Characterisation Of Individual Particles In PM_2.5 From Coal Combustion And Their Behaviour In Atmospheric Environment

Physicochemical characterisation of individual PM_2.5 particles from coal combustion, and their behaviour in the atmospheric environment, were studied using atmospheric chemistry and individual particle analysis. A coal combustion/dilution system was designed and used. PM_2.5 samples from bituminous and anthracitic coal combustion were collected. Particle size, morphology, chemical composition, and internal structure were determined by a Transmission Electron Microscope equipped with an Energy Dispersive X-ray Spectrometer（TEM-EDX）. Based on the chemical composition, morphology, and stability under an electron beam, individual particle classification criteria from coal combustion were established. The individual particles were classified into the following eight types: organic, soot, Si-rich, Ca-rich, S-rich, K-rich, Fe-rich and other particles. Existing morphologies and chemical composition profiles on coal combustion were supplemented and perfected. To understand the physical and chemical processes that individual particles undergo in the atmospheric environment, PM_2.5 samples were collected near a coal-fired power plant. Size distribution, morphology, chemical composition, and internal structure of individual aerosol particles were analysed by TEM-EDX. To further understand the physical and chemical mechanisms of long-range transport of individual particles from coal combustion, individual samples from typical cities（Beijing, Zhengzhou, and Shenzhen） were analysed in order to classify typical particles from a coal combustion source. The mechanisms were defined by comparing the characterisation of particles from coal combustion to that of particles in the atmospheric environment.Physicochemical characterisation of individual PM_2.5 particles from coal combustion was determined by TEM-EDX. The results show organic, soot, Si-rich, Ca-rich, S-rich, K-rich and Fe-rich were the main particles type. Soot from coal combustion tends to agglomerate and is chain-like. EDX analysis showed that elemental compositions of soot particles contain abundant carbon and minor oxygen, sulfur, and silicon. Most organic particle particles from household coal combustion are spherical or approximately spherical. EDX analysis showed that the element composition of organic particles mainly contains C and a small amount of O, sometimes contains minor Si, S, K and Cl element. Most Si-rich particles from household coal combustion are irregular. EDX analysis showed that the element composition of Si-rich particles mainly contains Si and O, contains a small amount of Mg, Fe, Na, Ca, S. Most Ca-rich particles from household coal combustion are irregular, but a few particles are plate. EDX analysis showed that the element composition of Ca-rich particles mainly contains Ca and O, contains a small amount of Na, Mg, Fe, S. Most S-rich particles from household coal combustion are bubble-shaped on the filter membrane because these particles are sensitive to the electron beam. EDX analysis showed that S-rich particles mainly contain S and O, and minor K, Si. Most K-rich particles from household coal combustion are irregular. EDX analysis showed that K-rich particles mainly contain K and O, and minor S, Si. Most Fe-rich particles from household coal combustion are irregular, but a few particles are spherical. EDX analysis showed that Fe-rich particles mainly contain Fe and O, and minor S、Ca, Zn, Mg, Mn.Individual spherical organic particles and soot are similar in morphology and chemical composition, but differ in structure. Soot particles are aggregates of spherules that consist of concentrically wrapped graphitic layers, whereas individual spherical organic particles neither possess a semi-ordered microstructure, nor do they form aggregates. Spherical organic particles have lower C and higher S values than soot particles. In addition, some organic particles contain the elements Cl and K.Morphologies, size, and chemical compositions of organic particles emitted household coal combustion were similar to tar balls from biomass burning. Relative number abundance of organic particles from household coal combustion was from 23% to 27%.Soot, organic, S- and Si-rich were the main particles in PM_2.5 samples from household coal combustion; the sum of their relative abundances was 83.8%. Bituminous coals emitted more soot and organic particles than anthracite coals because of their high volatile component. The relative abundance of S-rich particles from coal combustion was strongly positively correlated with the S content in coal.Relative number abundance of K-rich particles was 4.3%, 7.6% and 20.9% for bituminous chunk, anthracite chunk and anthracite briquette, respectively. Relative number abundance of S-rich particles was 12.2%, 40.7% and 30.0% for bituminous chunk, anthracite chunk and anthracite briquette, respectively. Relative number abundance of Ca-rich particles was lower 2.5% because low combustion temperature cannot make Ca evaporate in the course of household coal combustion.The diameter of most PM_2.5 particles from household coal combustion was below 0.6 μm. Particle size ranged from 0.3 to 0.4 μm when relative abundance of organic, and K-rich particles was highest. Soot, Si-, S- and Fe-rich particles were mainly between 0.2 and 0.3 μm.Si- and Ca-rich were the main particle types produced from coal-fired power plants and most were approximately spherical, while those emitted from household coal combustion were irregular. This is consistent with household coal combustion using relatively low temperatures which cannot melt minerals to produce spherical particles.Particle size, morphology, chemical composition, internal structure and mixing state of individual PM_2.5 particles around a coal-fired power plant were determined by TEM-EDX. The results show there were more spherical particles in downwind aerosol because of spherical Fe-rich particles and fly ashes from the coal-fired power plant. There were more S-rich and Ca-rich particles in downwind aerosol samples were likely related to the use of desulfurization agent to decrease SO2 emission. There were more core–shell structure particles downwind from sources, which suggests particles from the power plant, or from upwind, can easily react with other species and form a core–shell structure under suitable conditions, such as high sulphur dioxide and nitrogen oxide concentrations, and high relative humidity. Mineral particles containing Si usually mixed with those containing S, or with organic particles, whereas metal particles with S-rich, Cl-rich and organic particles formed an obvious core–shell structure in which the metal particles form the core and the S-rich, Cl-rich and organic particles form the shell.Particle size, morphology, chemical composition, internal structure and mixing state of individual PM_2.5 particles from three cities（Beijing, Zhengzhou, and shenzhen） were determined by TEM-EDX. The results show there were more S-rich particles in Beijing, Zhengzhou, and Shenzhen, notably in Shenzhen city the relative abundance of S-rich particles was 72.9 %. Fe-rich particles in the three cities were usually spherical with a large amount of Fe and small amounts of Na, Ca, Mg, Al, Si, and S. This structure is consistent with Fe-rich particles emitted from coal-fired power plants, suggesting the Fe-rich particles present are mostly from coal combustion. Fly ashes are emitted mainly from coal burning. They typically have a spherical structure and relatively high Al, Si, and O content. There were more fly ashes in Zhengzhou than in Beijing and Shenzhen, because Zhengzhou used more coal. There were more soot aggregates in Shenzhen than Beijing, probably because the Tanglangshan tunnel, near the Shenzhen sampling site, is used by large volumes of diesel car traffic. Diesel cars generate more soot aggregates than petrol vehicles. There were more soot aggregates in Zhengzhou than in Beijing and Shenzhen; these were mainly from coal burning and traffic sources. There were more organic particles in Beijing than Zhengzhou and Shenzhen because car ownership is higher there. In addition, Beijing burned more coal during the heating period. Coal and oil burning generate many organic gases, which can form organic particles under suitable conditions. The size distribution of PM_2.5 in Zhengzhou and Beijing was mainly in the range of 0.1 to 0.4 μm. The relative abundance of these particles was 70 % and 54 % respectively. PM_2.5 size distribution in Shenzhen was mainly between 0.7 and 1.0 μm, and relative abundance was 31 %. The size distribution range of fly ashes in Shenzhen, Beijing, and Zhengzhou was relatively narrow, less than 0.4 μm. The size distribution range of S-rich particles was relatively wide; the size distribution peak of S-rich particles in the three cities was respectively 0.7 to 1.0 μm, 0.4 to 0.7 μm, and 0.1 to 0.4 μm. This indicated that the size of S-rich particles was largest in Shenzhen, followed by Beijing and Zhengzhou, which was attributed to Shenzhen’s high temperature and high relative humidity; these meteorological conditions can accelerate hygroscopic growth of S-rich particles. The morphology of S-rich particles after hygroscopic growth was mostly spherical or approximately spherical.Physical and chemical characterisation of individual PM_2.5 particles from Beijing suburban in heating season was investigated by TEM-EDX. The results show relative number abundance of S-rich particles was highest in all particles types, but relative number abundance of fly ashes was lowest. There were less soot and organic particles in suburban samples than coal-combustion samples. The peak of particles was in the size range of 0.4-0.7μm. Fly ashes, organic and Si- rich particles were mainly between 0.1 and 0.4 μm. Soot, K-, Ca-, S- and Fe-rich particles were mainly between 0.4 and 0.7 μm. For the same particle type, there was more S content in most particles from atmospheric environment than from household coal combustion.The transformation of mineral particles, emitted from coal burning, to complex particles was explored in this dissertation. Firstly, the mineral particles absorb some water, resulting in a liquid surface on the mineral particle. Some acid gases can react with this liquid surface, leading to varied additional components of the mineral particle, which become a shell on the surface of the particle. Finally, the solution may be crystallized or undergo liquid-liquid separation, thereby forming complex particles. The newly-formed complex particles have a core–shell structure that may be homogeneous or heterogeneous.