Chemistry Reference
In-Depth Information
The literature suggests two other nucleation mechanisms, one involving oxidation of
terpenes or other organic compounds released from trees [
46
] and the other based upon
iodine oxides [
47
]. Heintzenberg et al. [
48
] summarised that new particle formation
events in the boundary layer generally show following distinct features. Firstly, a rapid
increase in number concentrations of particles below 20 nm due to nucleation and
subsequent particle growth into detectable sizes. Secondly, further particle growth
within the size range of the instrumentation, as a result of continuing condensation of
vapours. Finally, a gradual decrease in PNCs within several hours due to coagulation
and mixing with other air masses. Such a particle formation and successive growth is
informally called as “banana event” because of their appearance in two-dimensional
contour plots of PNCs as a function of time and particle diameter [
48
]. However, the
rate of nanoparticle formation varies in different environments. For example, Kulmala
et al. [
34
] concluded that the formation rate of 3 nm size particles within the boundary
layer, urban areas, coastal areas and industrial plumes can be in the range 0.01-10, up
to 100, 10
4
-10
5
and 10
4
-10
5
#cm
3
s
1
, respectively. They also reported that
particle growth rate varies between 1 and 20 nm h
1
.Thisgrowthrateismainly
driven by the ambient temperature, clumping, availability of condensable vapours,
condensation of other materials onto particle surfaces, deliquescence or hygroscopic
properties of particles (e.g. sea salt particles grow by collecting water) [
49
]. Ketzel
et al. [
50
] found that during periods with low primary particle emissions in
Copenhagen, particle formation events are due to nucleation in background air.
They observed the particle growth rate between 1 and 6 nm h
1
.Thesearesimilar
to those found at another European suburban background location in Prague where
the average value of particle growth rate was found to be about 5.4 nm h
1
[
51
]. These
are well within the range of 1-10 nm h
1
given for urban locations by Kulmala et al.
[
34
]. Ketzel et al. [
50
] also reported that total number of concentrations of particles
in Copenhagen increased by up to 5-10 times within a few hours in connection with
clean air and high solar radiation. Generally, new particle formation events occur in
defined conditions such as low wind speed, low relative humidity and high
global radiation. Other factors accompanying the new particle formation events are
lowered condensation sinks preceding the particle burst or low concentrations of SO
2
and NOx [
51
]. Detailed information on the formation and growth of particles in
different environments can be found in Holmes [
43
], Kulmala et al. [
34
] and the
references therein.
4.2 Anthropogenic Sources
Nanoparticles produced from anthropogenic sources can either be formed inadver-
tently as a by-product of combustion activities (i.e. emissions from road vehicles
and industries) or produced intentionally (e.g. ENPs) due to their particular
characteristics [
15
]. The following section describes the ENPs in detail and
provides only a brief overview of other anthropogenic sources.
