Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 184

25000

SSNE
LSNE

20000

15000

10000

5000

0

0123456789 0 1 2
Month

Figure 5.9 Monthly maximum number concentration of particles ranging from 11 to 30 nm
measured on days of new particle formation (1999-2001 dataset). SSNE: small scale nucle-
ation events; LSNE: large scale nucleation events. (Adapted from Charron et al. , 2008a)

refl ects the poorer dispersion during the winter or the more effi cient atmospheric
mixing during the hotter months of the year (Harrison and Jones, 2005). The favour-
able nanoparticle formation from vehicle exhaust at low temperature (Charron and
Harrison, 2003) and the shift of low volatility species to their gaseous phase during
the warmer period (Kuhn et al. , 2005) may also explain this urban seasonal pattern.

5.6 Measurement Methods for Atmospheric Nanoparticles

NPs present signifi cant measurement challenges, although for some descriptors,
such as number concentration, good quality methods have been available for many
years. Firstly, the point needs to be made that, depending upon the reason for the
measurements, NPs may be measured in terms of their number, surface area or mass
(Harrison et al. , 2000b). Largely because effective methods have been available for
number measurement, this has been far the most commonly used means of charac-
terisation, with surface area and mass commonly estimated by transformation of the
number size distribution data. There are, however, methods available capable of
directly measuring either surface area or mass. Since losses by diffusion or volatiliza-
tion could occur when sampling NPs, sampling protocols, including the quantifi ca-
tion of losses, should be well defi ned. In particular, the lengths and confi gurations of
sample inlets and sampling tubes are important parameters. Recent experimental
data showed that penetration effi ciencies dramatically decreased for NPs smaller
than 20 nm and losses increased when lengths of sampling tubes increased (Kumar
et al. , 2008). Theoretical calculations are given elsewhere (Hinds, 1999).
A summary of methods prepared in relation to occupational hygiene exposure
measurements appears in Table 5.2. A full section on measurement methods for
airborne NPs is also available in Chapter 8.