Characterization of PM2.5 Carbonaceous Particles with a High-Efficiency SEM: A Case Study at a Suburban Area of Xi’an

In this study, a computer-controlled scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (CCSEM-EDX) was used to analyze the morphologies and chemical compositions of more than 10,000 individual particles in daytime and nighttime PM2.5 samples collected in suburban Xi’an. The particles in the PM2.5 samples were divided into 7 categories according to their elemental compositions, among which carbonaceous particles in the submicron size range were predominant in all samples (> 90% in numbers). We found that about 20% (in numbers) of carbonaceous particles contained elemental fluorine. The weight percentage of fluorine in the particles ranged between 0.1 and 0.8%. The morphologies and elemental distribution of the fluorine-containing particles indicate the particles were very likely from fossil fuel combustion and waste burning. Results presented in this case study are worthy of in-depth study because of the predominance of these internally mixed particles in the atmosphere and their possible risk to people’s health and the environment.