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
