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The sphericity of particles must be considered when evaluating their effects on the climate and human health. Thus, to examine this property and its controlling factors, this study measured the scattering angular distributions of both thermodenuded and non-thermodenuded individual particles with a diameter of 500 nm in real time using a home-made polar nephelometer in Nagoya, Japan. Estimating the sphericities based on the depths of the local minima in the scattering angular distributions, we found the ambient aerosols to be external mixtures of at least two types of particles, one with relatively high and the other with relatively low sphericity. Although most of the particles exhibiting high sphericity were removed as they passed through the thermodenuder, approximately one-third of the fraction exhibiting low sphericity remained. During the daytime, the proportion of the low-sphericity particles decreased, whereas the average sphericity of the high-sphericity particles increased, which can be attributed to photochemical formation and/or aging processes. On days with extremely high relative humidity, the diurnal variation in the average sphericity displayed another peak during the early morning, which may have been due to the secondary formation of nitrate.

The primary measurement standard for the number concentration of liquid-borne particles adopted by the National Metrology Institute of Japan was upgraded to cover particle diameters in the range of 600 nm–10 µm. The main aim was to report the extension of the lower limit of the measurement range in particle diameters from 2 µm to 600 nm, and the reduction of the measurement uncertainty inherent in the previous method using the total-counting-type flow cytometric technique. To extend the measurement range, we adopted a mass-measurement-type optical particle counting method, which consisted of a pair of optical particle counters with a pulse height analyzer, an electronic balance, and a syringe sampler. The relative expanded uncertainties with a coverage factor of k = 2 for the 600 nm and 10 µm suspension measurements were 4.3% and 3.5%, respectively. The validity of the results was confirmed in both cases via comparison with the results obtained using an independent microscopic counting method. With the change in the measurement method, a reduction of the uncertainty was accomplished: e.g., from a 4.4% to a 3.5% relative expanded uncertainty (k = 2) for a 10 µm polystyrene latex particle suspension.