Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 172

ment station (Charron et al. , 2004) except in the case of secondary particle forma-
tion (Charron et al. , 2008b), while sulphur dioxide and PM 2.5- 10 concentrations were
substantially infl uenced. This suggests that number emissions from the coal-fi red
power plant (possibly peaking around particle diameters of 50- 100 nm as suggested
by available studies) are much smaller than those from vehicles on nearby roads.
The emissions of the gas burning plant may lead mostly to particles smaller than
11 nm that are not measured by the instrument deployed.
Biomass burning is another signifi cant source of NPs. Biomass burning includes
various types of natural and anthropogenic combustion, such as forest fi res,
agricultural burning, logging and land clearing slash and waste burning (Wardoyo
et al. , 2007 ). Morawska et al. (1999a) made approximately 200 measurements
during episodes of vegetation burning. They observed bimodal distributions with
an Aitken mode in the number size distribution between 40- 60 nm that was
considered to be representative of this source. Wardoyo et al. (2007) investigated
particle number size distributions emitted from the combustion of several grass
species. Differences were observed between controlled laboratory conditions and
airborne measurements. The latter showed smaller particles ranging from 30- 60 nm
during fast burning conditions and 60-210 nm during slow burning conditions, while
the former showed particles with diameters up to 80 nm depending on the age of
the smoke.

5.2.1.4 Natural Sources

The few studies available suggest that natural sources contribute little to primary
NPs except in remote environments. A recent study (Clarke et al. , 2006 ) has
shown that in the remote marine atmosphere 60% of particles smaller than
100 nm in diameter are sea salt produced by bubbles from coastal oceanic breaking
waves.
Pollen fragments and virus exist as airborne agglomerates of sizes below 100 nm
in diameter that contribute to the spread of disease or may be the cause of allergies
(Biswas and Wu, 2005 ).

5.2.2 Secondary Sources

NPs are also formed in the atmosphere from the nucleation of vapours of low vola-
tility. Semi-volatile gases involved in such processes may be either of biogenic or
anthropogenic origin and may be either inorganic (mainly sulfuric acid) or organic
compounds (mostly not yet identifi ed). Semi-volatile gases are formed by oxidation
of other gases in the atmosphere (sulfur dioxide, volatile organic compounds) and
reactants may include photo-oxidants such as hydroxyl radicals. An example of
particle number size distributions measured at Harwell, UK, on days with a large
scale nucleation event is presented in Figure 5.5. The occurrence of nucleation
events during the midday period and warmer seasons confi rms that photochemistry
plays a central role in these processes. Note that bursts of small particles of 50-
90 nm in diameter have also been observed during nighttime periods in urban areas
and characterization by real-time single particle mass spectrometry showed that
these ultrafi ne particles were entirely comprised of ammonium nitrate that has