HeeJin Hwang

Investigation of the Chemical Mixing State of Individual Asian Dust Particles by the Combined Use of Electron Probe X-ray Microanalysis and Raman Microspectrometry

In this work, quantitative electron probe X-ray microanalysis (EPMA) and Raman microspectrometry (RMS) were applied in combination for the first time to characterize the complex internal structure and physicochemical properties of the same ensemble of Asian dust particles. The analytical methodology to obtain the chemical composition, mixing state, and spatial distribution of chemical species within single particles through the combined use of the two techniques is described. Asian dust aerosol particles collected in Incheon, Korea, during a moderate dust storm event were examined to assess the applicability of the methodology to resolve internal mixtures within single particles. Among 92 individual analyzed particles, EPMA and RMS identified 53% of the particles to be internally mixed with two or more chemical species. Information on the spatial distribution of chemical compounds within internally mixed individual particles can be useful for deciphering the particle aging mechanisms and sources. This study demonstrates that the characterization of individual particles, including chemical speciation and mixing state analysis, can be performed more in detail using EPMA and RMS in combination than with the two single-particle techniques alone.

Single-particle characterization of summertime Antarctic aerosols collected at King George Island using quantitative energy-dispersive electron probe X-ray microanalysis and attenuated total reflection Fourier transform-infrared imaging techniques.

Single-particle characterization of Antarctic aerosols was performed to investigate the impact of marine biogenic sulfur species on the chemical compositions of sea-salt aerosols in the polar atmosphere. Quantitative energy-dispersive electron probe X-ray microanalysis was used to characterize 2900 individual particles in 10 sets of aerosol samples collected between March 12 and 16, 2009 at King Sejong Station, a Korean scientific research station located at King George Island in the Antarctic. Two size modes of particles, i.e., PM(2.5-10) and PM(1.0-2.5), were analyzed, and four types of particles were identified, with sulfur-containing sea-salt particles being the most abundant, followed by genuine sea-salt particles without sulfur species, iron-containing particles, and other species including CaCO(3)/CaMg(CO(3))(2), organic carbon, and aluminosilicates. When a sulfur-containing sea-salt particle showed an atomic concentration ratio of sulfur to sodium of >0.083 (seawater ratio), it is regarded as containing nonsea-salt sulfate (nss-SO(4)(2-)) and/or methanesulfonate (CH(3)SO(3)(-)), which was supported by attenuated total reflection Fourier transform-infrared imaging measurements. These internal mixture particles of sea-salt/CH(3)SO(3)(-)/SO(4)(2-) were very frequently encountered. As nitrate-containing particles were not encountered, and the air-masses for all of the samples originated from the Pacific Ocean (based on 5-day backward trajectories), the oxidation of dimethylsulfide (DMS) emitted from phytoplanktons in the ocean is most likely to be responsible for the formation of the mixed sea-salt/CH(3)SO(3)(-)/SO(4)(2-) particles.

The influence of collecting substrates on the single-particle characterization of real atmospheric aerosols.

This work investigated the influence of three different collecting substrate materials, Ag and Al foils and grids for transmission electron microscopy (TEM grid), on the morphological and chemical compositional analysis of individual particles collected at an underground shopping area in Seoul, Korea. The feasibility of using each substrate in a quantitative single-particle analysis was evaluated by comparing particle morphologies, X-ray spectra, and elemental quantification results obtained for the three substrates. The morphologies and the quality of X-ray spectra for crystalline mineral particles were very similar among the three substrates. However, water-soluble, CNO-rich aerosols showed different morphologies among the three substrates, mainly due to the differences in the hygroscopic properties of the substrates. The quality of the X-ray spectra of the CNO-rich particles was optimal when collected on the TEM grid. To reliably assess the characteristic X-rays of the CNO-rich particles collected on the Ag and Al foils, appropriate data analysis had to be applied. Especially, the X-ray spectra of the CNO-rich particles collected on Al foil required a new background subtraction procedure. The overall relative abundances of the chemical species, obtained from the three collecting substrates, were in good agreement with each other and single-particle characterization of the real aerosol sample was feasible on the different substrates. However, the TEM grid substrate was the most appropriate for single-particle analysis of the water-soluble CNO-rich particles as: (i) it retains the original morphology and size of the particles, (ii) it allows high contrast in the backscattered electron image (BSEI) mode, and (iii) it provides a high peak-to-background ratio (P/B) with small and correctable interferences in the X-ray spectra.

Molecular mass concentrations for a powdered SRM sample using a quantitative single particle analysis

In a quantitative single particle analysis, named the low-Z particle EPMA, number concentration data for chemical species encountered in aerosol sample are provided. However, it will be more useful if mass concentration data can be obtained from single particle analysis; i.e., the single particle analysis data for weight fractions of chemical species can be complementarily used in combination with the bulk analysis data, for more clearly understanding the behavior of airborne aerosols. In order to investigate how reliably mass concentration data can be obtained from the low-Z particle EPMA technique, a potassium feldspar powdered standard reference material (SRM), of which elemental weight fractions are well defined by various bulk analytical techniques, was analyzed using the low-Z particle EPMA technique. In this work, it is demonstrated that weight fractions of major elements in the powdered SRM sample obtained by the low-Z particle EPMA are within 8% to the certified values obtained by bulk analytical techniques, although the single particle and bulk analyses employ different approaches. Further, it is shown that the quantitative single particle analysis, i.e., low-Z particle EPMA, can provide molecular mass concentration data for chemical species, which is not easy to obtain using bulk analysis.

Single-particle Characterization of Aerosol Samples Collected at an Underground Shopping Area

A single particle analytical technique, named low-Z particle electron probe X-ray microanalysis, was applied to characterize four samples collected at an underground shopping area connected to Dongdeamun subway station, in January and May 2006. Based on the analysis of their chemical compositions of the samples, many distinctive particle types are identified and the major chemical species are observed to be soil-derived particles, iron-containing particles. sulfates. nitrates, and carbonaceous particles. which are encountered both in coarse and fine fractions. Carbonaceous particles exist in carbon-rich and organic. Soil derived particles such as aluminosilicates, AlSi/C, are more frequently encountered in spring samples than winter samples. Nitrate- and sulfate-con taming particles are more frequently encountered in winter samples, and those nitrate- and sulfate-containing particles mostly exist in the chemical forms of . Fe-containing particles which came from nearby subway platform are in the range of about 10% relative abundances for all the samples. It is observed that nitrate- and sulfate-containing particles and carbonaceous particles are much more frequently encountered in indoor aerosol samples than in outdoor aerosols, implying that , and VOCs at the underground shopping area were more partitioned into aerosol phase.