Epidemiological studies have shown that concentrations of airborne PM are correlated with hospital admissions and death rates. Initially, the mass concentration of airborne particles with a diameter of less than 10 μm (PM10) is identified as a key metric in relation to health outcomes. However, more recent research has suggested that smaller particles are more important. Attention has focused on particles having a diameter of less than 2.5 μm (PM2.5), although there is still a debate as to whether it is actually the mass of even smaller particles, or indeed a non-mass metric such as particle number (PN)7, that is primarily responsible for health effects.
In addition to health, airborne particles are responsible for a range of other adverse effects, including nuisance and visibility reduction.
Particles in diesel exhaust have a range of sizes, and the shape of the size distribution depends on whether the weighting is by number or by mass. There are three distinct size modes: the nucleation mode (also referred to as nuclei or nanoparticles), the accumulation mode, and the coarse mode. The nucleation mode has traditionally been defined as particles with a diameter of less than 50 nm. Accumulation mode particles range in size from around 50 nm to around 1 μm, with particles smaller than 0.1 μm being referred to as ‘ultrafine’. The nucleation mode contains many more particles than the accumulation mode, although because each particle is so small the total mass is lower. The coarse mode consists of particles larger than around 1 μm.
The main implication of the particle size distribution in vehicle exhaust is that the instruments used in testing need to be sensitive enough to measure particles in the relevant size.