Characteristics And Aging Process Of PM_2.5 Particles Emitted By Biomass And Coal Combustion

In recent decades,with the rapid development of economy and urbanization,China’s demand for energy has been growing.At present and over in the next few decades,the energy structure of China is still dominated by coal.Coal and biomass energy are still widely used as solid fuels in China.Bulk coal is mainly used for household heating in urban and rural areas in northern China,while biomass is mainly used for cooking in large agricultural areas and economically underdeveloped areas.Although the consumption of civil fuel is lower than that of industry,civil fuel has the characteristics of wide distribution of use,low thermal efficiency and high pollution emission.Particularly during the winter heating period in northern China,coal combustion emissions were a major source of air pollution PM2.5 and have received widespread attention in recent years.The aerosols emitted from the combustion of coal and biomass not only cause regional air pollution,but also affect the photochemical action to change the climate and human health.In this paper,the physicochemical characteristics of aerosol particles emitted from biomass and bulk coal combustion sources were studied by combustion experiments and smog chamber experiments.Biomass was selected from the three major agricultural crops in China,namely corn,wheat and rice,while bulk coal was selected from three common types of bulk coal used by residents in the plain area of Hanzhong.The morphology,quantity and concentration of primary single aerosol particles emitted from the simulated open burning of straw and the combustion of bulk coal in the stove were analyzed in laboratory,and the characteristics of single aerosol particles emitted by straw and bulk coal in different combustion stages were compared and analyzed.The effects of fuel type,combustion temperature and combustion stage on particulate emission were discussed.The photochemical smog chamber experiment was mainly used to simulate the aging and secondary aerosol formation of particulate matter emitted from biomass straw and bulk coal combustion in quasi-atmospheric environment.The aging rates and related influencing factors of particulate matter emitted from biomass straw and bulk coal were discussed,and the physicochemical characteristics and aging mechanism of particulate matter in actual environment were proposed based on the comparison between the actual atmospheric observation results and the simulation results.The micromorphology,particle type,particle size distribution,roundness measurement,mixing state,chemical composition and aging characteristics of the particles were analysed using the transmission electron microscopy with an energy-dispersive X-ray spectrometry(TEM-EDX)and Image processing systems of Leica,UK and Image-J.The emission experiment of biomass combustion shows that the biomass combustion stage was flaming stage and smoldering stage.The primary particles were classified into three types:organic particles,K-rich particles and soot particles.The organic particles include pure organic particles,organic-K particles and tar balls.The morphology of organic particles in flaming and smoldering stage was obviously different.The organic particles in the flaming stage were spherical and nearly spherical,and in the smoldering stage were gel-like.Most of the organic particles in the flaming stage contain potassium,which was considered to be KCl and K2SO4.The particle size in smoldering stage was obviously larger than that in flaming stage.The combustion emission experiment of bulk coal showed that the combustion of bulk coal includes ignition stage,fierce burning stage and char burning stage.Carbonaceous particles include:soot particles and organic particles;non-carbonaceous particles include:Si-rich,Ca-rich,Fe-rich,S-rich,Al-rich and other particles.The ignition stage was dominated by organic particles(90%),the fierce burning stage was dominated by soot(45%),and the char burning stage was dominated by mineral particles(70%).The occurrence frequency of S,Ca,Fe,Si,Al,Mg,Na and O elements in the particles of coal combustion was more than 50%.The heavy metal elements such as Ti,Mn,Zn,Ni and Cr were detected in the particles.In this study,the microstructure of the soot particles from bulk coal was clearer than that of the soot particles from biomass,indicating that the combustion temperature and combustion efficiency of bulk coal were higher.The smog chamber results showed that the particle size increased significantly,with an average increase of 0.2 μm,and the particle size distribution was narrow.The O:C ratio of organic particles increased during the 4-hour aging process,indicating that the aging degree of particulate matter was getting higher and higher.The K-rich particles were mainly KCl in the early stage of aging,and part of the particles gradually sulfurize and form K2SO4 in the aging process.In the aging process the percentage of tar balls increased continuously,from 3.6%to 43%.The organic particles were mainly low volatile organic compounds(LVOC).Raoult effect and polymerization effect make organic particles form tar ball particles.The smog chamber results showed that there were seven main types of particulate matter emitted from bulk coal:organic particles,soot particles,S-rich particles,Ca-rich particles,Fe-rich particles,Al-rich particles and secondary particles.After aging,the number of secondary particles increases sharply.The particle size increased by 0.6-0.8μm on average.S-rich particles become irregular shapes,and some S-rich particles were accompanied by a circular shell with an irregular shape inside and a bubble like structure on the surface.The mass ratio of O/C element in soot particles and organic particles increased from 9%to 12%on average,and the content of sulfur element in these particles increased.Most of the secondary particles were of core-shell structure and their mean core-shell ratios were:coal 1(0.55),coal 2(0.57)and coal 3(0.46).The formation-ageing process of secondary particulate matter in smog chamber was mainly through two pathways:one was the reaction of particulate matter with organic matter(VOCs,photo-oxidation products)to form a nucleoshell structure of secondary particulate matter,and the other was the surface of the particulate matter being acted upon by H2SO4 to form a shell of sulphate,which then combines with organic matter to form secondary particulate matter.The actual atmospheric observation show that the single particle can be divied into organic particles,soot particles,Si-rich particles,K-rich particles,Fe-rich particles,S-rich particle,mineral particles,mixed particles and secondary particles.The highest relative number percentages of secondary particles in the ambient atmosphere was approximately:44%.Organic particles and soot emitted from combustion sources and primary particulate matter in the ambient atmosphere were similar in morphology.Secondary particles in the ambient atmosphere were more aged,and have a higher sulphate content and more complex mixing than secondary particles in the smog chamber.The percentage of secondary particles in the smoke chamber was lower than that in the ambient atmosphere.The secondary particles in urban atmosphere were mainly core-shell structures,which mainly contain C,O and S.Organic particles with C,O,S and K elements and Fe,Si,S and K-rich particles in atmospheric environment may be come from biomass and coal combustion emissions.This indicates that SO2 in the atmosphere in winter mainly comes from coal combustion,which created sulfate and SOA in the atmosphere.Therefore,biomass and bulk coal combustion emissions were the main contributors to regional atmospheric pollutants.