Environmental Engineering Reference
In-Depth Information
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Dp (nm)
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Figure 5.5 Particle number size distributions measured at Harwell, UK; days of nucleation:
(a) 29 May 2001; (b) 10 September 2001.
undergone a gas-to-particle conversion at nighttime low temperature and high rela-
tive humidity (Tolocka et al. , 2004b ).
Bursts of photochemically-formed particles have been observed in various envi-
ronments with various degrees of pollution such as the marine boundary layer and
coastal sites (Allen et al. , 1999 ; O ' Dowd et al. , 2002a, 2002c), in polar areas
and boreal forests (Mä kel ä et al. , 1997); in Mediterranean forests (Kavouras and
Stephanou, 2002), in continental areas (Birmili et al. , 2003), in industrial plumes
(Brock et al. , 2002), in urban areas (Woo et al. , 2001 ; Alam et al. , 2003) and in the
tropopause region (Young et al. , 2007). Recently Kulmala et al. (2004) and Holmes
(2007) provided comprehensive reviews of experimental observations of new par-
ticle formation and growth in the atmosphere spanning the various environments
and possible mechanisms. They underline that processes responsible for new par-
ticle formation by nucleation and subsequent growth depend on the environment
and are still not entirely understood.
Various nucleation mechanisms have been proposed and demonstrated in the
literature. The most widely studied are the binary water/sulfuric acid nucleation
(Vehkamä ki et al. , 2002) and the ternary water/sulfuric acid/ammonia nucleation
(Napari et al. , 2002 ; Merikanto et al. , 2007). Observations made in the free tropo-
sphere were consistent with the binary water/sulfuric acid nucleation theory
(Kulmala et al. , 2004), while it is likely that only ternary nucleation would be able
to explain particle formation in the low troposphere (Kulmala et al. , 2000 ; Korhonen
et al. , 2003). Ion-induced nucleation (Yu and Turco, 2000) and the condensation of
semi-volatile organic compounds O' Dowd et al. , 2002b; Kavouras and Stephanou,
2002) are other demonstrated mechanisms. New particle formation from semi-
volatile organic compounds may include biogenic emissions, such as emissions of
organoiodine compounds from seaweeds at low tides (Allen et al. , 1999 ; O ' Dowd
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