Theoretical Studies On Atmospheric Species Involved Physicochemical Processes In The Gas Phase And At The Air-Water Interface

Physicochemical processes at the air-water interface are important in atmospheric chemistry.Due to the unique hydrogen bond environment,chemical processes at the air-water interface are obviously different from those in the gas phase.Relevant research has potential applications in atmospheric and environmental sciences,biology,and synthetic organic chemistry.Experiments have shown that many thermochemical and photochemical reactions can be significantly accelerated at the interface.However,the underlying microscopic mechanisms are still largely unknown.Therefore,it is desirable to theoretically study the physical and chemical processes at the water-air interface.In this dissertation,we use density functional theory,molecular dynamics simulation and quantum chemical calculation methods to study reaction mechanisms of some important compounds in the atmosphere.The first chapter mainly introduces the research background of gas-water interface.Aerosol widely exists in the atmosphere,and its large specific surface area plays an important role in atmospheric reaction.The physical and chemical process of gas-water interface is far different from the traditional gas-phase and liquid-phase reaction,and the surface structure of water has an important influence on the reaction.At the same time,the research progress and frontier problems of gas-water interface are also introduced.In the second chapter,we mainly introduce density functional theory,molecular dynamics technique,as well as the research progress of air-water interface atmospheric chemistry.In the first part,we briefly introduce the framework and development of density functional theory.Then,we briefly introduce the theoretical background of molecular dynamics(from classical molecular dynamics to first-principles molecular dynamics).In the third chapter,we focus on the reaction between Criegee Intermediates(CIs)and alcohol in gas phase and at the air-water interface.CIs are the products of olefins oxidation by ozone.Their monomolecular decomposition can produce OH radical,which affects atmospheric oxidation ability.At the same time,CIs can react with common species in the atmosphere(H2O,SO2,NO2,inorganic acid,organic acids,etc.)to produce different products.Some experiments show that alcohols are abundant in the atmosphere of tropical regions,and a large amount of α-alkoxyalkyl hydroperoxides(AAAHs)produced by the reaction between alcohol and CIs were detected,with a yield of about 30 Gg/year.We explored the reaction mechanism of alcohol and CIs at the air-water interface through ab initio molecular dynamics(AIMD)simulations and quantum chemical calculations.In the gas phase,the anti-CH3CHOO is more active than syn-CH3CHOO in the bi-molecular reaction with alcohol.When a water molecule is involved,a loop structure will be formed.Water molecules play a key role in the process of proton transfer,so atmospheric relative humidity has a significant impact on this reaction.We also studied the reaction mechanism of alcohol and CH3CHOO on the aerosol surface.According to the statistics of 100 AIMD trajectories,the loop reaction mechanism involving two water molecules accounted for 37%,which was the dominant process,and no direct bi-molecular reaction between CIs and alcohol was found.The time scales of all kinds of reactions are on the order of picoseconds,which is much faster than the effective rate constant of the gas phase.This also explains the high content of AAAHs in tropical areas,especially in tropical rainforests.In the fourth chapter,we mainly introduce the reaction mechanism of direct oxidation of hydrated SO2 by CIs to produce sulfuric acid.Oxidation of SO2 by CIs is one of the main ways to generate acid rain,and the recognized reaction mechanism has been verified experimentally and theoretically.CIs react with SO2 to form secondary ozonides(SOZ)ring compound firstly,which generates aldehydes and SO3(ringbreaking step)after decomposition,and SO3 hydrolyzes to produce sulfuric acid.However,S4+exists in various forms in atmospheric environment(SO2,HSO3-,H2SO3,SO32-),so it is necessary to consider the reaction path comprehensively.Meanwhile,the terminal oxygen atom of CIs is easy to be removed,which is greatly affected by substituents.So it is possible that CIs directly oxidise H2SO3/HSO3-/SO32-to produce sulfuric acid.Therefore,we study the reaction processes between 22 CIs and H2SO3/HSO3-/SO32-.It was found that substituents had an essential effect on the reaction.When substituents conjugated with Criegee’s C=O bond,the distance of O-O bond would be significantly extended,so that the end oxygen could be easily removed and directly combined with sulfur atoms of H2SO3 to form sulfuric acid.The minimum reaction energy barrier was as low as 2 kcal/mol which is smaller than the traditional model.So these new paths of sulfuric acid formation cannot be ignored.The ratio of C=O/O-O bond length can be used as a descriptor to calibrate such reactivity,and reasonable regulation of CIs substituents can have a certain effect on sulfuric acid in the atmosphere.In the fifth chapter,new mechanisms of the formation of ONSO3-in aerosols via reactions between NO2 dimer(ONONO2)and SO32-/HSO3-are introduced.The generation of sulfuric acid in aerosols is sensitive to the concentration of NO2.Since NO2 often exists in the form of dimer(N2O4,ONONO2)in the atmosphere,asymmetric dimer(ONONO2)is considered to have higher reactivity,and its hydrolysis to produce HNO3 and HONO is an important reaction in the atmosphere.We explored the reactivity of ONONO2 towards SO32-/HSO3-,and a new reaction mechanism was revealed.In the gas phase,the reaction between ONONO2 and SO32-/HSO3-is energy barrierless.In aerosols,water molecules participate in proton transfer in reaction between ONONO2 and HSO3-,which is accompanied with a charge transfer.The reaction between ONONO2 and SO3-is more direct.Compared with the previous hypothesis(HONO oxidizes HSO3-to produce ONSO3-),our proposed new path reveals the source of ONSO3-.Experimentally,ONSO3-is considered to be a key intermediate for the generation of greenhouse gas(N2O),through which HS04-and HNO are produced by hydrolysis of ONSO3-,and then HNO is dimerized to form N2O.We continue to explore the possible subsequent reaction process for ONSO3-.We find that the hydrolysis process of ONSO3-to produce HNO and HSO4-is not favorable in terms of free energy,especially in aerosols.So whether it is a key intermediate to generate N2O remains to be studied.At the same time,we found that ONSO3-hemolysis can produce NO and SO3-with a reaction energy barrier of 13.4 kcal/mol,which is likely to be the fate of ONSO3-.NO was oxidized in the environment when it was released.And SO3-will combine with another NO2 to produce NO2SO3-which undergos hydrolysis in aerosols to produce sulfuric acid and HONO again.This channel is undoubtedly a new sulfuric acid generation path.In the sixth chapter,reactions among hypohaloic acid(HOX),nitrogen halogen compound(XONO2),and hydrogen halide(HX)at the air-water interface are introduced.HOX and XONO2 are important halogen compounds in the atmosphere,which play a key role in the storage and circulation of atmospheric halogens.Recent experiments have shown that HOI and IONO2 react with halogens on aerosol surface to produce bihalogens(XY)at a fast rate.However,the reaction mechanism is still larglely unknown.So we explore the reaction mechanism through AIMD simulation.Both HOX and XONO2 form weak halogen bonds with HY,but form strong halogen bond with halogen ions.So,HY tends to dissociate firstly and then form halogen bonds with HOX and XONO2.Compitition bwteen halogen bonds and hydrogen bonds leads to a variety of binding modes of HOX and HY.HOX and HY form a ring structure with water molecules connected by hydrogen and halogen bonds,which can rapidly transfer protons to produce dihalogen XY.Single hydrogen bond interactions directly transfer protons to produce[H2O…X…OH2]+complexes.The time scales of the above reactions are on the order of picoseconds,which also indicates that the iodine cycle in the atmosphere on the aerosol surface to form XY is quick.Our AIMD simulations show that the di-halogen can not form halogen bond with a single water molecule in the gas phase,but it is possible on aerosol surface.This will change the XY charge distribution and X-Y bond length.And it can also strengthen the reactive adsorption of XY on aerosol surface and speed up the photolysis rate of X Y.