Chemistry Reference
In-Depth Information
Future Research Directions
Understanding of the safe design and use of airborne ENPs and their health and
environmental related impacts is still in its infancy. There are still numerous
questions to be answered. Few of them were highlighted by Kumar et al. [
4
] as:
(1) do the characteristics of the ENPs differ from those of other airborne
nanoparticles? (2) should the ENPs be regarded as an ENPs, especially in outdoor
environments? (3) what should be an appropriate measurement metric to represent
their health impacts? (4) can the same instruments be applied to measure airborne
ENPs and other nanoparticles? (5) are the dispersion characteristics of ENPs and
other nanoparticles similar? and (6) is exposure of ENPs a major concern? Further,
Morawska [
22
] extended the list of questions related to the exposure of ENPs to: (7)
are the particles in the nano-size range more toxic than larger particles of the same
material? (8) does the surface chemistry of the lung alter the toxicity of inhaled
particles? (9) do nano-fibres pose the same risk as toxic fibres such as quartz and
asbestos? and (10) do the currently deployed methods assess the health risk
appropriately? Currently, most of the above questions cannot be answered precisely
until more comprehensive information on these topics becomes available. Descrip-
tion of the various aspects of the ENPs is discussed in a nutshell here due to brevity
reasons but further details related to sources, characteristics, toxicity, physical and
chemical interactions of ENPs can be found in recent reviews by Handy et al. [
67
,
68
], Valant et al. [
69
], Bystrzejewska-Piotrowska et al. [
70
], Ju-Nam and Lead [
16
],
Kumar et al. [
4
] and Peralta-Videa et al. [
55
].
4.2.2 Road Vehicles and Other Anthropogenic Sources
Numerous studies based on the mass metric show that vehicular sources can
comprise up to 80% of total PM
10
and/or PM
2.5
in urban areas [
71
-
77
]. Similarly,
road vehicles dominate with their contribution towards the total PNCs in urban
environments. These can contribute up to 86% of total PNCs in the polluted urban
environment depending on the measurement location, meteorological and traffic
conditions [
2
,
18
,
19
,
78
]. A majority of these PNCs generally fall below 300 nm
diameter [
79
-
82
]. While most of the PNCs are contributed by particles in the
ultrafine size range (i.e.
100 nm), majority of the mass concentrations are
contributed by particles over 100 nm in size [
27
]. For instance, Charron and
Harrison [
31
] observed about 71-95% of total PNCs in central London in the
11-100 nm size range. There can be an appreciable contribution from particles
below 10 nm, mainly arising from secondary formation, towards the total PNCs
[
83
]. For example, Shi et al. [
84
] found this contribution between 36% and 44% at
roadsides in Birmingham, UK. This contribution was between 16% and 24% in the
3-10 nm range in Leipzig, Germany [
85
], and between 4% and 12% in Cambridge,
UK for the 5-10 nm size range [
44
].
Other anthropogenic sources of nanoparticles can include: brake and tyre wear
[
40
], industrial emissions such as from power plants [
86
], idling, taxiing and
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