Particle Emissions from Vehicles
Aerosols from vehicle engines originate from five sources: fuel, fuel additives, inlet air, lubrication oil and the mechanical breakdown of preexisting materials. The latter can also form from other sources in the vehicle (e.g., brake dust). Of those formed in the engine, there are four main types of aerosol: carbonaceous, organic, sulfate and ash. These usually appear in combination. Fuel and oil contribute to all four fractions; fuel additives, air and mechanical breakdown contribute to the ash fraction. A typical aerosol from a heavy-duty diesel engine is 41% carbon, 13% ash, 14% sulfate/water, 25% unburnt oil and 7% unburnt fuel (Kittelson, 1998).
The number-based size spectrum (Figure 15.1) of an engine aerosol normally includes some of at least three distinct, lognormal modes (Kittelson, 1998). Homogenous nucleation of volatile materials in the exhaust (or ash particles) can form the so-called nucleation mode, which is usually smaller than 30 nm in size, with a narrow geometric standard deviation (<1.5). After this comes the accumulation mode, which is normally between 60 and 200 nm, with a general standard deviation (GSD) between 1.5 and 2.0. This is where the carbonaceous (‘soot’) agglomerate particles are usually found. Particles larger than this are referred to as the coarse mode and consist mostly of material produced outside the engine, such as brake dust and reentrained soot ...