Environmental Engineering Reference
In-Depth Information
Table 5.2 ( continued )
Metric
Devices
Remarks
Mass directly
Size selective
static sampler
The only devices offering a cut-point around
100 nm are cascade impactors (Berner-type
low-pressure impactors or Micro-orifi ce impac-
tors). Allows gravimetric and chemical analysis of
samples on stages below 100 nm.
Sensitive real-time monitors such as the Tapered
Element Oscillating Microbalance (TEOM) may
be useable to measure nanoaerosol mass
concentrations on-line, with a suitable size
selective inlet.
Real-time size-selective (aerodynamic diameter)
detection of active surface area concentration
giving aerosol size distribution. Mass concentra-
tion of aerosols can be calculated, only if particle
charge and density are assumed or known.
Size-selective samples may be further analysed
off-line (as above).
Real-time size-selective (mobility diameter)
detection of number concentration, giving
aerosol size distribution. Mass concentration of
aerosols can be calculated, only if particle shape
and density are known or assumed.
TEOM
Mass by
calculation
ELPI
SMPS
.
5.6.1
Particle Number Concentration
Condensation particle counters are widely used to measure the total number count
of particles in the atmosphere. The instruments operate by drawing particles
through a zone within the instrument which is saturated with respect to a condens-
able vapour (Figure 5.10). Subsequent cooling of the fl ow leads to vapour conden-
sation on the particles and consequent growth to a size where they can be measured
by optical techniques.
For many years, most condensation particle counters used butanol as the con-
densable vapour; more recently a number of instruments based upon water vapour
have been developed. The lower size limit of such instruments is typically around
three nanometres (although some have lower cut points larger than this), which is
due to the limitations imposed by the Kelvin effect upon condensation on highly
curved surfaces. The particles, which have grown by condensation, are measured
optically either by counting pulses of scattered laser light as the particles exit the
condensation chamber through an optical chamber or, alternatively, at higher con-
centrations in a photometric mode in which light obscuration by the particle stream
is measured. As noted above, the technique is sensitive down to very small particle
sizes and has no upper limit other than that imposed by the entry of particles into
the sensing zone of the instrument. Consequently, when applied to the atmosphere,
it typically measures particles between a few nanometres diameter and perhaps
 
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