Page 242 - Fundamentals of Air Pollution 3E

II. Analysis and Measurement of Particulate Pollutants 205

charging of particles and their separation by passage through an electrical
field (11). By incrementing the electrical field strength, progressively larger
charged particles may be removed from a flowing air stream. The change
in the amount of charge collected by a electrometer grid is then related to
the number of particles present in a particular size increment. Instruments
based on this principle yield a number size distribution.
Light-scattering properties of particles are also utilized to determine a
number size distribution (12). Individual particles interact with a light beam
and scatter light at an angle to the original direction of the beam. The
intensity of the scattered light is a function of the diameter and the refractive
index of the particle. Inlet systems are designed to dilute a particle-laden
air stream sufficiently to permit only one particle in the beam at a time.
The intensity of the scattered light, as measured by a photomultiplier tube,
is proportional to particle size. The number of electrical pulses of each
magnitude is accumulated in a multichannel analyzer. By sampling at a
known flow rate, the number of particles of different diameters are counted
with this type of instrument.
The chemical composition of particulate pollutants is determined in two
forms: specific elements, or specific compounds or ions. Knowledge of
their chemical composition is useful in determining the sources of airborne
particles and in understanding the fate of particles in the atmosphere.
Elemental analysis yields results in terms of the individual elements present
in a sample such as a given quantity of sulfur, S. From elemental analysis
techniques we do not obtain direct information about the chemical form
2
of S in a sample such as sulfate (S0 4 ~) or sulfide. Two nondestructive
techniques used for direct elemental analysis of particulate samples are
X-ray fluorescence spectroscopy (XRF) and neutron activation analysis
(NAA).
XRF is a technique in which a sample is bombarded by x-rays (13). Inner
shell electrons are excited to higher energy levels. As these excited electrons
return to their original state, energy with wavelengths characteristic of each
element present in the sample is emitted. These high-energy photons are
detected and analyzed to give the type and quantity of the elements present
in the sample. The technique is applicable to all elements with an atomic
number of 11 (sodium) or higher. In principle, complex mixtures may be
analyzed with this technique. Difficulties arise from a matrix effect, so that
care must be taken to use appropriate standards containing a similar matrix
of elements. This technique requires relatively expensive equipment and
highly trained personnel.
NAA involves the bombardment of the sample with neutrons, which
interact with the sample to form different isotopes of the elements in the
sample (14). Many of these isotopes are radioactive and may be identified
by comparing their radioactivity with standards. This technique is not quite
as versatile as XRF and requires a neutron source.