Modeling aerosol formation in alpha-pinene photo-oxidation experiments

We present BOREAM ( Biogenic hydrocarbon Oxidation and Related Aerosol formation Model), a detailed model for the oxidation of alpha-pinene and the resulting formation of secondary organic aerosol (SOA). It is based on a quasi-explicit gas phase mechanism for the formation of primary products, developed on objective grounds using advanced theoretical methods, and on a simplified representation for the further oxidation of the products. The partitioning of the products follows a kinetic representation with coefficients estimated from vapor pressures calculated using a dedicated group contribution method. Particle phase and heterogeneous reactions are generally neglected, but the impact of peroxyhemiacetal formation in the aerosol is tested on the basis of laboratory estimates of the reaction rates. The model is evaluated against 28 laboratory experiments from 6 studies of alpha-pinene photo-oxidation covering a wide range of photochemical conditions. In contrast with previous modeling studies, the modeled and measured SOA yields agree to within a factor of 2 in most cases. The SOA yields are underestimated for the ozonolysis experiments of Presto et al. (2005a) when the standard version of the ozonolysis mechanism is used, presumably because of the lack of credible pathways for the formation of pinic and hydroxy pinonic acid. The underestimation is drastically reduced when the mechanism is modified to account for the formation of these compounds. Accounting for peroxyhemiacetal formation in the particle phase is found to further increase the SOA yields by about one third in high VOC ozonolysis experiments and to have a much smaller impact in all other cases. The model calculates that ozonolysis contributes about twice more to SOA formation than oxidation by OH, whereas NO3-initiated oxidation is negligible. In agreement with previous studies, low NOx conditions and low temperatures are calculated to favor aerosol formation, but the estimated temperature dependence is stronger than found in recent laboratory experiments.