Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 181

from 50 to 300 m). Very recently, large studies of spatial variation of particle numbers
concluded that particle number concentrations at various urban locations could not
be reliably estimated from central site measurements (Puustinen et al. , 2007 ; Hoek
et al. , 2008 ).
Measurements of vertical profi les of ultrafi ne particles on urban roads has dem-
onstrated the signifi cant roles played by the microscale topography and the wind
direction (parallel or perpendicular to the street) in the dispersion of vehicle emis-
sions (Figure 5.7). Higher concentrations were found within the building envelope
(Morawska et al. , 1999b) than at rooftops, where particle counts were decreased by
a factor fi ve compared to street level concentrations (Vä kev ä et al. , 1999 ) to reach
levels close to urban background concentrations. The decrease of particle number
concentrations with height is larger when the wind fl ow is parallel to the street than
in case of a vortex within the street canyon; the latter situation favours the upwards
advection of vehicle emissions (Longley et al. , 2004). Detailed examinations of
vertical profi les showed that particle number concentrations fi rst increased to reach
maximum concentrations around 2-7 m above the ground (Zhu and Hinds, 2005;
Kumar et al. , 2008) before decreasing to rooftop levels (Longley et al. , 2004 ; Zhu
and Hinds, 2005; Kumar et al. , 2008). Dry deposition and diffusion (controlled by
turbulence) are thought to be important processes in explaining vertical profi les
within street canyons, unlike horizontal profi les which are largely explained by
dilution by the wind speed (Zhu and Hinds, 2005; Kumar et al. , 2008 ).
Unlike primary NPs that are emitted from a source (mobile or stationary) and
subsequently diluted (and transformed) in the atmosphere, secondary NPs are
formed within the atmosphere and the spatial scale for nucleation will depend on
local and regional atmospheric characteristics. Nucleation events may occur simul-
taneously over large geographical areas of hundreds of kilometres if favourable
conditions for nucleation are homogeneously distributed over large distances
(gaseous precursor concentrations, meteorological concentrations, etc.). The exis-
tence of these so-called regional or large scale nucleation events was demonstrated

WIND

VORTEX VORTEX

B B L L N N B B L L N N

Figure 5.7 Schematic representation of a vortex situation in street canyon.