It has all basic aerosol processes, including nucleation, condensation (Bilde et al., 2015), coagulation and deposition. The model has been under constant development to include latest knowledge in secondary aerosol formation (Vuollokoski et al., 2010a and b, Hermansson et al., 2014).
In the model, particles are assumed to be spherical and to consist initially of sulphuric acid, water and organic compounds. The inputs include an initial size distribution, particle composition, ambient temperature, relative humidity and precursor gas concentrations. The condensation of an "unlimited" number of organic compounds is simulated by first estimating the pure liquid saturation vapour pressures of all oxidation products using the group contribution method described by Nannoolal et al. (2008) and the group contribution method SIMPOL (Pankow and Asher, 2008). By applying both of these two methods, we can evaluate how sensitive the modelled SOA formation is to the estimated saturation vapour pressures, which can vary orders of magnitudes depending on which method is used. The corresponding equilibrium vapour pressures for each particle size bin are derived with Raoult’s law, corrected for non-ideal mixing using activity coefficients calculated by the AIOMFAC thermodynamic model (Zuend et al., 2011), and the Kelvin effect. The densities of the condensing pure organic compounds are estimated based on their molecular mass and atomic volumes, taking into consideration the changes in volume due to intramolecular bonding (Girolami, 1994). The molecular diffusion coefficients of the vapours are based on a method described in Jacobson (2005). An exact estimate for the surface tension of the organic compounds is one of the most challenging topics and in previous studies we assumed a constant value of 0.05Nm−1 following Riipinen et al. (2010), which is a too simple approach and will be further investigated.
References (Publications from our group are highlighted in bold)