Theoretical Studies On The Transformation And Properties Of Sulfur Compounds In The Atmosphere

The studies on sulfur compounds have attracted the attention of atmospheric scientist for many years.These sulfur compounds,such as hydrogen sulfide(H2S),dimethyl sulfide(CH3SCH3,DMS)and sulfur dioxide(SO2),can been emitted into the atmosphere by natural sources and human activities.The transformation of these sulfur compounds in the atmosphere can produce the new sulfur compounds,thus making the studies on them more complicated.These emitted and produced sulfur compounds can affect the environment through their involvement in global warming,acid precipitation and cloud formation.Although the transformation of the sulfur compounds has been studied widely,the reaction mechanism in the process of their transformation has not been understood completely.Thus,the studies on their mechanism are necessary.In addition,understanding the properties of sulfur compounds can give more insight into the sulfur compounds and their transformation.Based on these,the dissertation will focus on the mechanism of transformation reaction of sulfur compounds and the properties of sulfur compounds.In the dissertation,we will study the transformation reaction and properties of some sulfur compounds(SO3,SO2 and methanesulfinic acid)using the ab initio,density functional theory(DFT)and molecular dynamics(MD)simulation methods.The main contents in the dissertation include the promotion of oxalic acid to the SO3 hydration reaction,the promotion of amines to the SO2 hydration reaction and the properties of these hydrated products,the oxidation mechanism of methanesulfinic acid by ozone,and the properties of methanesulfinic acid at the air-water interface.These contents can be summarized as follows:1.The role of oxalic acid in SO3 hydration reaction in the atmosphereThe hydration of SO3 to form H2SO4 is an important reaction in the atmosphere.The experimental and theoretical studies have shown that the reaction involving a single water molecule cannot occur in the atmosphere due to its high energy barrier.The inclusion of a second water molecule in the SO3 hydration reaction has been proven to reduce the energy barrier.Some species in the atmosphere have been reported to replace the second water molecule to promote the SO3 hydration.In this section,the oxalic acid-catalyzed SO3 hydration was studied using DFT and ab initio methods.The results show that the energy barrier of around 1 kcal mol-1 for SO3 hydration reaction catalyzed by oxalic acid(cTt,tTt,tCt and cCt conformers)is lower than that of 5.17 kcal mol-1 for water-catalyzed SO3 hydration,which means that the oxalic acid is more effectively than water molecule in promoting the SO3 hydration.According to the rate comparison between oxalic acid-catalyzed SO3 hydration and water-catalyzed SO3 hydration at different altitudes,it can be found that the rate constants of SO3 hydration reaction catalyzed by oxalic acid is larger than that catalyzed by water molecule.However,the concentrations of water is obviously higher than that of oxalic acid.Combined rate constants with concentration,it can be found that in the upper troposphere the rate of SO3 hydration reaction catalyzed by oxalic acid can reach one tenth of that catalyzed by water molecule.Thus,it can be conclude that in the upper troposphere the oxalic acid-catalyzed SO3 hydration can play an important role in promoting the H2SO4 formation.2.The role of amines in the SO2 hydration reaction and the properties of hydrated products in the atmosphereIn addition to the SO3 hydration,the SO2 hydration reaction also is important in the atmosphere.The SO2+ H2O →H2SO3 reaction has a barrier of greater than 30 kcal mol-1.Thus,the reaction is hard to occur.Studies have shown that there is still high energy barrier even if additional several water molecules are included.Some results from the theoretical calculation have indicated that the addition of some species(NH3,H2SO4 and H2SO3)can reduce the energy barrier of SO2 hydration more effectively than water molecule.In this section,the effect of amines(methylamine and dimethylamine)on the SO2 hydration reaction was studied using DFT and ab initio methods.The results show that the energy barrier of SO2 hydration in the presence of methylamine and dimethylamine is lower than that in the presence of NH3 or additional H2O molecules.For SO2+ 2H2O + DMA reaction,a pathway with near no energy barrier(0.10 kcal mol-1)can be found.These results indicate that methylamine and dimethylamine are effective for promoting the SO2 hydration.The MD simulations were used to analyze the properties of these hydrated products.The results from MD simulations show that these hydrated products are capable of self-aggregate into bigger clusters and are able to capture water and amine molecules.The properties of these hydrated products mean that they have the potential to promote new particles formation3.The oxidation mechanism of methanesulfinic acid by ozone in the atmosphereMethanesulfinic acid(CH3S(O)OH,MSIA)is an important intermediate in the oxidation of dimethyl sulfide in the atmosphere.DMS emitted by oceans have been proven to be the important natural source of sulfur compounds.Thus,the transformation of DMS in the atmosphere is significant for the understanding of atmospheric sulfur cycle.As MSIA is the reaction intermediate of DMS,the studies on the properties and reaction of MSIA should be given more attention.In this section,the DFT and ab initio methods were used to study the oxidation mechanism of MSIA by O3.Three pathways can be found for their reaction.In the most favorable pathway,the highest energy barrier is 13.02 kcal mol-1.According to the comparison of reaction rate between MSIA + O3 and MSIA+ OH within the range of 220 K to 298 K,it is clear that the rate of MSIA + O3 reaction is lower than that of MSIA + OH reaction.The result indicates that the oxidation of MSIA by O3 is minor important relative to its oxidation by OH radical.However,the oxidation of MSIA by O3 also can play an important role in some cases in which the concentration of OH radical is low.4.The properties of methanesulfinic acid at the air-water interfaceThe larger Henry’s law constant of MSIA leads to its higher propensity to stay in water phase.Thus,the MSIA produced from the air phase is easy to transport into the atmospheric aqueous-phase(cloud,fog,aerosol,etc).The first step of the transfer process is that the gas molecule is adsorbed at the air-water interface.In addition,the interfacial chemistry can play an important role in the atmosphere.Thus,the studies on the properties of MSIA at the air-water interface are meaningful for understanding its behavior and its heterogeneous oxidation.In this section,the MD simulations were implemented to study the free energy changes when the MSIA transport from the air phase into the aqueous phase,and to investigate its propensity to stay at the interface,and to analyze orientation and hydration status of MSIA at the interface.The result shows that the system free energy decreases when the MSIA molecule moves toward the interface,and reaches its minimum value at the interface,and increases first and then keeps stable when the molecule move from the interface into the aqueous phase.The result indicates the surface preference of MSIA.But the free energy well is shallow.MSIA needs to obtain only 0.76 kcal mol-1 of free energy when it crosses the well into the water phase.Based on the analysis of the additional 150 ns NVT simulation,it can be found that MSIA molecule have approximately 20%propensity to stay at the air-water interface.MSIA molecule tends to tilt at the interface with the sulfino group(-S(O)-OH)pointing toward the water phase and the methyl group directing toward the air phase.The characteristic of hydration status of MSIA at the air-water interface is favorable to its heterogeneous oxidation.According to the studies on SO3,SO2 and methanesulfinic acid,we have known the reaction mechanisms involving these sulfur compounds,the importance of these reactions in the atmosphere,and the properties of these sulfur compounds.The results obtained from the dissertation can make us know more about these sulfur compounds,and can give some useful information for the further study about sulfur compounds.