[1] We present BOREAM (Biogenic hydrocarbon Oxidation and Related Aerosol formation Model), a detailed model for the oxidation of a-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 a-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 NO 3 -initiated oxidation is negligible. In agreement with previous studies, low NO x conditions and low temperatures are calculated to favor aerosol formation, but the estimated temperature dependence is stronger than found in recent laboratory experiments.

Abstract.A prediction method based on group contribution principles is proposed for estimating the vapour pressure of α-pinene oxidation products. Temperature dependent contributions are provided for the following chemical groups: carbonyl, nitrate, hydroxy, hydroperoxy, acyl peroxy nitrate and carboxy. On the basis of observed vapour pressure differences between isomers of diols and dinitrates, a simple refinement is introduced in the method to account for the influence of substitutions on the vapour pressure for alcohols and nitrates. The vapour pressures predicted with this new method have been compared with the predictions from UNI-FAC (Asher et al., 2002). Given the large uncertainties of the vapour pressure data for the least volatile compounds, further experimental studies of subcooled vapour pressures of multifunctional compounds at ambient temperatures are required for better parameterizations. Among the α-pinene products identified to date, pinic acid and hydroxy pinonic acid are predicted to be the least volatile compounds, with estimated vapour pressures of 3×10 −6 torr and 6×10 −7 torr, respectively. The vapour pressure of the other primary products range from 10 −5 to 10 −3 torr, with hydroxy hydroperoxides presenting the lowest values. Noting that multifunctional carboxylic acids, in particular pinic acid, are believed to be mostly present as dimers in laboratory conditions, we suggest that the partial vapour pressure of the pinic acid dimer should be close to the experimental subcooled vapour pressure for pinic acid (estimated at ∼10 −6 torr) due to its large contribution to the total concentration (dimer+monomer) in experimental conditions.

Abstract. This paper presents a state-of-the-art gas-phase mechanism for the degradation of α-pinene by OH and its validation by box model simulations of laboratory measurements. It is based on the near-explicit mechanisms for the oxidation of α-pinene and pinonaldehyde by OH proposed by Peeters and co-workers. The extensive set of α-pinene photooxidation experiments performed in presence as well as in absence of NO by Nozière et al. (1999a) is used to test the mechanism. The comparison of the calculated vs measured concentrations as a function of time shows that the levels of OH, NO, NO 2 and light are well reproduced in the model. Noting the large scatter in the experimental results as well as the difficulty to retrieve true product yields from concentrations data, a methodology is proposed for comparing the model and the data. The model succeeds in reproducing the average apparent yields of pinonaldehyde, acetone, total nitrates and total PANs in the experiments performed in presence of NO. In absence of NO, pinonaldehyde is fairly well reproduced, but acetone is largely underestimated.The dependence of the product yields on the concentration of NO and α-pinene is investigated, with a special attention on the influence of the multiple competitions of reactions affecting the peroxy radicals in the mechanism. We show that the main oxidation channels differ largely according to photochemical conditions. E.g. the pinonaldehyde yield is estimated to be about 10% in the remote atmosphere and up to 60% in very polluted areas. We stress the need for additional theoretical/laboratory work to unravel the chemistry of the primary products as well as the ozonolysis and nitrateinitiated oxidation of α-pinene.

Abstract. A prediction method based on group contribution principles is proposed for estimating the vapour pressure of α-pinene oxidation products. Temperature dependent contributions are provided for the following chemical groups: carbonyl, nitrate, hydroxy, hydroperoxide, acyl peroxy nitrate and acid. On the basis of observed vapour pressure differences between isomers of diols and dinitrates, a simple refinement is introduced in the method, which allows to account for the influence of the substitutions on the vapour pressure for the hydroxy and nitrate functionalities. In general, the predicted vapour pressures of multifunctional compounds show a better agreement with experimental data (within a factor 2–3) than the UNIFAC method (Asher et al., 2002). Among the α-pinene products identified to date, pinic acid and hydroxy pinonic acid are predicted to be the least volatile compounds, with estimated vapour pressures of 3×10−6 torr and 6×10−7 torr, respectively. The vapour pressure of the other primary products range from 10−5 to 10−3 torr, with hydroxy hydroperoxides presenting the lowest values. Noting that multifunctional carboxylic acids, in particular pinic acid, are believed to be mostly present as dimers in laboratory conditions, we suggest that the partial vapour pressure of the pinic acid dimer should be close to the experimental subcooled vapour pressure for pinic acid (estimated at ~10−6 torr) due to its large contribution to the total concentration (dimer+monomer) in experimental conditions.

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