| Biogenic secondary organic aerosol (SOA), which has relatively strong polarity, hygroscopicity and solubility, plays an important role in optical properties, CCN activity and human health. SOA has been drawn more and more attention in recent years. Both on regional and global scales, biogenic terpenes are important precursors of SOA.In this study, using smog chamber system, SOA formation from the gas-phase ozonolysis of several typical biogenic terpene precursors (a-pinene,|3-pinene, a-cedrene,(+)-Ledene and (+)-Aromadendrene) was simulated. The number concentration, volume concentration, size distribution, optical properties, CCN activities and chemical composition were investigated under different reaction conditions. By changing the concentration of precursors, choice of OH radical scavengers (cyclohexane, ethanol and CO) and SCI scavengers (H2O, SO2, acetic acid and acetone), the effects of different reaction conditions on SOA product composition were explored. SO2, a crucial SCI scavenger, was added into reaction system of ozonolysis of sesquiterpenens to estimate SCI yield by the consumption of SO2. By quantification of the amount of H2SO4, the contribution of the reaction of SCI with SO2to atmospheric H2SO4was assessed. HRTof-CIMS, LC-MS and GC-MS were employed to measure gas-phase and particle-phase products, respectively. According to the products identified and the mechanisms of ozonolysis of terpenes reported by previous studies, possible formation mechanisms of products were deduced.The results show that the aerosol yields from sesquiterpenes are larger than monoterpenes. Adding SO2leads to the formation of sulfuric acid, which dramatically enhances the new particle formation. Since the majority (-88%) of CIs generated from the ozonolysis of a-cedrene is stabilized, the particle formation was significantly enhanced as a result of large amounts of sulfuric acid produced. Furthermore, the formation mechanism of sulfuric acid was revealed. In contrast, when H2O and CH3COOH were added as SCI scavengers, aerosol nucleation was inhibited. This suggests that SCIs play an important role in the nucleation process.From the identification of products of ozonolysis of monterpenes, it was found that aldehydes and ketones are main gas-phase products; organic acids are the dominant particle-phase products. However, in the ozonolysis of sesquiterpenes, SOZ were found to be the dominant gas-phase products, with organic acids being main particulate products. Considering formation mechanism, SOZ are generated from the unimolecule reactions; whereas organic acids are mainly formed from decomposition of CIs via hydrogen peroxide channels.In all experiments, SOA scattering coefficient increased with increasing particle size. In presence of acidic aerosol seeds, weak light absorption was observed at405nm, suggesting the production of "brown carbon" or organosulfate. From the study on CCN activity of a-cedrene SOA, we found that CCN activity increased with increasing oxidation state of SOA. Adding acetic acid significantly decreased hygroscopicity (κ values). When increasing relative humidity of reaction system, high κ value was observed. Furthermore, the hygroscopicity of particles was enhanced with the addition of SO2, due to the formation of sulfuric acid.This study revealed the characterics and formation mechanisms SOA from the biogenic VOCs. The results are important for developing relevant atmospheric chemistry models, and can further the understanding of the relationshop between plants and atmospheric pollutants, and the assessment of the direct and indirect climate impacts of biogenic aerosols. |
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