Page 45 - Fundamentals of Air Pollution 3E

24 2. The Natural versus Polluted Atmosphere

of water from sea spray; wind-borne pollen, fungi, molds, algae, yeasts,
rusts, bacteria, and debris from live and decaying plant and animal life;
particles eroded by the wind from beaches, desert, soil, and rock; particles
from volcanic and other geothermal eruption and from forest fires started
by lightning; and particles entering the troposphere from outer space. As
mentioned earlier, the true natural background concentration will never
be known because when it existed humans were not there to measure it,
and by the time humans started measuring particulate matter levels in the
air, they had already been polluting the atmosphere with particles resulting
from their presence on earth for several million years. The best that can
be done now is to assume that the particulate levels at remote places—-the
middle of the sea, the poles, and the mountain tops—approach the true
background concentration. The very act of going to a remote location to
make a measurement implies some change in the atmosphere of that remote
location attributable to the means people used to travel and to maintain
themselves while obtaining the measurements. Particulate matter is mea-
sured on a dry basis, thereby eliminating from the measurement not only
water droplets and snowflakes but also all vapors, both aqueous and or-
ganic, that evaporate or are desiccated from the particulate matter during
the drying process. Since different investigators and investigative processes
employ different drying procedures and definitions of dryness, it is im-
portant to know the procedures and definition employed when comparing
data.
There are ways of measuring particulate matter other than by weight per
unit volume of air. They include a count of the total number of particles
in a unit volume of air, a count of the number of particles of each size
range, the weight of particles of each size range, and similar measures
based on the surface area and volume of the particles rather than on their
number or weight. Some particles in the air are so small that they cannot
be seen by an optical microscope, individually weighing so little that their
presence is masked in gravimetric analysis by the presence of a few large
particles. The mass of a spherical particle is
w = §7rpr 3 (2-2)
where w is the particle mass (gm), r is the particle radius (cm), and p is
3
the particle density (gm/cm ).
The size of small particles is measured in microns (/-em)- One micron is
one-millionth of a meter or 10,000 A (angstrom units)—the units used to
measure the wavelength of light (visible light is between 3000 and 8000 A)
(Fig. 2-2) (2). Compare the weight of a 10-ju.m particle near the upper limit
of those found suspended in the air and a 0.1-jum particle which is near
the lower limit. If both particles have the same density (p), the smaller
particle will have one-millionth the weight of the larger one. The usual
gravimetric procedures can scarcely distinguish a 0.1-^tm particle in the