Mechanism Study Of Secondary Organic Aerosol Formation From Styrene And Limonene Ozonolysis

Secondary organic aerosols(SOA),formed from atmospheric oxidation of anthropogenic and biogenic volatile organic compounds(VOCs),comprise a large fraction of ambient particulate matter.Significant uncertainty exists in identifying the sources and mechanisms of SOA formation,making it difficult to understand its impact on global climate and the local air quality.Smog chamber has been a valuable tool to study the chemical mechanisms of SOA formation and to quantify SOA formation from selected VOCs in a controlled environment.However,a good understanding of the chemical reaction mechanisms involved requires both the micro-kinetic study of individual reaction and the macroscopic evaluation of the reaction mechanisms comparing with the actual atmosphere.Based on the chamber experiments,this thesis explores new unknown mechanisms using quantum chemistry calculations;and the new mechanisms have been evaluated by combining numerical simulations with chamber experiments.SOA is mainly formed from large VOCs with carbon number>6.Styrene(C8H8)and limonene(C10H16)are important VOCs emitted by anthropogenic and biological sources,respectively,and they are easily oxidized by oxidants to cause rapid degradation because of their C=C bonds.Based on the background above,the following research has been carried out:1.A FEP reaction chamber system with two reactors was designed and built.The chamber system was characterized with the key parameters of the system obtained.The result demonstrates that our smog chamber has the ability to simulate the gas-particle transformation process accurately.2.The influences of NH3 on SOA formation from ozonolysis of styrene have been investigated by chamber experiments.The chamber experiments reveal that the addition of NH3 led to significant decrease of SOA yield.The overall SOA yield decreased with the[NH3]0/[styrene]0 increasing.Gas phase reactions of CIs with aldehydes and NH3 of this reaction system were studied in detail by quantum chemistry methods.The calculated results showed that a secondary ozonide fromed through CIs with aldehydes could make important contribution to the aerosol composition.The addition of excess NH3 may compete with aldehydes,decreasing the secondary ozonide yield to some extent and thus affect the SOA formation.3.The influences of 2-butanol on SOA formation from ozonolysis of styrene have been investigated by chamber experiments.The experiment results showed that the introduction of 2-butanol results in a decrease of SOA yield.The bimolecular reaction rate constants of Criegee intermediates and aldehydes calculated by quantum chemistry calculations were applied to the zero-dimensional box model to simulate the SOA formation of styrene ozonolysis in the chamber.And it is shown that the secondary ozonide accounts for about half of the SOA yield in the absence of 2-butanol.While the introducing of 2-butanol will result in the decrease of[OH]and the increase of[HO2]/[RO2]ratio,which is proposed to be the main factor leading to the decreases of SOA yield.In addition,sensitivity analysis shows that the reaction of Criegee intermediate with aldehydes in this reaction system is dominant compared with other Criegee reactions,and the change of the reaction rate constant has little effect on the formation of SOA.4.The effect of NO2 on the SOA formation from ozonolysis of limonene has also been studied using both chamber experiments and box model simulations.Analysis of the reaction pathway and related aerosol chemical composition suggested that NO2 is not only related to the initial competition between O3 and NO3 oxidation of limonene,but also the competition between RO2 + HO2 and RO2 + NO2/NO3 following the ozonolysis of limonene.In the presence of NO2,peroxyacyl nitrates and other nitrates make a significant contribution to SOA formation.