Page 419 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

13.3 Properties of Nanoaerosol 399

13.3 Properties of Nanoaerosol
13.3.1 Number and Size of Nanoaerosol Particles

As shown Fig. 13.2, by number, nanoaerosol particles constitute 90 % or more of
ambient aerosols, although they only account for a very small fraction of the total
mass [48].
The small size of nanoparticles leads to a great surface area to mass ratio and
consequently other unique properties of nanoparticles. The high surface area leads
to a great surface reactivity. On a nano scale, both classic physics and quantum
physics play roles in the interfacial behavior of nanoaerosols.
Some researchers believe that high concentration nanoparticles in the air
agglomerate rapidly to form larger particles by chemical bonding and physical
reactions [51]. As a result, the lifespan of individual nanoaerosol particles is usually
short. Nanoparticles with the sizes of 1–10 nm have the lifespan of a few minutes to
hours [2]. Meanwhile larger nanoparticles are formed by agglomeration. As a result,
the size distribution of the nanoaerosol in a certain environment may change over
time. The agglomeration mechanisms of nanoaerosols are not yet well understood.
On the other hand, well-dispersed nanoaerosols at low concentration may remain
airborne for a long period of time; their setting velocity is extremely low because of
their small aerodynamic sizes. At low concentration, the chance for nanoaerosol
particles to agglomerate is low because of their great impact velocities.

Number Volume
Distribution Distribution

10 100 1000 10000
Particle diameter (nm)
Nanoaerosol

PM
2.5
PM
10
Fig. 13.2 Aerosol number and mass distribution versus size