Aerosol size distribution When the size distribution of particles in an aerosol is obtained, there is always a bimodal distribution. The small peak accounts for the particles that are in sub-nanometer size range and the larger peak accounts for the particles that are in nanometer to micrometer size range. Why does it show a bimodal distribution? What is the physical reason for this occurrence?  
 A: There may be several reasons for this.
In general the bimodal character of atmospheric aerosols is the owed to different lifetime of particles. These are dependent on the particle size and loss mechanisms predominantely associated with the respective sizes.
Usually atmospheric aerosols consist of four size modes. From smallest to biggest median, these are:

*

*Nucleation Mode: Particles originate by means of homohenous/heterogenous nucleation triggered by gas phase reactions. Particles within this mode rapidly grow out of this mode due to condensation, coagulation and especially diffusion (Diffusion is highest for small particles).


*Aitken Mode: Particle grow out of this mode due to coagulation. (soot particles)


*Accumulation Mode: Usually has the longest liftime of all modes ranging from a few days to a few years (dependent on the height (atmospheric layer)). Predominant loss mechanism is wet deposition. (e.g. smoke particles)


*Coarse Mode: Due to their size, these particles are usually heavy and drop out ambient air by means of sedimentation quiet quickly (e.g. Mineral dust, vulcan ashes, sea salt).
Dependent on the circumstances a mode may be more dominant than others, while another may hardly be present resulting in a bimodal character.
Concerning Sub Nanometer particles. I am not quite sure wheter these particles are considered to be aerosol particles still, as they approach molecular dimensions and physics may be different there.
