Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 182

by simultaneous measurements of the same nucleation event at different sites, such
as Melpitz/Leipzig in Germany (Birmili, 1999) and Vä rri ö /Hyyti ä l ä /Tahkuse in
Finland and Estonia (Vana et al. , 2004). During such events the growth of nucleated
particles as observed at a measurement site continues throughout the day, regard-
less of the wind direction. On the other hand, some nucleation events occur in a
relatively small domain in the atmosphere (a few kilometres) and are called small
scale nucleation events. Different types have been reported. Particles may be pho-
tochemically produced in a previously dispersed small plume of a few kilometres
in size, such as a sulfur dioxide rich plume from a power station (Brock et al. , 2002,
2003), an urban plume (Kerminen and Wexler, 1996; Brock et al. , 2003 ) or coastal
plume of biogenic iodine emissions (Allen et al. , 1999 ; O ' Dowd et al. , 2002a ). Small
scale nucleation events could also occur in a sunny region of the atmosphere sur-
rounded by clouds (Charron et al. , 2007 ).

5.5.2 Temporal Variations

The diurnal profi le of particle number concentrations at kerbside and urban centres
refl ects well that typical of traffi c generated pollutants (Figure 5.1). Weekdays and
weekends show different diurnal patterns with lower number concentrations at
weekends (Morawska et al. , 2002). Weekdays are characterized by a strong morning
rush-hour peak that is followed by a decrease in concentrations until a second,
much smaller, peak corresponding to the evening rush hour. Concentrations then
decline slowly throughout the night to a minimum at around 4 a.m. Saturdays and
Sundays do not exhibit the same diurnal pattern, even though the highest particle
numbers are often observed on Friday and Saturday nights (Charron and Harrison,
2003 ; Hussein et al. , 2005b). At Marylebone Road, London, a sharp peak that coin-
cides with heavy duty (diesel) traffi c is observed on Saturday mornings (Figure 5.1).
On weekday mornings and weekend nights, high emissions from heavy traffi c fl ows
are enhanced by poor dispersion (stable nocturnal layer), low temperatures and
possibly by a low pre-existing particles surface area.
In unoccupied conditions, indoor concentrations correlate with the outdoor con-
centrations but at a lower concentration. Both indoor and outdoor concentrations
were lowest during nighttime hours and showed a clear morning peak that coin-
cides with morning rush hour and also smaller peaks at lunch time and in the
evening (Hoek et al. , 2008). However, in occupied conditions, the diurnal pattern
of indoor nanoparticle concentrations is different from that of outdoor concentra-
tions and is more closely related to human indoor activities (Morawska et al. , 2003 ).
At certain urban sites and/or during the warmer seasons, another peak of total
particle number may be observed at midday due to photochemically produced
particles (Woo et al. , 2001). At remote/background sites such as Hyytiä l ä , Finland,
where new particle formation is frequent (Mä kel ä et al. , 2000), a diurnal pattern is
often observed with the highest concentrations at midday (around noon) and the
lowest concentrations during the night. This diurnal pattern of particle numbers is
associated with a diurnal pattern in particle size with the continuous and smooth
growth of newly-formed particles during the afternoon. At the rural site of Harwell,
UK, where new particle formation and local motorized traffi c explain well the