First Principles Studies On Complex Chemcial Reactions In Severely Polluted Atmosphere

In the last decades,the haze events frequently occur in northern part and eastern coastal areas of China,which poses great threats to environment and human health,such as the damage to respiratory and cardiovascular systems.The formation of haze is closely related to air pollution,and the problem of air pollution is widely concerned by the public.Based on the originality,the atmospheric pollutants can be divided into the primary pollutant and the secondary pollutants.Sulfur dioxide(SO2),nitrogen oxides(NOx)and volatile organic compounds(VOCs)are the main components of primary pollutants in the atmosphere,mainly from the combustion of fossil fuels,motor vehicle exhaust and industrial emissions.Through a series of complex physical and chemical processes,these pollutants can secondarily generate sulfate,nitrate and organic aerosol particles.The vapor pressure of these substances is relatively low,and they can easily gather and from the atmospheric aerosols,which are the main precursors of haze formation.In the study of haze chemistry,it is of great significance to study the molecular formation mechanism of secondary pollutants such as sulfate and the oxidation mechanism of organic matter to explore the causes of haze.In this dissertation,high-level quantum mechanical(QM)calculations combined with Born-Oppenheimer molecular dynamic(BOMD)simulations are used to study the mechanism of complex atmospheric chemical reactions related to the formation of haze.The results can provide theoretical support for the study of the causes of haze,the establishment for air quality model and the governance of atmospheric environment.The main conclusions are as follows:1.Based on high-level QM calculations and BOMD simulations,we proposed a possible unaccounted source of atmospheric sulfate formation,that is amine-promoted hydrolysis and non-radical oxidation of SO2.We found that amines,especially dimethylamine(DMA),can act as catalysts in the conversion of atmospheric SO2 to sulfate.In the presence of DMA,the endothermic and kinetically unfavorable hydrolysis reaction of gaseous SO2 and water vapor could become both exothermic and kinetically favorable.The hydrolysis product,bisulfate NH2(CH3)2+·HSO3-could be further oxidized by ozone(O3)under ambient conditions.Kinetic analysis suggested that the hydrolysis rate of SO2 assisted by DMA became comparable to the rate of the reaction between SO2 and OH· radical under the conditions of severely polluted air with high humidity.And the oxidants NOx acted as a much less significant role than O3 in the oxidation reaction of sulfite.The newly proposed oxidation mechanism of SO2 assisted by DMA and O3 could shed new light on the formation of sulfate in the atmosphere.2.The formation mechanism of atmospheric nitrated phenolic compounds(NPs)from o-cresol initiated by NO3-radical was systematically studied using high-level QM calculations.It was found all steps in nitration pathway of o-cresol could be easily overcome at the room temperature,except for a rate-determined step of Hydrogen-shift process with high energy barrier.However,water monomer(H2O),water dimer molecule((H2O)2),ammonia(NH3)and DMA,can drastically lower the activation energy barrier of the H-shift process.Among these molecular catalysts,DMA and(H2O)2 are more efficient than H2O and NH3 to promote the formation of NPs.Based on transition state theory(TST)analysis and the observed concentration of the participating atmospheric species,the estimated formation rate of methly-nitrophenol can reach?103 molecules·cm-3·s-1 at 298 K.The estimated saturated vapor pressure of NPs suggested that these compounds were mainly IVOCs and SVOCs,which meant that they were able to take part in the growth stage of aerosol particles.In addition,saltification between NPs and NH3/DMA can further reduce their vapor pressure and increase their contribution to participate in new particle growth.3.The oxidation mechanism of NPs initiated by OH· radical was investigated using high-level QM calculations.There were mainly two kinds of products in the oxidation of 2M6NP,one was small molecular fragments produced through ring-breakage reaction,the other was the peroxyacyl nitrates with higher oxidation state.Furthermore,nitrite molecule(HONO)was released during the oxidation of 2M6NP,which was a potential source of atmospheric HONO pollutants.Based on the empirical model,we estimated the saturated vapor pressure of the peroxyacyl nitrates.The results showed the volatility was greatly reduced after oxidation of 2M6NP,from IVOCs to LVOCs,which meant the peroxyacyl nitrates might participate in the formation of aerosol particles in the atmosphere,especially the growth process.In conclusion,we systematically explored the formation mechanism of sulfate and NPs and the oxidation mechanism of NPs in the atmosphere combined QM calculations and BOMD simulations.We believed that our findings would be helpful to understand the source of sulfate,the formation of and oxidation of NPs and the role of DMA in atmospheric chemistry.