Analysis Of The Three-dimensional Structure And Change Characteristics Of Urban Atmospheric Particulate Matter Based On ToF-SIMS Technology Optimization

Rapid urbanization has made cities the main settlements of our country’s population.While urbanization promotes the rapid development of human civilization,it also produces many air pollution problems.Atmospheric particulate matter or aerosol is an important component of atmospheric pollutants and plays a key role in many key processes related to the atmosphere.Factors such as the chemical reactivity of particulate matter,the formation of cloud condensation nuclei or ice nuclei,and the optical properties of particles can have profound effects on air quality,human health and climate.These important characteristics are closely related to the complex surface chemical distribution and internal mixing state of particles,and are of great practical significance for understanding the formation process of new particles,clarifying and analyzing their pollution characteristics,sources and formation mechanisms.In this study,Time of Flight Secondary Ion Mass Spectrometry（To F-SIMS）,a burgeoning characterization technique,was used to investigate the surface composition and mixing state of atmospheric particulate matter.The main conclusions and achievements obtained are as follows:（1）In this study,a PCA analysis software based on the free and open-source Python programming platform for To F-SIMS mass spectrometry data was designed and applied to the processing of complex atmospheric particulate matter data successfully.Combining the output data with the basic information of atmospheric particulate matter samples,the data of atmospheric particulate matter samples with different pollution degrees were analyzed.The important compositional differences between blank samples and lightly and heavily polluted samples were obtained,and it was speculated that the increase of Na NO₃ in heavy pollution events may be a typical feature of the Shanghai air pollution case.The development of this software greatly simplifies the analysis process of To F-SIMS complex data,improves the research efficiency,and facilitates the analysis and visualization of the complex chemical composition differences of atmospheric particulate matter.This software is suitable for mass spectrometry data analysis in the field of atmospheric particulate matter research,and plays an important role in the use and promotion of To F-SIMS in scientific research fields.（2）When To F-SIMS characterizes atmospheric particles on weakly conductive filter membrane substrates,the charging effect on the surface will interfere with the detection process.This study found that low-energy and low-current oxygen ion beam-assisted detection can improve and solve this phenomenon.Firstly,non-conductive and corrosion-resistant polytetrafluoroethylene（PTFE）and macromolecular sucrose samples were selected for the comparative experiment of oxygen ion beam assisted parameter optimization.The optimal parameter setting of oxygen ion beam assistance were determined:the current was around 11 p A,and the energy setting was 250 e V.Also,it was found that the oxygen cluster ion beam under this parameter setting was extremely destructive to macromolecules,which is suitable for the study of organic substances in atmospheric particulate matter.By comparing the oxygen cluster ion beam-assisted imaging of atmospheric particulate matter samples on the quartz fiber filter membrane,PTFE,tree root slices and cement materials with poor conductivity,it was found that their signal intensities and imaging effect under different scanning areas were greatly improved after the oxygen cluster ion beam assistance.The result show that the charging effect on the sample surface has been properly resolved,which verifies the feasibility of the method for the detection of a variety of non-conductive materials,and is suitable for fine characterization and analysis of atmospheric particles on various filter membrane substrates.Combined with the experimental results,it is speculated that the principle of this method to reduce the charging effect is that the oxygen cluster ion beam eliminates the electrons accumulated on the surface of the sample,and the positive charge inside the sample is excited to the surface,then the combination of electrons brought by the flood electron beam cancels each other,so that the sample returns to a neutral state.This method greatly improves the imaging quality and the accuracy of mass spectrometry data,and is suitable for reducing the charging effect of other weakly conductive samples in To F-SIMS detection.This study provides a new solution to the charging effect,which has great practical significance for the further development of To F-SIMS in the field of atmospheric particulate matter research.（3）In this study,To F-SIMS was used as the main research tool to analyze the three-dimensional structure of PM_2.5 in typical winter haze pollution events in Beijing.Mass spectrometry data and PCA output revealed the sources of particulates with different pollution levels.It is also found that particulate matter presents two different composition and mixed state evolution trends with the change of pollution degree.On the basis of the mass spectrometry analysis results,To F-SIMS three-dimensional imaging analysis was carried out on the internal mixing state of typical light pollution and heavy pollution particulate matter samples,and combined with positive and negative ion imaging results,the structural distribution and chemical composition of atmospheric particulate matter were analyzed.Studies have shown that the particles collected in lightly and heavily pollution form a three-dimensional structure of organic shell-inorganic core,a thinner organic covering layer is formed on the surface of lightly polluted particles,while a thicker covering layer of inorganic and organic mixed matter was formed on the surface of heavily polluted particulate matters.The source of ions in lightly polluted samples can be inferred by positive and negative ion imaging overlay,（NH₄）₂SO₄ and a small amount of KCl particles were found to be formed.In addition,the formation of large-area K_x（NH₄）_2-xSO₄ and NH₄Cl particles was observed in heavily polluted samples,and the formation of K_x（NH₄）_2-xSO₄ was further verified by SEM-EDX imaging.It was inferred that a strong ion exchange reaction occurred between（NH₄）₂SO₄and KCl in the actual process to form K_x（NH₄）_2-xSO₄ and NH₄Cl.The unique advantage of three-dimensional To F-SIMS imaging analysis is that it can reveal the surface composition and internal mixing state of atmospheric particulate matter,and determine the accurate chemical composition of sulfur compounds in the Beijing winter haze pollution event for the first time.The fine distribution characteristic information of atmospheric particles obtained by To F-SIMS can not only be associated to the water absorption efficiency of particles and the droplet nucleation ability,but also provide strong data support for the environmental system models,thereby further improving the accuracy of the models.Through the improvement of data analysis methods,the enhancement and upgrading of imaging technology,combined with a variety of other characterization approaches,this study conducted an in-depth discussion on the formation mechanism of the mixing state of atmospheric particles in typical haze pollution events in Beijing in winter,providing a new perspective and approach to explore the three-dimensional structure composition and change characteristics of atmospheric particles.