Physicochemical and optical properties of combustion-generated particles from a coal-fired power plant, automobiles, ship engines, and charcoal kilns

The physicochemical and optical properties of combustion-generated particles from various sources were investigated. Coal-fired power plants, charcoal kilns, automobiles, and ship engines were the major sources, representing combustions of coal, biomass, and two different types of diesel, respectively. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX) equipped with both SEM and HRTEM were used for physicochemical analysis. Light-absorbing properties were assessed using a spectrometer equipped with an integrating sphere. Particles generated from different combustion sources and conditions demonstrate great variability in their morphology, structure, and composition. From coal-fired power plants, both fly ash and flue gas were mostly composed of heterogeneously mixed mineral ash spheres, suggesting that complete combustion released carbonaceous species at high temperatures (1200–1300 °C). Both automobile and ship exhaust from diesel combustion show the typical features of soot: concentric circles comprising closely packed graphene layers. However, heavy fuel oil (HFO)-combusted particles from ship exhaust demonstrate more complex compositions containing a different morphology of particles than soot; for example, spherically shaped char particles composed of minerals and carbon. Regarding soot aggregates, particles from HFO combustion have different chemical compositions; carbon dominates but Ca (29.8%), S (28.7%), Na (1%), and Mg (1%) are also present, which were not found in particles from automobile emission. This indicates that fuel properties and combustion conditions are important in determining the fate of particles. Finally, from biomass combustion, amorphous and droplet-like carbonaceous particles with no crystalline structure were observed, and are generally formed by the condensation of low-volatile species at low-temperature (∼300–800 °C) combustion conditions. Significant differences in optical properties depending on the combustion source were observed. Diesel combustion particles from automobile and ship showed wavelength-independent absorbing properties, whereas the particles from coal and charcoal kiln combustion showed enhanced absorption at shorter wavelengths, which is a characteristic of brown carbon. Main light absorbers at short wavelengths are proposed for organics and minerals (e.g., iron oxide) for biomass and coal combustion, respectively. Our findings suggest that source-dependent properties and distributions across the globe should be considered when their impacts on climate change and air qualities are discussed.