Quantum Chemical Study On Atmospheric Transformation Pathways And Kinetics Of Volatile Amines

Ambient volatile organic compounds(VOCs)are closely related to the ozone(O3),toxic conversion products and secondary organic aerosol(SOA)pollution.Amines are an important type of VOCs.There are more than 150 kinds of amines have been detected in the atmosphere,which are emitted from a variety of natural and anthropogenic sources.In addition,postcombustion CO2 capture(PCCC)units will become an important source of anthropogenic amines in the atmosphere.Revealing the transformation behaviors of these amines in the atmosphere and their potential impacts on the SOA formation are significant for the prevention and control of air pollution by amines.In this study,quantum chemical calculations and kinetics modeling were employed to investigate the atmospheric transformation pathways and kinetics of amines by selecting the typical amines in PCCC units as model compounds.Furthermore,the contribution of atmospheric oxidation by radicals and participation in sulfuric acid nucleation to the transformation of amines were discussed.Main research contents and results are as follows:(1)Atmospheric oxidation pathways and kinetics of monoethanolamine(MEA)initiated by Cl were revealed.MEA is the most promising chemicals in amine-based PCCC technology.·Cl is a strong oxidizing radical in the atmosphere,which has received extensive attention in recent years.Hence,·Cl initiated reactions with MEA were investigated by CCSD(T)/aug-ccpVTZ//MP2/6-31+G(3df,2p)method and kinetics modeling.Results showed that ·Cl reaction with MEA mainly leads to the formation of N-centered radicals,and finally transform into several products including carcinogenic nitrosamine/nitramine via further reaction with O2/NOx in their subsequent reactions.Combining the reaction rate constants initiated by ·Cl and ·OH,the concentration of Cl and ·OH and the N-centered radicals yields,the contribution of ·Cl to the transformation of MEA and the carcinogenic nitrosamine/nitramine formation is about 5～50%and 25～250%of that of ·OH,respectively.If·Cl is not considered,the environmental risks of MEA will be underestimated.This research has theoretical significance and practical value for accurately assessing the fate and environmental risks of MEA.(2)Reaction pathways and kinetics of the oxidation of cyclic diamine piperazine by ·Cl were clarified.Piperazine is an alternative to MEA,and its concentration level in the atmosphere is ppt.Using CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p)method and kinetics modeling,it was found that ·Cl reactions with piperazine mainly leads to N-centered radicals,that mainly react with NO to produce nitrosamine in their further reactions.Together with the reactions of ·OH with piperazine,the yield of N-centered radicals from piperazine oxidation initiated by ·OH and ·Cl is lower than that of the corresponding MEA reactions.However,the nitrosamine yield of piperazine is higher than that of corresponding MEA when the concentration of NO is lower than 5 ppb.The results indicate that the carcinogenic nitrosamines yields cannot be assessed based solely on the N-centered radicals yield,and further consideration needs to be given to the subsequent competitive reactions of N-centered radicals with O2/NO.This research is helpful to understand the fate and en vironmental risks of different amines.(3)Contribution of piperazine(PZ)participating in sulfuric acid(SA)nucleation to the transformation of PZ were evaluated.By employing DLPNO-CCSD(T)/aug-ccpVTZ//coB97X-D/6-31++G(d,p)method and Atmospheric Cluster Dynamics model,the formation pathways and kinetics of(PZ)x(SA)y(x=0-4,y=0-4)clusters were studied.Results showed that PZ plays a similar role to monoamine in stabilizing SA and can enhance the SA nucleation at the ppt level.The potential of PZ to promote SA nucleation is stronger than that of MEA and dimethylamine,which is considered to be one of the strongest agents to enhance SA nucleation.At 278.15 K,the participation in SA nucleation contributes 50%～97%to the total removal of PZ.In addition,the reevaluating risk of nitrosamine formation of PZ and the relative risk of nitrosamine formation of PZ to MEA is greatly decreased.This research has theoretical significance for understanding the contribution of amines in the formation of SOA.(4)Atmospheric oxidation pathways and kinetics of N-containing peroxy radicals(NRO2·)formed from atmospheric oxidation of trimethylamine(TMA)and triethylamine(TEA)were clarified.TMA and TEA may be released in the PCCC units,and their atmospheric concentration is on the order of ppt to ppb level.Using UCBS-QB3//M05/aug-cc-pVTZ method and kinetics modeling,it was found that both NRO2· formed from α-C centered radicals of TMA and TEA oxidation can undergo autoxidation mechanism(a chain reaction of H-shift followed by O2 addition),even on the high NO/HO2· concentration conditions.However,different from TMA,the pathway dissociating into fragmental products can compete with the autoxidation pathway for TEA system.More importantly,TEA reaction system cannot lead to the formation of products with high O/C ratio.In addition,it is initially revealed that the NRO2·chemistry also depends on the type of C-centered radicals formed in the initial reactions of tertiary amines.This research helps to enrich the understanding of atmospheric fate of amines.In summary,based on quantum chemical calculations and kinetic modeling,this study reveals the transformation pathways and kinetics of’ representative amines in the atmosphere,their potential contribution to the SOA formation and toxic conversion products.The results will help to further understand the atmospheric behaviors and risks of related pollutants,and promote the development of the discipline of atmospheric environmental chemistry.