Theory Study On Mechanism Of O₃-initiated Atmospheric Photooxidation Of Terpenes

Terpenes are a group of closely related compounds with isoprene as their basic structural unit including isoprene polymers and their derivatives and are the world's largest emitter of biogenic non-methane hydrocarbons.Because a majority of biogenic VOCs contain more than one double bonds,they are believed to be the important precursors of photochemical ozonization and formation of secondary organic aerosols(SOA).They have high reactivity with major atmospheric oxidants, ozone(O₃),hydroxyl radical(OH),and nitrate radical(NO₃)in aerosphere,and products are typically multifunctional photochemical oxidants containing aldehyde, ketone,organic acid,and alcohol moieties.Most of obtained products in the atmosphere are oxygenous non-volatile and semi-volatile compounds and can be transformed into secondary organic aerosols by physical and chemical adsorption. Because of the small size,long average lifetime and adverse environment effects of SOA,they not only degrade visibility,change global climate,but are a severe threaten to human.Ozonolysis of terpenes is one of the most important processes for secondary organic aerosol formation.A series of quantum chemistical calculations based on density functional theory have been carried out to study several different monoterpenes(limonene,linalool, ocimene and myrcene).All results were obtained at the UB3LYP/6-311+G(3df,2p)//UB3LYP/6-31G^**level using Gaussian O3 program. Major valuable results are listed as follows:1.The information of the stationary points including the reactants, intermediates,transition states and products is calculated which is useful to further understand the chemical formation process of SOAs.The adducts of limonene with O₃ in internal- or external-ring have different energy,and the latter has a higher barrier than the former one.It is natural that the adduct in internal-ring takes a major proportion.The adduct of linalool with O₃ in＞C=C(CH₃)₂ takes a major proportion. Ocimene and myrcene are as same as linalool,which agree with the experimental results. 2.On the basis of direct kinetics calculations of limonene ozonolysis using Transition State Theory(TST)and Canonical Variational Transition State Theory (CVT)with Zero-Curvature Tunneling Correction(ZCT)and Small-Curvature Tunneling Correction Method(SCT),we come to the conclusion that the variational effect is significant for every elementary reaction and the effect grows stronger with temperature increases.However,tunneling effect is unnoticeable and decreases with temperature rise.3.In the subsequent reaction,H₂O molecule acts as an activator of OH transfer in some elementary reaction,which promotes the chemical process fast.The O abstraction reactions from peroxy hydrocarbons to NO molecule are favorable in both thermodynamics and dynamics.4.Several furan derivatives are formed in the reaction of linalool with O₃, which is in agreement with the experimental results.5.According to the Gibbs free energy of the elementary reaction,the oxidation process takes spontaneously once initiated by O₃.The obtained products have high polarity and water-solubility which contribute to the formation of SOA through the hydration or absorption reaction on aerosol surface.