Effect Of Water Molecules On The Mechanism Of Atmospheric Reaction Involving Reactive Halogen Species

Reactive halogen species(RHS),including X.?X2?XY?XO?OXOO?HOX?XONO2,(X?Y denotes halogen atom)are involved in many atmospheric chemical processes,affecting the source and sink of many significant species.RHS play an important role in tropospheric chemistry,especially in ozone depletion.CIO and BrO derived from halon,methyl bromide and other anthropogenic sources are the most concentrated RHS in the atmosphere,which can cause damage to the ozone layer through the formation and photolysis of dimers.However,recent studies have shown that the chemical cycle of dimer formation and photolysis alone cannot explain the O3 depletion induced by chlorine catalysis completely.RHS may react with other reactive molecules and radicals,thus determining the fate of RHS in the atmosphere.Water is ubiquitous in the troposphere in the form of water vapor,liquid phase(clouds and fogs),ice as well as aerosol.It is potentially important in affecting the transformation and transport of atmospheric pollutants.Having the ability to act as both a hydrogen bond donor and acceptor,water is known to form hydrogen-bonded complexes with other radicals and polar moleucles in the gas phase,thus changing the reaction barrier,affecting the reaction kinetics,and even fundamentally changing the Cl/Br radical cycle.Therefore,it is essential to explore the influence of hydrate formation on the mechanism and kinetics of RHS reaction and the key role of hydrate in ozone depletion.To investigate the effect of water and water cluster on the RHS gas phase reaction,the detailed analysis of the reaction between CIO and important atmospheric species(HCHO/HO2)in the presence of water and water dimer were car:ried out from mechanism and kinetics points of view using quantum chemical calculations.The calculations were performed at the CCSD(T)/aug-cc-pVTZ//B3LYP-D3/aug-cc-pVTZ level.For the reaction between HO2 and CIO,four main channels:starting from HO2…H2O + ClO,H2O…HO2+ ClO,ClO…H2O + HO2,H2O…ClO + HO2 in the presence of a single water molecule were investigated.The path starting from H2O…HO2 + ClO was found to be the most favorable channel due to the lowest barrier height and the highest concentration of H2O…HO2.HOCl and O2 can be formed both without and with barrier,among which the barrierless path is efficient.Results show that HOCl + O2 can not be formed barrierlessly in the presence of water,and the barrier height for all the paths are increased.For HCHO + ClO,the reaction proceeds through four different paths without water and eleven paths with water,producing H + HCO(O)Cl,Cl + HC(O)OH,HCOO + HCl,and HCO+ HOC1.Results indicate that the formation of HCO + HOCl is predominant both in the presence and in the absence of water.In the absence of water,all the reaction paths proceed through formation of a transition state configuration,while for some reactions in the presence water,the products were directly formed via barrierless proton transferThe rate constant of each elementary process was determined using harmonic transition state theory and assuming pseudo-steady state approximation on the pre-reactive complex.The rate constants for the two reactions show positive temperature dependence.Differently,for the HO2 + ClO reaction,even the rate constants for the most favorable path are 6-10 orders of magnitude slower than the reaction in the absence of water.While for the reaction between HCHO and ClO,the rate constant for the formation of HCO + HOCI in the absence of water is 2.62 × 10-16 cm3 molecule-1 s-1 at 298.15 K In the presence of water,the rate constants for the barrierless paths towards formation of HCO + HOCl are 6-8 orders of magnitude higher than that of the reaction without water The current study demonstrates that atmospheric water retards the reaction of HO2 with ClO under atmospheric conditions,while the HCHO + ClO reaction is effectively catalyzed by water and water cluster.It is found that atmospheric water will affect the rate and degree of ozone depletion by affecting the RHS gas phase reaction activity.The present theoretical prediction may supplement the laboratory and field observations of RHS hydrate and lead to a better understanding of ozone depletion mechanism involving water catalysis.