Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 168

high fraction of volatile material (Kittelson, 1998). Kittelson et al. (2006b) investi-
gated particle number emissions from seven petrol vehicles operated on-road and
on a chassis dynamometer. Particle number emissions were highly dependent on
driving mode. Higher emissions were observed during high speed highway cruise
conditions, acceleration and at cold start temperatures. Contrary to previous fi nd-
ings, no typical particle signature was found. However, they showed that many of
the particles were smaller than 10 nm and the relative contribution of light duty
petrol vehicles to particle number emissions increased as particle size decreased.
Such very small particles are rarely measured in the roadside atmosphere because
of instrument limitations. Kittelson et al. (2006b) also showed that high mileage
petrol vehicles emitted a greater number of nuclei mode particles compared to the
lower mileage petrol vehicles. Interestingly, they demonstrated that when a per
vehicle basis is used heavy duty vehicles produced much greater particle number
concentrations than light duty vehicles (as indicated above), while particle number
production is only slightly higher when a fuel-specifi c basis is used. Despite the
smaller contribution of petrol engines than diesel engines, apportionment results
show that weekend production of particle number in the United States was attrib-
utable to light duty petrol vehicles (Kittelson et al. , 2006b ).
A fraction of NPs emitted by on-road vehicles corresponds to non-exhaust emis-
sions. However, very few studies are currently available and the contribution of
non-exhaust emissions to current (and future) NPs emissions requires further
investigation. Brake wear particulate matter emissions were determined by Garg
et al. (2000). On average, 35% of the brake pad mass loss was emitted as airborne
particulate matter with on average 33% (in mass) of the airborne particulate matter
smaller than 100 nm. Sanders et al. (2003) showed a larger proportion of brake wear
debris being airborne (50-70%) but a much smaller contributions to ultrafi ne par-
ticle mass. However, their work demonstrated that semi-metallic and low metallic
brake linings generated high numbers of particles smaller than 100 nm, particularly
under harsh braking conditions.
Signifi cant amounts of ultrafi ne particles are produced at the road- tyre interface,
as observed in a road simulator study (Dahl et al. , 2006). The particles most
likely consisted of mineral oils from the softening fi ller and fragments of the
carbon - reinforcing fi ller material (soot agglomerates). The mean particle number
diameters were between 15 and 50 nm and emission factors were found to be
3.7

1
respectively. This means that even though emission factors for particles originating
from the road-tyre interface were 10 2 - 10 3 lower than those of current light-duty
vehicles, the road-tyre interface might possibly be a signifi cant contributor to
nanoparticle emissions when cleaner vehicles are used. The characteristics of
both exhaust and non-exhaust emissions of on-road vehicles are summarized in
Table 5.1 .

1 0 11 and 3.2 1 0 12 particles vehicle − 1 k m − 1 at speeds of 50 and 70 km h

−

× ×

5.2.1.2 Indoor Sources

Indoor nanoparticle concentration levels are also infl uenced by vehicular traffi c, as
the main outdoor source in urban areas, and by some major indoor sources, particu-
larly cooking and smoking.