Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 166

of the diesel vehicles (or over the diesel particle fi lter involving an oxidation cata-
lyst) when sulfur rich fuels are used (Maricq et al. , 2002 ; Vogt et al. , 2003 ; Vaaraslahti
et al. , 2004 ; Giechaskiel et al. , 2005 ; Ntziachristos et al. , 2005 ; Uhrner et al. , 2007 )
and preferentially during high speed operations (Maricq et al. , 2002 ; Vogt et al. ,
2003 ; Giechaskiel et al. , 2005 ; Ntziachristos et al. , 2005 ; Uhrner et al. , 2007 ). Strong
acceleration (Uhrner et al. , 2007), high engine rotational speed (Uhrner et al. , 2007 )
and high load (Vaaraslahti et al. , 2004) would also be parameters that infl uence the
nucleation of sulfates. High particle number concentrations are measured when
diesel vehicles are run with high sulfur fuel (Casati et al. , 2007 ; Maricq et al. , 2002 ;
Vogt et al. , 2003), but a large nucleation mode has also been observed with low
sulfur fuel (sulfur content < 40 ppm according to Ntziachristos et al. , 2005 ; < 7 ppm

according to Uhrner et al. , 2007 ).
In the absence of an oxidation catalyst, Maricq et al. (2002) measured particle
sizes in the exhaust of a light duty diesel lorry following a single log normal distri-
bution with a number mean diameter of 70-83 nm. They concluded that the oxida-
tion catalyst, whose role is to remove hydrocarbons and carbon monoxide, oxidizes
sulfur dioxide into sulfur trioxide, which is in turn transformed into sulfuric acid in
the presence of water. The nucleation of sulfuric acid as the exhaust cools in the
atmosphere may be favoured by the low specifi c particle surface area available for
condensation in the case of soot particle removal with a diesel particle fi lter (DPF).
The formation of NPs is also found to correlate positively with the lubricant
sulfur content (Vaaraslahti et al. , 2005 ). Kittelson et al. (2006c) compared two diesel
particulate matter abatement devices, the continuously regenerating diesel particle
fi lter (which consists of a diesel oxidation catalyst followed by an uncatalysed fi lter)
and the catalysed continuously regenerating trap (which consists of a diesel oxida-
tion catalyst followed by a catalysed fi lter). The fi rst device produced high concen-
trations of nuclei mode particles depending on the exhaust temperature (in
agreement with fi ndings of Vaaraslahti et al. , 2004), while the second reduced
number counts to levels which were not detectable. Grose et al. (2006) confi rmed,
based upon measurements of particle volatility and hydroscopicity, that NPs
emitted by a diesel engine equipped with a catalytic trap are primarily comprised
of sulfates.
There is also evidence that the nucleation of hydrocarbons can be responsible
for high nanoparticle concentrations in the exhaust of diesel vehicles. In the absence
of a continuously regenerating diesel particulate fi lter, the emissions of a heavy
duty diesel engine showed size distributions with a nucleation mode only at low
load, independent of the fuel sulfur content (sulfur levels of 2- 40 ppm) and inde-
pendent of the lubricant sulfur level, and NPs were thought to be mainly comprised
of hydrocarbons (Vaaraslahti et al. , 2004; 2005 ). R ö nkk ö et al. (2006) measured the
emissions of a heavy duty diesel vehicle equipped with an oxidation catalyst that
met the Euro III emission standards. They showed that at low engine torque hydro-
carbons have an important role in the nucleation processes, whilst at high torque
the processes would be sulfur-driven. The most likely sources of material for such
particles are engine lubricating oil partially burned during the combustion process
and unburned fuel (Tobias et al. , 2001 ). Sakurai et al. (2003) confi rmed that diesel
exhaust nanoparticle composition had a volatile component that comprised more