Assessment of the Sphericity Characteristics of Submicron Particles Using a Single-particle Polar Nephelometer at an Urban Site in Japan

ABSTRACT Sphericity is one of the essential properties of particles in evaluating their effects on climate and human health. With the use of a home-made polar nephelometer, real-time measurements of the scattering angular distribution of individual particles with a diameter of 500 nm, with and without passing through the thermodenuder (TD), were conducted in Nagoya, Japan, to examine particle sphericity and its controlling factors. Particle sphericity was estimated from the depths of local minima in the scattering angular distributions, and ambient aerosols were found to be external mixtures of at least two types of particles with relatively high and low sphericities. Although most particles with higher sphericity were removed with passage through the TD, approximately one-third with lower sphericity remained. In the daytime, the proportion of particles with low sphericity were lower and the average sphericity of particles with high sphericity increased, which can be explained by the photochemical formation and/or aging processes. For days when the relative humidity was extremely high, another peak in the diurnal variation of the average sphericity was found in the early morning, which is possibly due to secondary formation of nitrate.