| As an important constituent of the earth's atmosphere, atmospheric particles or aerosols have great effects on ambient air quality, regional and global climate change, and human health. High emission loads, as well as cross-contamination with a variety of roles, making the transformation mechanism and mixing state of secondary aerosol extremely complex in urban atmosphere in China. This kind of complex mixing state of aerosols can change the aerosol particle size, chemical composition, hygroscopicity, absorption and reflection of radiation significantly, thereby leading to the Earth's energy budget imbalance or global climate change. Therefore, the mixing state of single-particle in atmosphere research is one of the focus and cutting-edge topics in atmosphere research in recent years. The emergence of single-particle aerosol time of flight mass spectrometry(ATOFMS) makes it possible for the study mentioned above, providing a powerful analysis technique for single particle size and chemical composition measurement in real-time. Using this technology, we have conducted extensive analysis and studied on particle type, mixing state, aging processes, and formation mechanisms of secondary aerosols as well as particle source appointment in the past few years in Shanghai.This thesis will focus on four topics as follows:â‘ Particle classification and data analysis. With the huge amount of data collected by ATOFMS everyday, we need a strong, effective data analysis method to analyze these data. In this thesis, two different methods:ATOFMS marker classification method and Art-2a classification method are developed in ATOFMS data processing. Using marker methods, combined with previous studies and experience, total particles will be divided into primary aerosols and secondary aerosols. Primary aerosol include four major categories:mineral dust aerosols (Dust), sea salt aerosols (Sodium), carbonaceous aerosols (Carbon) and biomass burning aerosols (Biomass burning). The remaining aerosols mainly containing sulfate, nitrate and organic will be classified as secondary aerosols (Secondary). Comparing marker classification results with mass spectrometral features obtained by Art-2a classification, we can clearly distinguish which particle is the primary or the secondary particle type. Meanwhile, we can get some special groups of particle, such as heavy-metal containing particles.â‘¡Mass spectral patterns of some typical primary particles. In China, biomass burning, in particular crop straw burning is one of the major primary particle sources. In this thesis, laboratory experiments were carried out to simulate the Chinese crop straw burning. The results show that, by using of the combination of potassium [K]+ and potassium compounds clusters [K2Cl]+, [K2Cl]+, [K2SO4]+, levoglucosan ions [CHO2]-, [C2H3O2]-, [C3H5O2]- and organic nitrogen tracer peak [CN]-, we can easily extract biomass burning particles from ambient atmosphere. The characteristics of mass spectra between rice straw and corn straw burning show no significant difference. The results of our experiments also show that the Marker ions classification and Art-2a classification are consistent well with each other. The number of typical biomass burning particles recognized by these methods accounted for 88%of the total number particles in simulation experiment.â‘¢Formation mechanism of important secondary particles. On the basis of understanding of classification and typical mass spectral patterns of primary particls, this study focuses on formation mechanism of oxalate containing particles. Through single-particle mass spectrometry analysis of oxalate containing particles in summer 2007, biomass burning was revealed as a primary source of oxalic acid which contributed more than 20% of the oxalate-containing particles. Evidences for two different formation pathways of oxalic acid were observed in our experiment. The number fraction of oxalate-containing particles is correlated with that of sulfate particles and the changes of air parcel backward trajectories, suggesting that in-cloud processing played important roles in oxalic acid formation. The diurnal patterns of dust and sea salt particle counts fitted well with the ambient relative humidity variation, suggesting that heterogeneous reactions occurring in hydrated/deliquesced aerosols also contributed to the production of oxalic acid.â‘£Mixing state of particle and its environmental impact. This chapter will fully discuss the evolution and mixing state of aerosols and their impacts on atmospheric visibility. The size and chemical composition of individual aerosol particles was measured using ATOFMS during the period December 14-23,2008. Results showed that carbonaceous particles are the dominant particle type during the entire experiment. No tight correlation was observed between visibility and ATOFMS particle counts, suggesting that the visibility impairment during this experiment was not due to the high number concentration of fine particles at least not in the sampling scope of ATOFMS. However, the mixing state of carbonaceous particles showed dramatic changes from fresh elemental carbon or biomass burning particles to deeply aged ones with addition of secondary components, mostly ammonium nitrate, during heavy haze events. In the periods of heavy haze time, peaks of carbon and nitrate (mainly ammonium nitrate) peaks appear simultaneously all the time in mass spectrum, which means that in high relative humidity conditions, these soluble compounds are easily mixed with the hydrophobic carbon. Our real-time single particle data strongly suggest that the changes of internal mixing state and the hygroscopic index of particles dramatically increase the light extinction efficiency of aerosols and cause the visibility degradation.
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