Page 46 - Fundamentals of Air Pollution

III. Participate Matter 25

presence of a 10-/nm particle. To measure the entire size range of particles
in the atmosphere, several measurement techniques must therefore be
combined, each most appropriate for its size range (Table 2-4). Thus the
smallest particles—those only slightly larger than a gas molecule—are mea-
sured by the electric charge they carry and by electron microscopy. The
next larger size range is measured by electron microscopy or by the ability
of these particles to act as nuclei upon which water vapor can be condensed
in a cloud chamber. (The water droplets are measured rather than the
particles themselves.) The still larger size range is measured by electron or
optical microscopy; and the largest size range is measured gravimetrically,
either as suspended particles separated from the air by a sampling device
or as sedimented particles falling out of the air into a receptacle.
By measuring each portion of the particle size spectrum by the most
appropriate method, a composite diagram of the size distribution of the
atmospheric aerosol can be produced. Figure 2-3 shows that there are
separate size distributions with respect to the number, surface area, and
volume (or mass) of the particles. The volume (mass) distribution is called
bimodal because of its separate maxima at about 0.2 and 10 /urn, which result
from different mechanisms of particle formation. The mode with the
0.2-/Ltm maximum results from coagulation and condensation formation
mechanisms. These particles are created in the atmosphere by chemical
reaction among gases and vapors. They are called fine particles to differenti-
ate them from the particles in the 10-/u,m maximum mode, which are called
coarse. These fine particles are primarily sulfates, nitrates, organics, ammo-
nium, and lead compounds. The mode with the 10-/u,m maximum are
particles introduced to the atmosphere as solids from the surface of the
earth and the seas, plus particles from the coagulation-condensation mode
which have grown larger and moved across the saddle between the modes
into the larger size mode. These are primarily silicon, iron, aluminum, sea
salt, and plant particles. Thus there is a dynamism that creates small parti-
cles, allows them to grow larger, and eventually allows the large particles
to be scavenged from the atmosphere by sedimentation (in the absence of
precipitation), plus washout and rainout when there is precipitation.
The majority of particles in the atmosphere are spherical in shape because
they are formed by condensation or cooling processes or they contain core
nuclei coated with liquid. Liquid surface tension draws the material in the
particle into a spherical shape. Other important particle shapes exist in the
atmosphere; e.g., asbestos is present as long fibers and fly ash can be
irregular in shape.
The methods just noted tell something about the physical characteristics
of atmospheric particulate matter but nothing about its chemical composi-
tion. One can seek this kind of information for either individual particles
or all particles en masse. Analysis of particles en masse involves analysis
of a mixture of particles of many different compounds. How much of