The Oxidation Mechanisms And Kinetics Of Limonene And Two Terpene-derived Acids In Atmospheric Aqueous Phase

Secondary organic aerosols are a type of aerosol that have an impact on visibility,climate change,and human health.More and more studies have revealed that focusing solely on gasphase secondary organic aerosols（gas-SOA）is insufficient to appropriately estimate global SOA levels and to explain various aerosol features.As a result,researchers began to focus on the formation of aqueous phase secondary organic aerosols（aq-SOA）.Studying the production pathways of low-volatility products and the aqueous-phase reaction mechanism is very important to predict the formation of SOA,which can be disseminated in aerosols and even participate in the formation of new particles.In this study,we applied calculations from density functional theory to examine the transformation mechanism of limonene degradation by NO₃·in the atmospheric aqueous phase,providing a theoretical basis for the formation of nocturnal SOA and organic nitrates（ONs）.And we examine the oxidation mechanism of limononic acid（LA）with·OH,and the oxidation mechanism of β-caryophyllonic acid（BCA）with·OH and O₃ in the atmospheric aqueous phase.The reaction mechanism and reaction rate constant were studied at the base group level of M06-2X/6-311++G（3df,3dp）//M06-2X/6-311++G（d,p）.The contents and conclusions are as follows:1.Study on transformation mechanism of limonene degradation by NO₃·in atmospheric aqueous phaseNO₃·is the main oxidant in the nighttime atmosphere,and its reaction with biogenic volatile organic compounds is an important source of nitrogen-containing organic compounds in the atmosphere and a key pathway for the formation of SOA.This study mainly explores the transformation mechanism and kinetics of limonene by NO· in the atmospheric aqueous phase.helping to understand the formation of nocturnal SOA and ONs.The reaction between limonene and NO₃·presents a competitive relationship between addition and extraction reactions,and the initiation reactions are exothermic reactions that can occur spontaneously,indicating that NO₃·has a strong oxidation effect on limonene.The subsequent products will produce SOA precursors such as hydroxyl,epoxide,hydrogen peroxide products,and organic nitrates containing hydroxyl,hydrogen peroxide,and carbonyl groups,which can reduce the saturated vapor pressure and promote its reaction in the atmospheric aqueous phase or particle surface and solid phase.promoting the formation of overnight SOA.2.Oxidation mechanisms and kinetics of LA by·OH in atmospheric aquous phaseLimononic acid（3-isopropenyl-6-oxoheptanoic acid,LA）is from the oxidation of limonene in the atmosphere,which has a great contribution to the formation of SOA.Due to a large Henry’s law constant,LA can conduct oxidation reaction in the aqueous phase under the actual atmospheric conditions.However,the products and intermediates during oxidation processes of LA in the aqueous phase are not yet clear.In this study,the stable structure of LA is used to study its oxidation mechanism initiated by·OH in the atmospheric aqueous phase,and to investigate the initiation reaction constants through kinetic calculations.Then,the subsequent reaction between intermediate and other components（such as O₂,·OH.HO₂·,and H₂O）were investigated.The reaction mechanisms of different paths were discussed,and intermediate products were paid attention to.This study revealed the reaction mechanism.intermediates,or final products,which can’t be found in the previous experiment.It shed light on the oxidation processes of LA and its contribution to SOA in atmospheric aqueous.3.Study on the reaction mechanism of BCA with·OH and O₃ in atmospheric aqueous phaseBCA（β-caryophyllonic acid,BCA）is similar to LA and has a higher molecular weight than LA.It can form highly oxidized low volatile products in the atmospheric aqueous phase by breaking the main carbon chain and increasing oxygen-containing functional groups.In this study,the reaction mechanism and kinetics of BCA with·OH and O₃ were investigated by quantum chemical calculations.In addition to the termination reaction between BCA and·OH and H₂O.subsequent intermediate intermediates will also undergo a reaction with O₂ and pass through the stages of RO₂·and RO·,and finally undergo a cracking reaction to generate small molecular substances.The reaction of BCA with O₃ can generate primary ozone oxides and the Criegee Intermediates oIM2,which can undergo rich subsequent reaction processes through self isomerization,and the reaction with·OH,H₂O,and O₂.The main reaction products also include keto-BCA,as well as other small molecule SOA precursors.The entire reaction process improves the O/C ratio of SOA in the aqueous phase and generates products of small molecules such as hydroxyacetone.which plays an important role in the formation of SOA.The reaction rate constant of the initiation reaction between BCA and·OH is 2.24×10¹⁰M^-1 s^-1,and the initiation reaction rate constant with O₃ is 1.85 × 10⁵ M^-1 s^-1.This indicates that the pathway of BCA oxidation by·OH has advantages.but the atmospheric aqueous phase process of BCA oxidation by O₃ is also crucial.