Kinetic Studies Of The Reactions Of Criegee Intermediates(CH₂OO,syn-CH₃CHOO) With Atmospheric Trace Gases

Carbonyl oxides(R1R2COO),also known as Criegee Intermediates(CIs),are highly reactive intermediates,which are produced from the reactions of unsaturated hydrocarbons with ozone in the atmosphere.The CIs formed from ozonation olefin reactions are in the state of internal vibration excitation and then would undergo prompt unimolecular decomposition or be stabilized by bath gas,forming stabilized CIs(SCIs).The stabilized CIs have a sufficient lifetime to go through slow decompose to OH,HO2 radicals,or react with atmospheric trace species.The unimolecular dissociation process of SCIs is an important source of OH radicals in the atmosphere,and the products of bimolecular reaction can form various low volatile polymers that have a great influence on the formation of secondary organic aerosols and play an important role in the atmospheric environment.At present,kinetic experiments on CIs are conducted employing photoionization mass spectrometry and UV absorption spectroscopy method.The ionization source used in the former method is the high-repetition-rate ultraviolet light source and it's difficult to satisfy the conditions.The latter detection process is easily disturbed by multiple reactants.To overcome the above limitations of experiment,a set of gas-phase experimental apparatus are independently designed and built for laser photolysis to generate free radicals and laser induced-fluorescence(LIF)method to detect free radicals,which based on the characteristics of high detection sensitivity,fast time response,and no interference to the tested area of the laser-induced fluorescence method.Using temperature and pressure control devices,the kinetic study of the reaction of free radicals with molecules can be measured at microsecond resolution in a wide temperature(278 K?320 K)and pressure(1?760 Torr)range.Based on the above device,we used the OH LIF method to carry out the kinetic studies of the cycloaddition reactions of CH2OO+SO2 and CH2OO+CH3CH2CHO at first.The reaction rate coefficient of CH2OO+SO2 at 300 K and 10 Torr was measured to be(3.88±0.13)× 10-11 cm3 s-1,which agrees well with previous results obtained by different experimental methods,and it was proved that our OH LIF method is feasible in the study of CIs chemical kinetics.Temperature-dependence of the reaction of CH2OO with CH3CH2CHO was observed to be k(T)=(1.04±0.57)×10-13 cm3 s-1 exp[(1.99±0.23)kcal mol-1/RT]by the OH LIF method.The high-pressure limit of rate coefficient of CH2OO+CH3CH2CHO was measured to be(3.23±0.49)×10-12 cm3 s-1 at 298 K and pressure higher than 50 Torr,in good accord with a previously reported theoretical value of 2.44 × 10-12 cm3 s-1.Then,we used the OH LIF method to study the insertion reactions of CH2OO+NH3 and CH2OO+(H2O)2.The negative temperature-dependence of the reaction rate coefficient of CH2OO with NH3 was observed to be k(T)=(2.76±0.87)×10-14 cm3 s-1 exp[(0.48±0.12)kcal mol-1/RT].The reaction rate coefficient of CH2OO+NH3 at 298 K and 50 Torr is(5.64±0.56)×10-14 cm3 s-1,which verified the theoretical value 5.36×10-14 cm3 s-1 from Misiewicz with perturbative correction method,and resolved the divergence between the early results using perturbative correction method and density functional theory.Owing to the difference in temperature dependence between OH+NH3 and CH2OO+NH3 reactions.CIs may play a role in reacting with NH3,especially at a higher elevation,at night-time,and in the winter months.The reaction rate coefficient of CH2OO+(H2O)2 at 300 K and 60 Torr was measured to be(7.53±0.8)×10-12 cm3 s-1,which agrees well with the results of the UV absorption method from Smith et al.,Chao et al.,and Sheps et al.Using the result from the OH LIF method,it can be calculated that the consumption rate of CH2OO by the reaction with(H2O)2 is about 400?10500 s-1,which is dominating over the loss of CH2OO in the atmosphere.Finally,because the unimolecular decomposition of syn-CH3CHOO could produce OH radicals but anti-CH3CHOO could not,the OH LIF method can optionally detect the kinetic study of syn-CH3CHOO,which can completely exclude the interference coming from anti-CH3CHOO.The rate coefficient of syn-CH3CHOO reacting with H2O was measured to be(3.47±0.94)×10-16 cm3 s-1 at 298 K and 50 Torr,which is three to five orders of magnitude larger than the existing theoretical results,and the distinct difference has proved that the reaction of syn-CH3CHOO+H2O may have a significant influence on the consumption of syn-CH3CHOO in the atmosphere.Considering the effect of(H2O)2 on the role of syn-CH3CHOO with H2O reaction system,the upper limit of the reaction rate coefficient of syn-CH3CHOO+(H2O)2 at 298 K is obtained as 1×10-12 cm3 s-1.The rate coefficient of syn-CH3CHOO with HCl was measured to be(4.77±0.72)×10-11 cm3 s-1 at 298 K,which is consistent with the computed value 3.5×10-11 cm3 s-1 by Kaito.The Arrhenius fitting to the temperature-dependent rate coefficients of syn-CH3CHOO+HCl reaction gives k(T)=(3.19±0.7)×10-13 cm3 s-1 exp[(2.98±0.12)kcal mol-1/RT].By studying the kinetic of the reaction of CIs(CH2OO,syn-CH3CHOO)with several representative atmospheric trace gases,we found that the oxidation effect of CIs on the consumption of atmospheric trace gases was lower than that of OH radicals,and the loss of CIs by the extremely high concentration of water was dominant in the atmosphere.