Study On Morphology,Composition And Hygroscopicity Of Fine Primary Particles From Residential Coal Burning

In suburbs and rural regions of China,coal is the dominant fuel for heating and cooking during wintertime.However,coal in residential stoves is incompletely combusted without any air pollution control devices and emits lots of particulate matters.Therefore,primary emissions from residential coal burning have been the main contributor to the formation of Chinese haze during wintertime.Various bulk technologies have been used to obtain mass concentrations and compositions of primary particles from residential coal burning.However,they cannot provide detailed information on morphology,composition,and mixing state of individual primary particles,which is crucial to analyze sources of aerosol particles,to study their ageing and reaction processes in the atmosphere,and further to clarify the mechanism of Chinese haze formation in winter.To our knowledge,few studies on the comprehensive features of individual primary particles directly derived from coal burning have been reported.Therefore,it is necessary to study the morphology,composition and hygroscopicity of fine primary particles emitted from residential coal burning.In this study,nine types of raw coal with a wide range of maturity,including lignite,bituminous coals and anthracite,were selected and burned in the residential stove of the dilution sampling system.The individual primary particles were collected during flaming and burn-out stages,and were analyzed by transmission electron microscopy with an energy-dispersive X-ray spectrometer(TEM-EDS),scanning electron microscopy(SEM)and NanoSIMS 50L.PM2 5 samples were collected during flaming stage,and were analyzed by OC/EC analyzer,ion chromatography,inductively coupled plasma mass spectrometry and inductively coupled plasma optical emission spectrometer.An individual particle hygroscopic(IPH)system was used to observe the hygroscopic properties of individual primary particles.Bulk analyses show that the relative abundance of EC emitted from low-maturity coals were much higher than that from other maturity coals.The mass fractions of OC from medium-maturity coals were higher than 70%among OC,EC and ions,which reveals that OC were the dominant compound.Compared to low-and medium-maturity coals,the relative abundance of inorganic ions from high-maturity coals were much higher.We found that SO42-CI-and NH4+ were the dominant ions in PM2 5 from residential coal burning.In addition,Ai.Ca and Na were the dominant metals,accounting for 57%-80%of total metals;Zn and Pb were the dominant heavy metals,accounting for 54%?91%of total heavy metals.Individual particle analyses show that the individual primary particles from residential coal burning can be divided into six types:organic matter(OM),OM-S,soot-OM.S-rich.metal,and mineral particles based on their morphology and composition.According to the different morphology of OM and soot-OM particles,OM particles were further divided into spherical OM,dome-like OM and irregular OM;soot-OM particles were further divided into chain-like soot-OM and cluster-like soot-OM.TEM observations show that residential lignite burning mainly emitted cluster-like soot-OM particles in the flaming stage,and dome-like OM particles in the burn-out stage.As for residential bituminous coals burning,gas coal from Shaanxi mainly emitted chain-like soot-OM particles in the flaming stage,and soot-OM and OM particles in the burn-out stage;Non-caking coal from Shaanxi emitted abundant dome-like OM particles in both flaming and burn-out stages;Non-caking coal from Inner Mongolia mainly emitted OM particles,including abundant spherical OM particles in the flaming stage,and dome-like OM particles in the burn-out stage;Fat coal from Shandong emitted abundant OM particles in both flaming and burn-out stages;Meager coal from Shanxi mainly emitted OM-S particles in both flaming and burn-out stages.Under residential burning condition,anthracite and natural coke from Shanxi also emitted abundant OM-S particles in both flaming and burn-out stages.In addition.TEM observations reveal that metal elements were mainly occurred in mineral,some carbonaceous particles.K-rich,S[K]-rich and metal particles(Fe-rich,Zn-rich,and Pb-rich particles).In conclusion,individual particle analyses and bulk analyses both reveal that low-maturity coals dominantly emitted black carbon(BC:;also names as EC or soot):the medium-maturity coals emitted abundant OM;and high-maturity coals emitted abundant OM mixed with inorganic ions(OM-S particles).We found that soot particles were mainly formed in the flaming stage of low-maturity coals under higher burning temperatures.We concluded that coal maturity and burning temperature both determine particulate properties in coal emissions.In addition,we found that residential coal burning was an important source of the primary sulfate and chlorine in the atmosphere.We found that OM and soot particles from residential coal burning displayed extremely weak hygroscopicity.and OM-S particles displayed a dramatic size growth as the relative humidity(RH)increase.Therefore,inorganic salts within individual particles determined particle hygroscopic growth.Emissions from residential coal burning have effects on human health,air quality.and regional climates.Our results suggest that air quality improvements can benefit substantially from the replacement of low-and medium-maturity coals with high-maturity coals,natural gas or electricity in rural areas.