There are a wide variety of aerosols with different physical and chemical properties in the atmosphere. Being generated from various sources, these aerosols can influence public health and global climate both directly and indirectly. Every year, numerous studies are carried out to assess the chemical compositions of aerosols in various places around the world. Most of those studies rely on bulk analysis procedures which provide only the average compositions of particulate samples. Since atmospheric particles are chemically and morphologically heterogeneous, information concerning the average composition and aerodynamic diameter is not sufficient to describe the population of the particles. Thus, microanalytical methods have proven to be a reliable option for studying atmospheric particles.
A quantitative single-particle analytical technique, low-Z particle electron probe X-ray microanalysis (low-Z particle EPMA), has been successfully applied to characterize various types of individual aerosol particles. The low-Z particle EPMA based on SEM/EDX can be used to provide information on morphology and elemental concentrations. Herein, low-Z particle EPMA was applied for the characterization of various airborne particles such as lead-containing, airborne subway, floor dust, and magnetic subway particles.
In this dissertation, the first chapter presents the analytical results of ambient sinlgle particles collected near a lead smelter. Aerosol particle samples were collected over a 24-hour period, starting from 8 pm on 31 May 2002, using a high volume TSP sampler. For this near source sample, 73 out of 377 particles (accounting for 19.4%) were identified as lead-containing particles mixed with other species (S, Cl, K, Ca, and/or C), which should have been emitted from a nearby lead smelter. Lead-containing particles of less than 2 μm size in the near source sample accounted for the relative abundances of 42%. SEM-EDX analysis of individual standard particles, such as PbO, PbS, PbSO4, PbCl2, and PbCO3, was also performed to assist in the clear identification of lead-containing aerosol particles. Lead-containing particles were frequently associated with arsenic and zinc, indicating that the smelter had emitted those species during the non-ferrous metallurgical process. The frequently encountered particles following the lead-containing particles were mineral dust particles such as aluminosilicates (denoted as AlSi), SiO2, and CaCO3. Nitrate- and sulfate-containing particles were encountered frequently in 2 ? 4 μm size range, mostly in the forms of Ca(NO3, SO4)/C, (Mg, Ca)SO4/C, and AlSi+(NO3, SO4). Particles containing metals (e.g., Fe, Cu, and As) in this near source sample had relative abundances of approximately 10%. Although the airborne particles collected near the lead smelter contained elevated levels of lead, other types of particles, such as CaCO3-containing, carbonaceous, metal-containing, nitrates, sulfates, and fly-ash particles, showed the unique signatures of samples influenced by emissions from the lead smelter.
The second chapter presents the analytical results of subway particle samples collected at four underground subway stations in Seoul. To clearly identify indoor sources of subway particles, four sets of samples collected in tunnels, at platforms, near ticket offices, and outdoors were investigated. For the samples collected in tunnels, Fe-containing particles predominated with relative abundance of 75-91% at the four stations. The amounts of Fe-containing particles decreased as the distance of sampling locations from the tunnel increased. In addition, samples were also collected at the platform in subway stations with platform screen doors (PSDs) that limit air-mixing between the platform and the tunnel. The analysis of those samples showed marked decreases in relative abundances of Fe-containing particles, clearly supporting the possibility that Fe-containing subway particles are generated in the tunnel. PM10 mass concentration levels are the highest in the tunnels and decreased systematically with the increasing distance from the tunnel. The extent of the decrease in PM10 in stations with PSDs is also larger than those without PSDs. The results clearly indicate that Fe-containing particles originating from tunnels predominate in the indoor microenvironment of subway stations, resulting in high indoor PM10 levels. As such, PSDs are found to play a significant role in reducing Fe-containing particles at platforms and near ticket offices.
In the third chapter, floor dust subway particles were investigated to examine chemical composition of Fe-containing particles in details. Floor dust particles were collected at five sampling locations of an underground subway station, Jegi station, in Seoul, Korea. Size-segregated floor dusts were divided into magnetic and non-magnetic fractions using a permanent magnet. Using X-ray diffraction (XRD) and SEM/EDX, iron metal, which is relatively harmless, was found to be the dominating chemical species in the floor dusts of the < 25 μm size fractions with Mg, Al, Si, Ca, S, and C as minor components. From SEM analysis, the floor dusts of the < 25 μm size fractions collected on railroad ties appeared to be smaller than 10 μm, indicating that their characteristics should be comparable to those of airborne particles in the tunnel and the platform. As most floor dusts are magnetic, PM levels at underground subway stations can be controlled by removing indoor magnetic particles using magnets. In addition, airborne subway particles, most of which were smaller than 10 μm, were collected using permanent magnets at two underground subway stations, Jegi and Yangjae stations in Seoul, Korea. XRD and SEM/EDX analyses showed that most of the magnetic aerosol particles collected at Jegi station was iron metal, whereas those at Yangjae station contained a small amount of Fe mixed with Na, Mg, Al, Si, S, Ca, and C. The difference in composition of the Fe-containing particles between the two subway stations should be attributed to the different ballast tracks used.
As the fourth chapter in this dissertation, the speciation of mineral particles was performed on a single particle level for 24 mineral samples, including kaolinite, montmorillonite, vermiculite, talc, quartz, feldspar, calcite, gypsum, and apatite by the combined use of attenuated total reflectane FT-IR (ATR-FT-IR) imaging and low-Z particle EPMA techniques. Although the low-Z particle EPMA is powerful for characterization of individual particles, its limited capability for unambiguous molecular speciation is sometimes disadvantageous. FT-IR is a powerful technique for functional group analysis and molecular speciation of organic and inorgainc chemical compounds. Our previous work (Ryu and Ro, 2009) demonstrated the potential of the combined use of ATR-FT-IR imaging and a quantitative ED-EPMA, low-Z particle EPMA, techniques for the characterization of individual aerosol particles. These two single particle analytical techniques provide complementary information, the ATR-FT-IR imaging on mineral types and low-Z particle EPMA on the morphology and elemental concentrations, on the same individual particles. This work demonstrates that the combined use of the two single-particle analytical techniques can powerfully characterize externally heterogeneous mineral particle samples in detail, while offering great potential for the characterization of airborne mineral dust particles.