Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 178

5.4 Fate and Behaviour of Atmospheric Nanoparticles

As with other particles, nanoparticle growth occurs through coagulation, conden-
sation of low volatility components and surface reactions. However, because their
dimensions are similar to those of gas molecules, the smallest ones show unusual
physical properties (Kelvin effect, properties of the free molecular regime,
Brownian motion for coagulation, etc.) that need to be taken into account (Lehtinen
and Kulmala, 2003). On the other hand, nanoparticle shrinkage occurs through
evaporation (because of their volatile nature). Condensation and evaporation lead
to a change in the particle diameter and particle mass but not to the overall particle
number concentration. On the other hand, coagulation and fragmentation lead to
a change in the particle number concentration and change in the particle diameter
but the particle mass is conserved. Parallel to these transformations, NPs may also
be lost from the atmosphere by dry and wet deposition. Deposition processes lead
to a reduction in particle number concentration and a shift in the size distribution
to larger diameters, since both processes are more effi cient for small particles than
for coarser particles. All these processes may occur during the aging and transport
of fresh emissions and generally lead to a shift in the particle size distribution to
larger diameter, even though shifts to smaller diameters are also observed at urban
scales. The fundamental theories of these processes are not the purpose of this
chapter and the reader is referred elsewhere for details (Seinfeld and Pandis,
1998 ).
Condensation corresponds to the surface deposition of gases of low volatility and
this partitioning between gas and particle phases to reach the equilibrium concen-
trations at atmospheric conditions is reversible in the absence of surface reaction.
This means that when the vapour phase concentrations are low, NPs can also shrink
by evaporation. The formation of involatile oligomers from more volatile organic
monomers in the aerosol phase after condensation was demonstrated by Vesterinen
et al. (2007) and such reactions help in understanding observed growth rates in the
atmosphere. The examination of condensation and evaporation processes for NPs
requires consideration of the Kelvin effect (the saturation pressure goes to infi nity
as the particle radius goes to zero). Thus, saturation pressure increases as the par-
ticle size decreases and this means that the smaller the particles the higher the
supersaturation ratios required for condensation. Similarly, the smaller the particles
the faster the evaporation due to the Kelvin effect. Consequently, for the smallest
NPs (below 10 nm in diameter), the Kelvin effect prevents condensation of any
semi-volatile compounds in suffi cient abundance for producing substantial growth,
and heterogeneous reactions are thought to play a major role in the rapid growth
of such small particles and to facilitate further condensation of organic vapours
(Zhang and Wexler, 2002; Anttila and Kerminen, 2003).
Coagulation is the process in which two particles collide and stick, which is
slow except at high number concentrations. In a polydisperse aerosol, coagulation
is more effective between particles of different sizes, since large particles provide
a large surface for absorption and small particles have a greater diffusion rate.
For accumulation mode particles, every collision would result in coalescence or