Aerosol particle size distributions with one two three four and even five modes are called unimodel bimodal quadramodal or pentamodal respectively. Distributions may include one or two nucleation modes one or two accumulation modes and a coarse mode. A nucleation mode is a mode with mean diameter smaller than 0.1nm . Thus nucleation mode particles are nanoparticles or ultrafine particles. The nucleation mode contains newly nucleated particles (those formed directly from the gasphase ) or small emitted particles. Nucleated particles are between 1and 3 nm in diameter because they consist of a small cluster of gas molecules that have changed phase to a liquid.
Two nucleation modes often exist in the case of vehicle emissions. One mode has a peak smaller than 10 nm