Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 402

TABLE 15.2
Approximate Composition of Dry Air (by Volume)

Concentration

Component Symbol Percent (%) Parts per Million (ppm)

Nitrogen N 2 78.084 780,840

Oxygen O 2 20.9476 209,476

Argon Ar 0.934 9340

Carbon dioxide CO 2 0.0314 314

Neon Ne 0.001818 18.18

Helium He 0.000524 5.24

Methane CH 4 0.0002 2

Sulfur dioxide SO 2 0-0.0001 0-1

Hydrogen H 2 0.00005 0.5

Krypton Kr 0.0002 2

Xenon Xe 0.0002 2

Ozone O 3 0.0002 2

Note: Figures are taken from p. 6.1 of ASHRAE's Handbook of Fundamentals ,

based on an atomic weight of carbon of 12.0000. The handbook also
reports that the molecular weight of dry air is 28.9645 g/mol based on
the carbon-12 scale.

15.5.3 p artiCulate m atter
Typically, in actual practice, the terms particulates (or particles ) and particulate matter are used
interchangeably. According to 40 CFR 51.100-190, particulate matter is defined as any airborne
finely divided solid or liquid material with an aerodynamic diameter smaller than 100 micrometers
(micro = 10 -6 ). Along with gases and water vapor, Earth's atmosphere is literally a boundless arena
for particulate matter of many sizes and types. Atmospheric particulates vary in size from 0.0001 to
10,000 microns. Particulate size and shape have direct bearing on visibility. For example, a spheri-
cal particle in the 0.6-micron range can effectively scatter light in all directions, reducing visibility.
The types of airborne particulates in the atmosphere vary widely, with the largest sizes derived
from volcanoes, tornadoes, waterspouts, burning embers from forest fires, seed parachutes, spider
webs, pollen, soil particles, and living microbes. The smaller particles (the ones that scatter light)
include fragments of rock, salt and spray, smoke, and particles from forested areas. The largest por-
tion of airborne particulates is invisible. They are formed by the condensation of vapors, chemical
reactions, photochemical effects produced by ultraviolet radiation, and ionizing forces that come
from radioactivity, cosmic rays, and thunderstorms. Airborne particulate matter is produced either
by mechanical weathering, breakage, and solution or by the vapor-to-condensation-to-crystalliza-
tion process (typical of particulates from a furnace of a coal-burning power plant).
We know very well that anything that goes up must eventually come down. This is typical of
airborne particulates also. Fallout of particulate matter depends, obviously, mostly on their size—
less obvious on their shape, density, weight, airflow, and injection altitude. The residence time of
particulate matter also is dependent on the atmosphere's cleanup mechanisms (formation of clouds
and precipitation) that work to remove them from their airborne suspended state. Some large par-
ticulates may only be airborne for a matter of seconds or minutes. Intermediate sizes may be able
to stay afloat for hours or days. The finer particulates may stay airborne for a much longer duration:
days, weeks, months, and even years.