Studies On The Decomposition Of Criegee Intermediates And Their Reactions With Typical Atmospheric Pollutants By Using OH LIF Technique

In recent years,haze weather has appeared in many cities in China.Air pollution has seriously affected people’s life and health,and thus has received extensive attention.Secondary Organic Aerosols(SOA)is an important cause of haze and is the main component of PM2.5(Particulate Matter).Terpene is one of the typical volatile organic compounds(VOCs),and its ozonolysis process in the atmosphere is one of the important pathways to generate SOA.As an important intermediate during the ozonolysis of alkene,Criegee intermediates(CIs)play a key role in the formation of atmospheric SOA.Meanwhile,as an important atmospheric oxidant,the reactions of CIs with SO2 and carboxylic acids contribute to the formation of atmospheric sulfate aerosols and SOA.In addition,previous studies have shown that the unimolecular reaction of syn-CIs is an important source of atmospheric OH radical,affecting the atmospheric oxidation capacity.Therefore,the kinetic studies of CIs are conducive to understand the formation mechanism of SOA and to evaluate the impact of CIs on the atmospheric oxidation capacity,and thus provide theoretical basis for air pollution control.CH2OO and syn-CH3CHOO are the CIs with the simplest structure and the most studied.Among them,syn-CH3CHOOO is the model system for studying syn-CIs.However,due to the interference of anti-CH3CHOO,it is very challenging to perform conformer-specific study and obtain the accurate experimental kinetic data of syn-CH3CHOO,leading to the failure to accurately evaluate the impact of syn-CIs on the atmospheric environment.In this paper,we firstly applied the high-repetition-rate OH laser-induced fluorescence(OH LIF)method to study the kinetics of syn-CH3CHOO reactions,and further discussed the atmospheric implications based on the experimental results.The primary research contents and results are as follows:(1)The OH(v"=0)laser-induced fluorescence(LIF)method was used to directly measure the unimolecular reaction rate coefficient of syn-CH3CHOO.The measured rate coefficient of syn-CH3CHOO+SO2 reaction is in good agreement with previously reported values,which confirms the feasibility of applying the OH LIF method to the rate coefficient measurement of syn-CH3CHOO.On this basis,the unimolecular reaction rate coefficients of syn-CH3CHOO between 10-100 Torr pressure were measured,and it was found that this reaction has already reached a high-pressure limit at 25 Torr.At 298 K,the measured unimolecular reaction rate coefficient of syn-CH3CHOO is 182±66 s-1,which is significantly higher than the previously reported results of ozonolysis of alkene,and is close to the recent theoretical result.This work confirms experimentally that the unimolecular reaction accounts for the major loss of synCH3CHOO and the OH is the primary product for the syn-CH3CHOO reaction.(2)The OH(v"=0)LIF method was applied to kinetic studies for the reaction of synCH3CHOO with a typical hydrofluoroolefins-CF3CH=CH2.At pressure of 50 Torr,the measured rate coefficient for syn-CH3CHOO+CF3CH=CH2 reaction at 283,298,308,and 318 K are(2.12 ± 0.45)× 10-14,(2.42±0.51)× 10-14,(2.63 ± 0.55)×10-14 and(2.79±0.59)× 1014 cm3 molecule-1 s-1,respectively.A weak positive temperature dependence was observed,with an activation energy of 1.41±0.12 kcal mol-1.By selectively detecting the OH(v"=1)signal,it is proved that the OH(v"≥l)generated by the unimolecular decomposition of syn-CH3CHOO is negligible and has no effect on the OH(v"=0)kinetic model and the measurement of the rate coefficient,which has reference to the future kinetic study of CIs reactions with other atmospheric pollutants by OH(v"=0)LIF method.The results suggest that the syn-CH3CHOO+CF3CH=CH2 reaction has little contribution to the direct degradation of CF3CH=CH2(τ>20 years).(3)The kinetics for the reactions of syn-CH3CHOO and CH2OO with acrolein,a typical unsaturated carbonyl compound in the atmosphere,were studied by using the OH(v"= 0)LIF method,respectively.At 298 K,the rate coefficients for CH2OO and syn-CH3CHOO re actions with CH2CHCHO are(1.63± 0.19)× 10-12 and(1.17 ± 0.16)× 10-13 cm3 molecule-1 s-1,respectively.No obvious dependence of the rate coefficients on pressure was observed for both reactions under 5-200 Torr,but both reactions exhibit negative temperature dependence at 281318 K.The obtained respective activation energies are(-1.70 ± 0.19)and(-1.47 ± 0.24)kcal mol-1 for CH2OO and syn-CH3CHOO reacting with acrolein,which suggests that the ratedetermining step of these reactions is barrierless and negative temperature-dependent.The results showed that the main reaction channel between CIs and unsaturated carbonyl compounds was C=O bond addition pathway,and the CIs reactions with acrolein have a little impact on the atmospheric degradation of CH2=CHCHO.