Theoretical Study On Air-Water Interface Reaction Mechanisms Between Typical Atmospheric Pollutants And Cl/ClO Radicals In Marine Boundary Layer

In addition to making up 70%of the earth’s surface,the marine and its sprays,droplets,and aerosols provide good environmental media for reactions at the air-water interface.The airwater interface has unique interfacial properties,which can reduce the reaction barrier,accelerate the reaction rate and even change the existing homogeneous atmospheric chemical reaction mechanism.It provides a new reaction site for atmospheric chemical reactions,which is important to understand the heterogeneous process of atmospheric chemical reactions.Chlorine(Cl)and chlorine monoxide(ClO)radicals are typical halogen radicals with high concentration in the marine boundary layer(MBL)and contribute significantly to the atmospheric oxidizing ability of the MBL.However,due to the limitations of many factors,such as the variety of pollutants,the lack of standards in the experiment,and the challenge in the experimental detection of small molecular clusters,the oxidation mechanism of typical atmospheric pollutants at the air-water interface is still unclear,influencing the precise control strategies formulation of pollutants and secondary particulates pollution.In recent years,ab initio molecular dynamics(AIMD)simulation has played a pivotal role in the study of air-water interface reactions.AIMD can observe reactions on short-time scales and provide details of the reactions,which can help better understand the influence of the air-water interface on atmospheric chemical processes.At present,studies on the air-water interface behavior of Cl/CIO radicals and the mechanism of the reaction between Cl/ClO radicals and atmospheric pollutants are still lacking.In this study,the air-water interface behavior of Cl/ClO radicals and the reaction mechanisms of typical atmospheric pollutants induced by Cl/ClO radicals at the air-water interface in the MBL were studied by combining AIMD and Density Functional Theory(DFT)methods.The main contents and conclusions of this study include the following three aspects:1.Theoretical study of the interaction between Cl/CIO radicals and water in the gas-phase and air-water interfaceThis study used Born-Oppenheimer Molecular Dynamics(BOMD)to investigate interactions between Cl/ClO radicals and water molecules at the air-water interface.Radical mobility,radial distribution functions,coordination,and population analyses were conducted to investigate the surface preference,bonding pattern,and tracking Cl/CIO radicals in the water droplets.In addition,to compare the results of the air-water interface,the DFT method was adopted to study the interaction of Cl/CIO radicals with 1-6 water molecules in the gas phase.The results illustrate that Cl/ClO radicals tend to remain near the air-water interface in water droplet systems,which is consistent with that outside of water clusters in gas phase systems.ClO radical can form O*-H and Cl-O bonds with water molecules;however,neither an O*-O hemibond nor a Cl-H bond can be detected in all systems.The(H2O)0-Cl-O*-(H2O)1 is the dominant structure at the air-water interface and(H2O)1-Cl-O*-(H2O)1 is the main structure in the gas phase.Moreover,Cl radicals form no or only one Cl-O bond with water molecules at the interface and the Cl*-(H2O)1 is the dominant structure.This study comprehensively demonstrates the behavior of Cl/ClO radicals at the air-water interface and is expected to improve the understanding of the oxidizing ability and the causes of air pollution in the marine atmosphere.2.Theoretical study on the mechanisms of oxidation reactions between Cl/ClO radicals and formic acid at the air-water interfaceIn this study,the oxidation mechanism of Cl/ClO radicals with formic acid(FA)at the airwater interface was investigated by using BOMD and Constrained Molecular Dynamics(CMD)simulations.It is shown that the oxidation of FA by ClO radical follows a double-hydrogen abstraction mechanism.The abstraction of formyl hydrogen by ClO radical and hydroxyl hydrogen by water molecule occurs almost simultaneously,resulting in CO2 remaining at the air-water interface stably.The free energy barriers of the double-hydrogen abstraction process without the participation of any water bridge are nearly zero,indicating that the air-water interface can catalyze the rapidly oxidative decomposition of FA by ClO radicals.Contrary to ClO radical,FA follows the proton transfer reaction at the air-water interface in the presence of Cl radical.The hydroxyl hydrogen of FA is transferred to water molecules in the droplet to form hydronium ion,which affects the acidity of the droplet.This study can deepen the understanding of the heterogeneous oxidation mechanism of FA at the air-water interface,put forward a new mechanism of CO2 emission,and highlight the role of the air-water interface in accelerating the atmospheric chemical reaction rates,which is expected to provide accurate input parameters for carbon emission prediction models.3.Theoretical study on the mechanisms of oxidation reactions between Cl/ClO radicals and methanol at the air-water interfaceIn this work,the mechanism of methanol(MeOH)oxidation by Cl/ClO radicals at airwater interface was studied by using BOMD and CMD simulations.The results show that there are two reaction pathways for the oxidation of MeOH by ClO radical.ClO and MeOH can form methyl diol anion and hydronium ion through hydrogen abstraction followed by proton transfer,or form formaldehyde and water molecule through dihydrogen abstraction.However,Cl radical cannot oxidize MeOH at the air-water interface.The comparison shows that the oxidizing ability of ClO radical is stronger than that of Cl,and the reactivity of C-H group with radical is stronger than that of O-H group in organic molecules.This research enriches the study of heterogeneous reactions of MeOH,proposes a new source of formaldehyde and the influence of droplet acidity,and fills in the blank of the reaction mechanism between VOCs and radicals at the air-water interface for aerosol model.4.HONO formation from the oxidation reactions of ClO,NO,and water in the gas phase and at the air-water interfaceHere,BOMD and metadynamics(MTD)simulations and DFT calculation were performed to study the formation mechanism of HONO from the oxidation reactions of ClO radical and NO with the addition of water molecules.Monohydrated system((ClO)(NO)(H2O)1)and dihydrated system((ClO)(NO)(H2O)2)as well as the air-water interface system were constructed.This study shows that HONO formation follows a single-water mechanism in gasphase as well as air-water interface system.The free-energy barrier of the(ClO)(NO)(H2O)1 system was calculated and found to be 2.31 kcal mol-1,whereas the(ClO)(NO)(H2O)2 system was a barrierless reaction.HONO formation at the air-water interface is faster than that in monohydrated and dihydrated systems.Although the concentration of ClO radical in MBL is two orders higher than that of Cl radical,the production rates of HONO in the(ClO)(NO)(H2O)1 system was six orders lower than that in(Cl)(NO)(H2O)1 system,which means that Cl radicals dominate HONO formation rather than ClO radicals in MBL.These results can deepen the understanding of the HONO formation mechanism,provide more accurate input parameters for atmospheric chemical models,and set the theoretical basis for reducing HONO emissions and establishing HONO-control strategies.