Modeling of Secondary Organic Aerosol partitioning for the Isoprene system at Low and High RH
The secondary organic aerosol (SOA) produced from isoprene oxidation by ozone with and without inorganic seed is modelled using an equilibrium partitioning model based on AIOMFAC and EVAPORATION. The model results are compared with measurements at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber. For the model we used experimental data on SOA yields and molecular composition of first generation products from isoprene oxidation under conditions comparable to those of the experiments. The model uses surrogate molecular species such as monomers and dimers to represent the SOA formed. Experimental SOA mass concentration and levels of isoprene reacted were used to determine branching ratios for the model predictions. The branching ratios are adjustable parameters used to estimate the percentage of semi-volatile or low volatility monomer and dimer compounds formed from compounds predicted by the Master Chemical Mechanism. Model results for the yield and mass concentration are compared with the experiments for cases with and without Ammonium sulfate seed. The goal is to study the hygroscopic growth and partitioning of aerosols at varying RH for both seeded and non-seeded cases. To match the SOA mass yield and mass concentration calculated by the model with the experimental data for the seeded case, the branching ratios of the monomers needed to be adjusted. Results and possible reasons for the discrepancies will be discussed.