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take-off from aircraft at airports [
87
,
88
], ship journeys from ports or harbours [
89
],
construction, demolition or recycling of concrete [
29
,
90
,
91
] or cooking [
92
],
biomass burning, fuel combustion during gardening, waste incineration, agriculture
processes, cigarette smoke and fugitive emissions [
23
]. We have not covered these
sources in detail here as their influence is either more localised or site specific. Their
individual contribution is likely to be relatively small in urban areas in comparison
with strong nanoparticle sources (e.g. road vehicles). Figure
2
illustrates results of
two recent source apportionment studies indicating typical contributions of various
sources towards the total PNCs along the roadside at Marylebone road in London
[
19
] and urban background site in Barcelona [
18
]. Further details on aforemen-
tioned sources can be seen elsewhere [
2
,
21
,
23
].
5 Airborne Nanoparticle Concentrations in European Cities
A review of recent nanoparticle studies is carried out, covering about 45 different
locations within 30 and 15 European cities and countries, respectively. The
reviewed PNC data have been classified into two categories: (1) roadside (i.e.
measurement locations situated along the freeways, roadsides, curbsides,
crossroads in city centres or street canyons) and (2) urban background (i.e. mea-
surement locations situated at 10s of metres away from the roads, highways or at the
rooftops of buildings). Table
1
provides a comprehensive summary of the name of
country, study location and year, measured size range and instrument used along
with the original source of this information. Figures
3
and
4
give a summary of
average PNCs in roadside and urban background environments in various European
cities, respectively. Birmingham (1.70
10
4
cm
3
)
sites in the UK and Switzerland, respectively, appear to demonstrate the most
polluted roadside sites; Zurich (3.50
10
5
cm
3
) and Zurich (8.0
10
4
cm
3
) also falls in the same category
for an urban background site, although these are now relatively old studies and it is
likely that PNC have decreased since that time. Antwerp (1.24
10
4
cm
3
)
and Prague (0.72
10
4
cm
3
) seem to have least polluted roadside and urban
background sites, respectively. Average PNCs at roadside are about 2.4 times
higher than the average PNCs at the urban background sites. This ratio is based
on a global average over all the European sites and is comparable to those found
during individual local studies. For instance, a recent study by Reche et al. [
93
]
reported PNCs at roadside (Marylebone Road) and urban background (North
Kensington) sites in London; their roadside to background PNC ratio turns out to
be about 1.83.
Average roadside PNCs (i.e. 3.82
0.91
10
4
cm
3
) show a great range of
variability, showing over an order of magnitude (~14 times) difference between the
minimum and maximum PNCs (Fig.
4
). This difference, however, comes down to
~5 times if we ignore the PNC values measured at the Birmingham site during
1996-1997. This variability can be explained by many dissimilar factors acting on
different sites such as: position of sampling head, type of fuel used in the vehicles in
3.25
