The Formation Mechanism Of Sulfate Aerosol On The Surface Of CaCO₃ Composite Particles

The secondary inorganic aerosols(sulfate,nitrate,ammonium,etc.)lead to haze pollution events in China to a great extent,especially the explosive growth of sulfate concentration.Most of the aerosols exist in the form of multi-component(composite particles).The heterogeneous reaction of atmospheric particles with various trace gases can produce secondary inorganic aerosols.At present,the simulation of heterogeneous reactions in the atmosphere is mostly focused on single component particles and single or multiple trace gases,and the heterogeneous reactions of composite particles and the interaction between particle components are less involved.Therefore,in this study,the heterogeneous reaction of SO2 on the surface of composite particles is studied,which is very important to understand the formation mechanism of secondary inorganic aerosols,clarify the formation process of haze pollution and design effective mitigation measures.Taking CaCO3 as carrier,NH4NO3 and polycyclic aromatic hydrocarbons(fluoranthene and anthracene)as loading objects,CaCO3-NH4NO3 composite particles,CaCO3-fluoranthene and CaCO3-anthracene composite particles were prepared on the basis of CaCO3.Taking single particles and composite particles as the difference,the process of SO2 transformation into secondary inorganic aerosol on the surface of different particles was explored.The main conclusions are as follows:(1)Compared with single component particles(CaCO3),the conversion effect of SO2 on the surface of CaCO3-NH4NO3 composite particles is enhanced,and increases with the increase of relative humidity.The presence of NH4NO3,a strong hygroscopic component in composite particles,maked the surface moisture of particles increase,thus having more active sites,which can greatly promote the absorption and chemical transformation of SO2.With the increased of relative humidity,more water can be stored on the surface of composite particles,forming a liquid film on the surface of particles,which was conducive to more SO2 dissolving on the surface of composite particles for chemical reaction and ion migration,so that SO2 was finally converted into SO42-.(2)When the relative humidity(RH75%)was constant,the conversion effect of SO2 increases with the increase of NH4NO3 content in CaCO3-NH4NO3 composite particles,and NH4NO3 had some effects on the final productThe presence of NH4NO3 changed the hygroscopicity of the composite particles,causing more water to adhere to the surface of the particles,increasing the number of SO2 adsorption sites,greatly promoting the chemical adsorption of SO2,and the reaction produced H2SO3.H+was produced by hydrolysis of H2SO3,so that the particle surface was acidic.NO3-has strong oxidability under acidic condition,and NO3-oxidized S(?)to S(?).The results of XRD and FTIR showed that the SO2 gas formed CaSO4-2H2O on the surface of CaCO3-NH4NO3 composite particles through a series of reactions.The main reason was that the strong hygroscopicity of NH4NO3 increased the surface water of particles,and then affected the crystallization of the insoluble material CaSO4,which existed in the form of CaSO4·2H2O.The conversion of SO2 to CaSO4·2H2O achieved the fixation of gaseous SO2.(3)It was found that fluoranthene and anthracene on the surface of CaCO3 can promote the transformation of SO2 to SO42-to a certain extent only under the condition of high relative humidity and ultraviolet light,but not under the condition of low relative humidity or no light.The possible reason is that the high relative humidity promotes the adsorption of SO2 on the particle surface,O2 and PAHs generate 1O2 under the action of light,and 1O2 oxidizes s(?)to s(?).