Individual particle analysis of high concentrated fine particulate matters (PM2.5) in Seoul using automated SEM-EDS

Abstract

Airborne particulate matter is composed of various molecules adhering to particles, including organic compounds, minerals, and metals. These particles come in different sizes and shapes depending on their emission source. It is widely recognized that smaller particles tend to contain higher concentration of heavy metals, underscoring the importance of understanding the composition and structure of these compounds in order to assess their potential harm to human health [1,2]. Consequently, scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) has garnered significant attention as a non-destructive method for analyzing the morphology and composition of individual particles [3,4]. Furthermore, the utilization of automated SEM-EDS analysis enables the examination of thousands of particles over a large area, facilitating statistical analysis [5,6]. In this study, our primary objective was to provide insights into the composition and structure of individual particles in fine dust, enhancing our understanding of the potential health risks associated with these airborne pollutants. Specifically, we focused on analyzing particulate matter smaller than 2.5 micrometers (PM2.5) in Seoul. The collection of airborne PM2.5 samples took place in Korea Institute of Science and Technology, Seoul, Republic of Korea between 2021 and 2023 during the winter season. Particle analysis was conducted on days with high concentrations of PM2.5 using automated SEM-EDS, with a specific focus on the morphology and compound forms of metal-based particles. The analysis employed a SEM instrument (Regulus 8230, Hitachi) equipped with a cold-field emission gun and EDS (Aztec 6.0, Oxford). The data collection process was automated using the Aztec Feature of the Oxford Aztec EDS software. For each sample, approximately 350 frames at a magnification of 10,000X were collected and analyzed, documenting 1,000 particles, irrespective of the number or size of regions of interest on the corresponding polycarbonate filters. Based on the analysis of 1,000 particles in each sample, the high-concentration PM2.5 samples collected in Seoul predominantly contain elements like Al, Si, Ca, Mg, Na, as well as small amounts of heavy metals, including Fe, Pb, Zn, Ti, Mn, and As. However, particles observed during days affected by yellow dust exhibit distinct morphological characteristics compared to those found in high-concentration PM2.5 samples. The yellow dust-affected particles are generally larger in size and tend to have aggregated small heavy metal particles. The automated SEM-EDS analysis method facilitates efficient and comprehensive particle characterization, offering valuable insights into their origins. This information can be used for various purposes, such as tracking pollution sources and predicting the impact of air quality on health.