Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 176

Recent research using aerosol mass spectrometry and measurements of
hygroscopicity and volatility (Smith et al. , 2005 ; Sakurai et al. , 2005 ) showed that
new particles (6-15 nm) in Atlanta, USA, were entirely made of ammonium sulfate
since no other compounds were detected in these small particles. This fi nding is in
agreement with either ternary nucleation of particles or rapid neutralization of
sulphuric acid by atmospheric ammonia. Evidence of new particle formation from
semi-volatile organic compounds has also been acquired in various atmospheres
(Section 5.2.2). Zhang et al. (2004a) identifi ed the species that contributed to the
condensational growth of new particles larger than 33nm in Pittsburgh, USA,
using an Aerodyne aerosol mass spectrometer. Sulfate was always the fi rst species
that contributed to the growth and NPs appeared to be acid during the initial stage
of nucleation. They gradually become neutralized by ammonia (10- 40 minutes
after sulfate). Finally, the contribution of organics often commenced around 11:00
when the photochemistry was more intense and mainly oxygenated organic com-
pounds were involved in the growth process during the latter part of the event.
This fi nding is in agreement with that of Wehner et al. (2005), who showed that
sulfuric acid largely explains the growth of NPs until their diameter is around
10-20nm; before the condensation of secondary organic compounds. Oxidation
products of terpenes have been identifi ed as possible secondary organics involved
in the growth process. Laaksonen et al. (2008) demonstrated that during and after
nucleation in Hyytiä l ä, Finland, secondary organic compounds, likely oxidation
products of terpenes, were a major constituent of NPs, in agreement with the
fi nding of Boy et al. (2003) .
It is currently known that NPs from diesel exhaust emissions are of two modes,
a nucleation mode below 30nm (sulfate and/or hydrocarbons) and a soot mode
around 60nm (elemental carbon and adsorbed organic compounds, metals). The
relative contribution of sulfate and hydrocarbons in the nucleation mode depends
on many parameters (Section 5.2.1.1). Meyer and Ritovski, (2007) demonstrated,
using a volatilization and humidifi cation tandem differential mobility analyser
(VH-TDMA), that ternary nucleation of sulfuric acid, ammonia and water played
a role in the initial formation of diesel NPs produced at high loads, while at low
loads diesel NPs were hydrophobic, suggesting that they were predominantly made
of hydrocarbons. Schneider et al. (2005) showed that under specifi c conditions (high
fuel sulfur content, high engine load) sulfate represented 90% in mass of nucleation
particles (above 20 nm) emitted by a light duty diesel vehicle. On the other hand,
much recent research has shown that lubricating oil hydrocarbons contribute
greatly to the organic fraction of NPs emitted by heavy duty diesel vehicles. Tobias
et al. (2001) and Sakurai et al. (2003) showed that branched alkanes and alkyl sub-
stituted cycloalkanes from lubricating oil and unburned fuel represented more than
95% in mass of particles ranging from 25 to 60 nm, while the sulfate was less than
5% in mass.
A few studies have focused on nanoparticle emissions from petrol vehicles.
Sodeman et al. (2005) examined the composition of individual particles ranging
from 50 to 300 nm (aerodynamic diameters) emitted by 28 light duty petrol vehicles
using an Ultrafi ne Aerosol Time - of - Flight Mass Spectrometer (UF - ATOFMS).
Particles below 100nm contained a large amount of elemental carbon and the