Aerosol particles in the atmosphere influence global climate by changing the radiative budget, i.e., they scatter and absorb sunlight. The magnitude of their scattering and absorption depends on the amount, sizes, compositions, mixing state, and shapes of particles. The effects of particle shape on their optical properties are difficult to estimate since the techniques to quantify and evaluate the shape of tiny particles are limited. In this report, electron tomography technique, which can show the three-dimensional (3D) particle shape, is reviewed. Electron tomography makes 3D digital data of materials smaller than ~1 micrometer from a series of transmission electron microscopy (TEM) images. Instead of measuring particle height, electron tomography obtains the 3D information from the tilting angles of material of interest. Such 3D digital data can be used to evaluate its optical properties using discrete-dipole approximation (DDA). The technique has been used for soot particles, which commonly have complicated structures that influence their optical properties. Measurements of 3D shape and optical properties of aerosol particles using electron tomography and DDA improve the estimation of their optical properties and evaluation of their climate effects.
Environmental electron microscopy is a new method that enables in-situ dynamic observation of nano-scale behavior of aerosols and materials. Two types of instruments, environmental transmission electron microscope (ETEM) and environmental scanning electron microscope (ESEM), have been developed and applied to various experiments since the 1990’s. The present article briefly describes the basic principles and instrument systems of ETEM and ESEM, and then shows four examples of ESEM experiments on the hygroscopic behavior of aerosol and artificial small particles, i.e., the influence of nitrate and sulfate on hygroscopicity of Kosa particles, the deliquescence of sea-salt particles, the characteristics of small ice crystals on several kinds of ice nuclei, and the change in hygroscopicity of nitrate particles caused by polymer coating. In conclusion, ETEM and ESEM have a big potential for applications in the fields of aerosol research and engineering.
Epifluorescent microscopy, coupled with fluorescence staining techniques, is useful for observations and quantitative estimation of airborne microbial communities (bioaerosols). Irradiation of short-wavelength excitation light onto a specimen results in luminescence of longer-wavelength fluorescence from materials in the specimen. Various fluorescent chemicals are used as stains bound to specific biomaterials. DAPI staining enables detection of all microbial cells in aerosol samples. Live/Dead staining method can estimate concentrations of both living and dead cells among microbial assemblages. Fluorescence in situ hybridization (FISH method) can be used as nucleotide probes for specific microbial species among bioaerosols. Epifluorescent microscopic observations provide information essential for elucidation of ecological characteristics of bioaerosols in atmospheric environments.
In this report, recent advances in micro Raman spectroscopy and related research on atmospheric aerosols have been compiled. Special emphasis is on the effective use of the electron based X-ray micro analysis (e.g. SEM, TEM-EDX and EPMA) in combination with the micro Raman spectroscopy on individual mineral dust aerosols. Also, successful applications of micro Raman spectroscopy on the detection and identification of bioaerosols are described. There have been remarkable progresses in the advanced Raman scattering techniques such as the resonance Raman spectroscopy, Surface Enhanced Raman Scattering (SERS), and the ‘state-of-the-art ’near-field Raman spectroscopy, that hold considerable promise in the future application on aerosol research.
Non-destructive simultaneous analytical method for multi-elements in fine particulate matter (PM2.5) was developed using the latest type of energy dispersive X-ray fluorescence spectrometry (EDXRF) equipped with three-dimensional polarization optics and secondary targets. Analytical results obtained by EDXRF for S, Al, K, Ca, Ti, Cr, Fe, Cu, and Zn agreed well with the certified values of SRM2783 (Measured/Certified＝0.8-1.2). Analytical results obtained by EDXRF for Al, K, Ca, V, Cr, Fe, and Zn in PM2.5 collected in Yokohama, were also in good agreement with those obtained by ICP-MS method (EDXRF/ICP-MS＝0.8-1.2). Furthermore, analytical results for all target elements (Al, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, and Pb) obtained by modified EDXRF method using the matching library technique agreed very well with those obtained by ICP-MS method (EDXRF/ICP-MS＝0.9-1.1). We achieved the multi-elemental analysis by using EDXRF coupled with fundamental parameter (FP) quantification as fast as 900 s (15 min) per sample. EDXRF method does not require any sample pretreatments prior to analysis. EDXRF method developed in this study would be quite useful for analyzing a large number of PM2.5 samples for detailed monitoring purposes.