The global environment is suffering from air pollution due to excess particulate matters, resulting in huge societal and economic costs. Air quality is usually characterized by the mass concentration of fine particulate matters with aerodynamic diameters smaller than 2.5 µm (PM2.5), which is mainly contributed by micron-sized particles, whereas the hazard induced by ultrafine particulates (with diameters smaller than hundreds of nanometers) remains seriously underestimated. In addition to their mass distribution, the size distributions of airborne particulates are becoming increasingly important for the evaluation of air hazards.