Environmental Engineering Reference
In-Depth Information
Atmospheric NPs may be either solid or liquid. They may consist partially or
totally of semi-volatile material and may be involved in gas- to - particle equilibria
during their evolution within the atmosphere. Atmospheric NPs may be perfect
spheres or of irregular shape and may also be fractal-like agglomerates. Additionally,
particles smaller than 100 nm may occupy different modes (nucleation mode for
the smaller ones, Aitken mode and accumulation mode for the larger ones) and, as
a consequence, they may have different sources or physical and chemical properties
if their size is below 10 nm or close to 100 nm, as discussed throughout the text.
5.2
Sources of Atmospheric Nanoparticles
As with other air pollutants, NPs can be directly emitted in the atmosphere or can
be formed within the atmosphere itself. The former ones are called primary NPs
while the latter are called secondary NPs.
Literature review suggests that a major source of outdoor NPs is emissions from
vehicles and a second major source for ultrafi ne particles in outdoor air appears to
be photochemically produced particles. Major primary sources of ultrafi ne particles
in the indoor atmosphere appear to be smoking and cooking.
5.2.1
Sources of Primary Nanoparticles
Combustion processes generate large numbers of particles (Morawska and Zhang,
2002). According to AQEG (2005), 94% of emissions of particles less than 100 nm in
diameter (by mass) for 1998 in the United Kingdom corresponded to combustion
processes; including two-thirds from road transport as the major contributor (62%).
The contribution of road traffi c is much larger for ultrafi ne particles than for PM 10 .
Recent work has also shown that some indoor sources generate very high levels of NPs.
5.2.1.1
Mobile Sources
Traffi c is the major source of primary outdoor NPs. The contribution of traffi c to
particle number concentrations in urban areas is clearly underlined by large con-
centrations and diurnal variations of particle numbers measured near roads or in
streets canyon (Harrison et al. , 1999 ; Wehner et al. , 2002 ) as illustrated in Figure 5.1
and discussed below. In roadside measurement studies, particle number size distri-
butions of traffi c infl uenced aerosols are generally bimodal with the strongest peak
generally between 10 and 40 nm (Morawska et al. , 1999a ; Moln à r et al. , 2002 ; Wehner
et al. , 2002), with a second mode often merged into the fi rst one and around 50-
90 nm (Figure 5.2). Diesel engines are major sources of NPs; while petrol engines
have also been reported to emit NPs (Kittelson, 1998).
Variable particle number emission factors* are reported in the literature since
they depend on a large number of parameters, such as the dilution ratio, the air
* An emission factor defi nes the relationship between the amount of pollution produced by a source and a parameter
indicating the source activity, for example for mobile sources either the vehicle distances travelled or the amount of
fuel burned.
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