Composition of Semi-volatile Particles from Diesel Exhaust

Vehicle exhaust particles from diesel passenger vehicles were studied in terms of volatility and chemical composition. Condensation of semi-volatile compounds leads to particle growth during exhaust dilution and cooling. The particle growth was observed to be particle surface related. At higher vehicle speed and load some of the semi-volatile material forms nucleation particles that are dominating the particle number concentration. The nucleation mode is completely volatile at 180°C and consists mainly of sulfate. The amount of organic material is smaller. The organics/sulfate ratio is larger for the soot mode indicating an earlier condensation process of organics before they are incorporated in the nucleation process. Under typical atmospheric dilution conditions most of the semi-volatile material is present in the soot mode. The semi-volatile material evaporates at temperature between 130°C and 180°C. Thermal treatment using a thermodenuder enables complete evaporation of the nucleation particles, however not all material from the soot particles is removed.