Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 190

which measure the charge associated with particles depositing at different points
within the differential mobility analyser. This charge is converted to a current which
can then be related to the number of particles depositing per unit time. Because
such instruments are capable of measuring different size ranges simultaneously, as
there is no change in the voltage with time, they are capable of very fast response
measurements of airborne particle number size distributions. Although developed
primarily for engine exhaust particle measurement applications, such instruments
are applicable to the measurement of atmospheric particle size distributions. In
particular, the UFP 330 instrument has been developed for routine monitoring
stations since it is easy to use and needs little maintenance. The six size classes
(20- 30, 30 - 50, 50 - 70, 70 - 100, 100 - 200, 200 nm) measured by the UFP 330 and data

from classical mobility spectrometers (SMPS/DMPS) have been compared. In
general, a good agreement was found for particles ranging from 70 to 200 nm and
the largest disagreements were found for particles below 30 nm and for those above
200 nm (Zschoppe et al. , 2007 ; Hillemann et al. , 2007 ; Wehner et al. , 2007 ).

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5.6.2 Surface Area

Although not intuitively obvious, there is more than one way of expressing the
surface area of the particles within an aerosol. One means of expression is simply
the total geometric surface of the particles. The meaning of surface area expressed
in such a way is straightforward if particles have totally smooth surfaces but
becomes far more complex when the surfaces are rough. As an analogy, consider
measuring the coastline of the British Isles. If this were measured off a low resolu-
tion map, then it would appear much shorter than if it were measured using a much
fi ner scale from much higher resolution maps. In other words, coarse resolution
maps obscure a lot of the detail which can nonetheless be measured if the spatial
resolution is increased. The same is likely to be true of airborne particles with
uneven surfaces. Consequently, even when expressing a simple geometric surface
area, there is always an underlying complexity unless the particles have entirely
smooth surfaces.
The other way of expressing surface area derives from its measurement by
attaching species to the surface whose concentration can be determined. The rate
of attachment of absorbing or condensing species is a simple function of particle
surface area only for very small particles where the dominant process is molecular
bombardment of their surface (Hinds, 1999). Larger particles, however, maintain a
rather static boundary layer of gas molecules above their surface and access of
adsorbing or condensing molecules depends upon diffusion through that laminar
boundary layer prior to attachment to the particle surface. This diffusion process
is not a simple function of particle surface area and, consequently, refl ecting this
change of physical processes, the mathematical expressions describing gas adsorp-
tion of condensation on particle surfaces is strongly particle size dependent.
However, the situation is simpler for NPs as they lie within the regime described
by molecular bombardment processes and consequently surface area, be it expressed
geometrically or as inferred from molecular attachment processes, should be the
same.