The Origins of Nanoparticle Modes in the Number Distribution of Diesel Particulate Matter

The measurement of the number distribution of nanometer size particles (nanoparticles) in the diesel exhaust emission is important in order to evaluate their environmental and health impact, and to develop new types of diesel particulate filters (DPFs). Generally, the distributions of diesel particulate matter are complicated in the nanometer size regime that contains some peaks depending on the various particle formation mechanisms. They are strongly influenced by the homogeneous and heterogeneous nucleation of heavy hydrocarbons in the dilution process, and nanoparticles are also generated even in the aerosol chargers used for the electrical aerosol measurement system.

In this study, we directly sampled particulate matters emitted from a diesel truck mounted on the chassis dynamometer by a flow separator and dilution system, and measured the nanoparticles (size range; 3 to 200nm) using two types of differential mobility analyzers combined with a Faraday cup electrometer (FCE) and a condensation nucleus counter (CNC). The particle size distributions were analyzed and the origins of the nanoparticle modes are clarified by changing the experimental parameters such as engine load, dilution ratio, temperature and the gas concentrations. As a result, the measured distributions proved to be bimodal or trimodal depending on the experimental conditions.