Particulate matter present in air is divided into different categories
depending on the size of the particles (aerodynamic diameter).
Fine particles are airborne particles which are smaller than
coarse particles. They have an aerodynamic diameter of 2.5 µm or less
(PM2.5). The fine particles which are smaller than 0.1 µm are
referred to as ultrafine particles (PM0.1).
Fine particles are largely formed from gases.
Ultrafine particles are formed by nucleation, which is the initial
stage in which gas becomes a particle. These particles can grow up to a size
of 1µm either through condensation, when additional gas condensates on the
particles, or through coagulation, when two or more particles combine to
form a larger particle.
Please note that ultrafine particles (PM0.1) are part of the fine