Formation of nanometer-sized particles by α-ray radiolysis in SO₂/H₂O/N₂ mixtures was experimentally investigated. The size and charge distributions of generated particles were measured simultaneously using a differential mobility particle sizer (composed of a Vienna-type differential mobility analyzer, a particle-size magnifier (PSM), and an optical particle counter) and a ultrafine aerosol visualization system (composed of the PSM and an observation cell), respectively. The size and number concentration of the generated particles increased with increasing SO₂ and H₂O concentrations, the irradiation time, and the strength of ²⁴¹Am α-source. When the SO₂ concentration was 1 ppm, maximum values for average particle diameter and number concentration were 5 nm and 2.5 × 10⁵ cm^-3, respectively. The fraction of charged particles was less than 0.1. Our results showed that ion-induced nucleation was a minor mechanism and that binary nucleation of H₂O molecules with H₂SO₄ molecules, produced by SO₂-OH radical reactions, was a major mechanism.

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Light absorption and extinction coefficient of atmospheric aerosols at and near the Uji campus of Kyoto University were measured with the integrating plate method and the telephotometer. Combustion processes were a major source of the aerosols, and a considerable light absorption is remarkable at the ground level of Uji area. The estimated single scattering albedo during this measuring period (September 13 to 22, 1993) for the aerosols was ranged from 0.6 to 0.7, and less than the critical single scattering albedo, 0.85.

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Morphological changes of two-component (Pb/NaCl) aerosol particles undergoing Brownian coagulation in a flowing system were studied by image processing of two-dimen-sional scanning electron micrographs of the particles. From the relationship between radii of gyration and perimeters of the particles, fractal dimensions were calculated considering the aggregates as multifractal (lower dimension d_l and higher dimension d_h). Changes in fractal dimension d_h were correlated to the degree of agglomeration.
 Chemical composition of individual particles was calculated by using diameter of primary Pb particles, side length of primary cubic NaCl particles and density of each constituents. Number distribution of the two-component particles was determined as a function of volume equivalent diameter and mass (or volume) fraction of the constituent, and time evolution of the distributions showed conversion of externally mixed aerosols to internally mixed ones due to Brownian coagulation. Mutimodality in NaCl mass distribution measured by cascade impactor could be reproduced by conversion of volume equivalent diameter to aerodynamic diameter using the primary particle sizes and density of two constituents.

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In order to reveal compositional differences between airborne road dust and airborne soil particles and to evaluate their local emissions in the early wintertime, the determination of geological profiles becomes a crucial and essential work. In this study, road dust and soil particles collected at the 7 sites of southern Kanto plain were prepared for the chemical analyses of apparent elemental carbon (Cae), apparent organic carbon (Cao), carbonate (CO₃^2-), elements and water soluble ions. Though the main components, Al, Fe, Mg, Na, Ti and K, were at the same levels of road dust with these of soil particles,the concentrations of Cae, Cao, Ca and CO₃^2- in the road dust samples were observed about one order higher than those in the soil samples. Cae, Cao, Ca and CO₃^2- can be the better indicators used for separating the contribution of road dust from soil particles with the Chemical Mass Balance (CMB) method. As the equivalent ratios of [Ca²⁺] / [CO₃^2-] for road dust samples were near 1.0, but ones for soil samples were about 10. Thus, the most of Ca²⁺ and CO₃^2- in road dust are probably existing as a form of CaCO₃ particle which has neutralizing effect on ambient aerosol acidity as a major alkaline component. A satisfied explanation for spatial differences and diurnal variations of ambient aerosol acidity (H⁺) was given using the CMB results in this study.

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