Kinetic and Mechanism Studies for Oxidation Reactions of a Series of Cis-3-Hexenyl Esters in the Atmosphere

Author:Zhang Qi Lei

Supervisor:zhang wei jun gai yan bo

Database:Doctor

Degree Year:2018

Download:85

Pages:131

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It is now well recognized that a wide variety of volatile non-methane organic compounds are emitted into the atmosphere from vegetation.It is estimated that the world’s emissions of biogenic volatile organic compounds(BVOCs)amount to 1150 TgC/year,accounting for 90%of the annual global emissions of VOCs.BVOCs are mostly short-lived,highly chemically active unsaturated substances that easily react with atmospheric oxidizing agents such as OH,NO3,and O3 to form peroxy radicals.The further reaction of peroxy radicals on the one hand promotes the conversion of NO to NO2,resulting in the accumulation of O3 concentration.On the other hand,the generated low-volatile substances tend to form secondary organic aerosols,which may have an important impact on air quality and human health.In addition,BVOCs are important contributors to hydrocarbons in the atmosphere,and their photochemical degradation also has an important impact on the global carbon cycle.Green leaf volatiles(GLVs)are an important part of BVOCs.They mainly include a series of C5 and C6 alcohols,aldehydes,esters,etc.,which are produced in plant cells by the conversion of fatty acids,and are largely released into the atmosphere during plant growth and development or under external stimulation.As representatives of green leaf volatiles,cis-3-hexenyl esters have been widely detected in recent years.For example,cis-3-hexenyl esters is one of the major components of plant emissions,sometimes accounting for more than 40%of total emissions.In addition to cis-3-hexenyl formata,cis-3-hexenyl butyrate and cis-3-hexenyl isovalerate are also considered to contribute significantly to total BVOCs emissions.The research content of this paper mainly includes the following parts:(1)A flow reactor syetem has been built.And absolute rate measurements ozonolysis reaction with a-pinene and isoprene were performed on atmospheric pressure and 298 K to characterize this system.The rate constants for reactions of O3 with four cis-3-hexenyl esters have been measured using absolute method in a flow tube reactor at 298K and atmospheric pressure.The measured rate constants(in 10-17 cm3 molecule-1 s-1)were 4.06±0.66 for cis-3-hexenyl formate,5.77±0.70 forcis-3-hexenyl acetate,7.62±0.88 for cis-3-hexenyl propionate,and 12.34±1.59 for cis-3-hexenyl butyrate,respectively.Theoretical calculations were also carried out for the title reactions to better understand their kinetics and mechanism using density functional theory(DFT)and transition state theory(TST).Geometry optimizations,energy and the harmonic vibrational frequency calculations were performed for all of the stationary points at the BHandHLYP/6-311+G(d,p)level of theory.The calculated rate constants are in good agreement with the experimental values.The results showed that the reactivity of the studied compounds towards O3 is obviously dependent on their chemical structure,such as the nature of the substituent,and the relative positions of the double bond and the substituent.(2)The gas-phase reaction rate constants of a series of cis-3-hexenyl esters with OH radicals were obtained using a relative rate method in a chamber at 296±2 K and an atmospheric pressure,The measured rate constants(in 10-11 cm3 molecule-1 s-1)were 7.41 ± 0.74 for cis-3-hexenyl formate,8.84 ± 0.29 for cis-3-hexenyl acetate,9.84±0.96 for cis-3-hexenyl propionate,and 10.16 ± 0.69 for cis-3-hexenyl butyrate,respectively.The measured rate constants for cis-3-hexenyl formate and cis-3-hexenyl acetate with OH were consistent with the literature values within the error range.There are no other reports on the other two reactions.Reaction products were determined using PTR-MS,and common contaminants such as formaldehyde,acetaldehyde,and propionaldehyde were mainly found in all studied reactions.(3)The gas-phase reaction rate constants of cis-3-hexenyl formate,cis-3-hexenyl acetate and cis-3-hexenyl propionate with Cl atom were obtained using the relative rate method in a chamber at 296±2 K and an atmospheric pressure.The measured rate constants(in 10-10 cm3 molecule-1 s-1)were(2.42±0.11),(2.76±0.25),(4.08±0.25),respectively.To the best of our knowledge,the reaction rate constants for cis-3-hexenyl acetate and cis-3-hexenyl propionate with Cl atom are reported for the first time.(4)Finally,the rate constants measured were used to evaluate the relative atmospheric importance of these reactions in the degradation of these unsaturated esters by comparing their lifetimes.The results showed that reactions with OH,NO3 radicals and O3 are the main degradation process of these esters in the atmosphere.OH radicals play a major role in the daytime,while NO3 plays a leading role at night.O3 is involved in the degradation of these esters during both the day and night.In coastal areas or polluted areas with high Cl concentration,reaction with Cl atoms may also be important.