Aerosol formation via a reaction from low concentration SO₂ (sub-ppm level) was carried out using a flow-type reactor and the change in gaseous species was simulated by a numerical calculation. Particle generation was observed when 248 nm laser beam was irradiated to the gas mixture containing SO₂, O₃ and H₂O vapor. The amount of SO₂ was reduced by 21 % at initial concentrations of SO₂ = 179 ppb and OH = 183 ppb, and the number concentration of generated particles ranged from 6 × 10⁶ to 9 × 10⁶ cm ^-3. The generated particles were less than 100 nm in diameter and SO₄^2- concentration was 70 wt% at the first stage of nucleation, which decreased rapidly to less than 10 wt%. Sulfuric acid molecules were detected as a precursor of nucleation by a mass spectrometer. It was suggested that sulfuric acid molecules efficiently attracted not only water molecules but also water clusters and small water droplets during the gas-to-particle nucleation process.

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Single-scattering albedo describes the relative amount of scattering and absorption of light by aerosol particles and is one of the most important parameters for the evaluation of the effect of the atmospheric aerosol on climate. Despite this fact, there are very few data on this property and in most cases the existing data do not take into account the effects of relative humidity. Moreover, the absorption coefficient, which is used to calculate single-scattering albedo, usually lacks a method dependent calibration. We performed a closure study on the aerosol extinction coefficient by measuring extinction directly in the atmosphere and compared it with simultaneously measured scattering and absorption coefficients for five days in October and November 1999 in Kyoto, Japan. We found that relative humidity plays an important role in an accurate determination of the single-scattering albedo. An error analysis showed an increase of uncertainty for decreasing values of the single-scattering albedo and the resulting error depended strongly on the method of calculation. Closure tests showed that extinction coefficients obtained from in-situ methods compared to ambient values are mostly within error bars.

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Natural radionuclides, i.e., thorium isotopes, in dry and wet deposition samples were measured at Tsukuba, Japan, as were their surface air concentrations, in order to understand dry and wet removal processes of the natural radionuclides. The dry deposition velocities of thorium isotopes, calculated from surface air concentrations and dry deposition fluxes, showed difference among isotopes and temporal variations. The dry deposition velocities of thorium isotopes were greater than that of the anthropogenic radionuclides. The wet deposition velocities of thorium isotopes for individual rainfall events varied largely. The wet deposition velocity is given as the product of washout ratio and rainfall rate. Typically it was found that the washout ratios of thorium isotopes show a temporal variation. In order to explain this phenomenon, we examined the relationship between the washout ratios and rainfall rate and found a negative correlation between washout ratios of thorium isotopes and rainfall rate. The result suggests that factors controlling the wet removal of the thorium isotopes for individual rainfall events is surface air concentrations, particle sizes and rainfall rate rather than precipitation amount, which is in agreement with previous understandings. As a result, fractionation among thorium isotopes in surface airborne particles is partly controlled by wet and dry removal processes.

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We previously reported that ultrafine particles of the Y-Fe-O system (1-100 nm in size) prepared by a radio-frequency thermal plasma process contained a new compound (cubic in crystal system, ferri- or ferromagnetic). Although its crystal structure was not yet clarified in the present study, the composition was estimated to be YFe_xO_1.5(1+x)(x≅3.5, const.) by high accurate lattice constants and Fe/Y of products measured by ICP-ES. The relationship between Fe/Y ratio of precursors and by-product formations and the replacement of Y by each of Er, Ho, Dy and Nd were also examined. The content of hexagonal-YFeO₃ (h-YFeO₃ : by-product) was found to be minimized at around Fe/Y = 4. Therefore the replacement was performed at this metal ratio because by-product was easily removed. Er, Ho and Dy formed almost a single phase of similar compounds, and their lattice constants could be determined on the basis of the cubic crystal system. Nd, however, resulted in the formation of mixtures of orthorhombic-NdFeO₃, α-Fe₂O₃ and unidentified compounds. Er, Ho and Dy are nearly equal to Y in ionic radius, Whereas Nd is larger than Y, indicating that the rare-earth ions with almost the same size as Y-ion well match the crystal lattice of the new compound.

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