number concentrations of particles per unit volume of air
(Morawska et al., 2008), and these particles can originate directly
by combustion (primary particles) or gas–to–particle conversion
(secondary particles). The details about this topic can be found in
the studies of Morawska et al. (2008) and Kumar et al. (2010).
These authors also reported that the sizes of particles depend on
the multiplicity of sources and processes that lead to their
formation, and therefore, on the material from which the particles
were formed.