Fresh organic and soot particles from crop straw burning: Morphology, composition and size distribution

Abstract

Particulate matter from biomass burning is considered to be one of the major sources of air pollution in China. To examine the characteristics of the particles emitted from the burning of crop straw, an open burning experiment is designed. Individual particles are collected in the flaming and smouldering stages, and analysed by individual particle analyses represented by high‐resolution transmission electron microscopy equipped with the energy‐dispersive X‐ray spectrometer. Five types of particles are identified, namely, pure organic particle, organic particle‐K (OP‐K), tar ball, K‐rich and soot particles. Of these, the OP‐K particles have the highest relative number percentage in all analysed particles in both the flaming and smouldering stages. Interestingly, the microscopic morphology of organic particles is completely different in the flaming and smouldering stages, being near‐spherical in the flaming stage and gel‐like in the smouldering stage. According to the elemental compositions, most of OP‐K particles contained potassium compounds in the form of KCl and K2SO4. High combustion efficiency and temperature promote the formation of crystal structure, resulting in the onion‐like microstructure of soot particles. Apparently, the soot microstructure is a fingerprint feature that helps identify combustion sources. In addition, the particles emitted in the flaming stage are smaller than those in the smouldering stage, implying that high temperature combustion tends to be associated with high combustion efficiency. The relationship between burning temperatures and particle sizes can help to understand the emission of particles from crop straw burning and find new ways to solve the problems caused by straw burning. The results of this study can provide more detailed ‘fingerprint’ characteristics of the particulate matter emitted from biomass‐burning sources, and an understanding of their potential impact on the atmospheric environment.