Page 302 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanoparticle Transport, Aggregation, and Deposition 287

Conventional diesel exhaust (in the absence of after-treatment) emits
5
10–100 times more particle mass and up to 10 times more particle
numbers than gas engines. For instance, soot produced from the burn-
ing of diesel fuel has been characterized as consisting of agglomerated
spherical particles with a mean diameter between 20 nm to several
microns. The primary particles here are homogeneous small particles
with a mean size distribution of 25 nm. Soot had a surface area of
2
2
175 m /g, compared to 11 m /g for commercial carbon black. The core of
the soot was characterized as containing disordered polycyclic aromatic
hydrocarbons, which have been reported to act as nuclei for soot for-
mation [117]. The small primary particle size and high surface area, and
presence of adsorbed hydrocarbons all contribute to the high reactivity
of these particles [115]. Furthermore, absorbed material in diesel
exhaust particulate matter is specifically responsible for adverse health
effects; particles in the smaller size fraction of the particulate matter
may have a larger fraction of absorbed material.
The size distribution of combustion-derived atmospheric nanoparti-
cles evolves as they are dispersed from the point source and is a func-
tion of the characteristics of the system and the particles [116]. System
properties that have been found to be particularly significant include the
meteorological conditions (wind speed, wind direction, atmospheric tem-
perature, and relative humidity), particle concentration, presence and
concentration of trace gases (e.g., NO ), and the concentration of mate-
x
rials that may induce coagulation, condensation, and evaporation
processes [113]. The variation in particle size depends on the balance
between growth through coagulation and condensation and shrinkage
by evaporation. Nanoparticles may aggregate either with each other
(self-coagulation) or with other larger background particles, a process
known as heteroaggregation [116]. Aggregation of atmospheric nanopar-
ticles is typically a rapid process [116] and accounts for the more sub-
stantial concentrations of atmospheric nanoparticles in the immediate
vicinity of combustion or other generation sources.
Given the dependence of particle size distribution on meteorological
conditions, or actually the “solution chemistry,” it is reasonable to expect
that the characteristics of the nanoparticles will have seasonal varia-
tions. Minoura et al. [113] observed that nanoparticles had on average
a larger peak diameter in the summer months compared to those in the
winter months. This difference may be attributed to a variety of factors
as has been discussed here. Additionally, it was found that nanoparti-
cles in the winter were determined to come from a point source, while
those in the summer were thought to come from both a point source and
through photochemical nucleation, which takes place primarily in
summer. The more favorable formation and growth conditions (e.g.,
higher humidity and particle concentrations) in summer months also