Influence of non-ideality on condensation to aerosol
Secondary organic aerosol (SOA) is a complex mixture of water and\norganic molecules. Its composition is determined by the presence\nof semi-volatile or non-volatile compounds, their saturation vapor\npressure and activity coefficient. The activity coefficient is a\nnon-ideality effect and is a complex function of SOA composition.\nIn a previous publication, the detailed chemical mechanism (DCM)\nfor α-pinene oxidation and subsequent aerosol formation BOREAM was\npresented. In this work, we investigate with this DCM the impact\nof non-ideality by simulating smog chamber experiments for α-pinene\ndegradation and aerosol formation and taking the activity coefficient\ninto account of all molecules in the aerosol phase. Several versions\nof the UNIFAC method are tested for this purpose, and missing parameters\nfor e.g. hydroperoxides and nitrates are inferred from fittings to\nactivity coefficient data generated using the SPARC model. Alternative\napproaches to deal with these missing parameters are also tested,\nas well as an activity coefficient calculation method based on Hansen\nsolubility parameters (HSP). It turns out that for most experiments,\nnon-ideality has only a limited impact on the interaction between\nthe organic molecules, and therefore on SOA yields and composition,\nwhen water uptake is ignored. The reason is that often, the activity\ncoefficient is on average close to 1 and, specifically for high-VOC\nexperiments, partitioning is not very sensitive on the activity coefficient\nbecause the equilibrium is shifted strongly towards condensation.\nStill, for ozonolysis experiments with low amounts of volatile organic\ncarbon (low-VOC), the UNIFAC parameterization of Raatikainen et al.\nleads to significantly higher SOA yields (by up to a factor 1.6)\ncompared to the ideal case and to other parameterizations. Water\nuptake is model dependent, in the order: ideal > UNIFAC-Raatikainen\n> UNIFAC-Peng > UNIFAC-Hansen ≈ UNIFAC-Magnussen ≈ UNIFAC-Ming. In\nthe absence of salt dissolution, phase splitting from pure SOA is\nunlikely.