Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 179

aggregation; however, that may not be the case for very small particles because of
the possible competing effects of van der Waals attraction and Born repulsion
(Slama Lighty et al. , 2000 ).
To explain the evolution of freshly emitted small particles near roadways, one of
the most important processes is likely to be dilution with cleaner air. Thus, for
example, particles from traffi c mix upwards with less polluted air, leading to a
reduction in the number concentrations and, generally, a shift of the size distribu-
tion towards larger size diameters simply because the dilution air contains coarser
particles. Dilution of volatile gases at increased distances from the freeway may be
responsible for the shrinking of the smaller particles by evaporation to reach new
equilibrium, since the vapour phase concentrations are lower (Zhang et al. 2004b ;
Zhang and Wexler 2004 ) .
Different studies close to major highways or with models have drawn different
conclusions concerning the predominant process. However, as a common trend it
seems that condensation/evaporation and dilution are processes that have a more
signifi cant effect upon the particle size distribution than coagulation and deposition
(Gidhagen et al. , 2004a, 2004b; Zhang et al. , 2004b ; Zang and Wexler 2004 ), except
in particular conditions such as low wind speed and in the absence of turbulent
mixing when coagulation and deposition could contribute to, respectively, 10% and
50% of particles episodic losses (Gidhagen et al. , 2005 ). Also, Kerminen et al. (2007)
showed that coagulation may be signifi cant in the high concentrations of the
morning rush hour. Because condensation, evaporation, coagulation and dilution
alone fail to explain the evolution pattern of nanoparticle size distributions near
roadways, more complex scenarios involving the shrinking or fragmentation of NPs
have been proposed. Jacobson et al. (2005) suggest that small ( 15 nm) liquid NPs

shed semi-volatile organics almost immediately upon emission. This shrinking
enhances the coagulation rates of particles by over an order of magnitude.
Measurements in the urban atmosphere of Kawasaki, Japan, supported this hypoth-
esis (Fushimi et al. , 2008 ).

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5.5 Atmospheric Concentrations

5.5.1 Spatial Variations

Because of their nature (mostly volatile) and their small size, ultrafi ne particles
have a shorter lifetime than coarser particles. As a consequence, particle number
concentrations generally exhibit a much stronger variability than particle mass
concentrations (Van Dingenen et al. , 2004 ; Weijers et al. , 2004 ). Van Dingenen et al.
(2004) observed a gradual decrease in particle number concentrations of particles
ranging from 10 to 30 nm moving from kerbside, to urban site, to rural site, to
background site in Europe. When they compared the annual average particle
number concentrations of these sites to their annual average PM 2.5 concentrations,
they observed a factor of 10 between the minimum concentrations and the maximum
concentrations for particle number and a factor of three for PM 2.5 . Observations
tend to show that the further the measurement site from vehicular emissions, the
lower the concentrations.