Page 43 - Fundamentals of Air Pollution

22 2. The Natural versus Polluted Atmosphere

TABLE 2-2
The Gaseous Composition of Unpolluted Air
(Wet Basis)
ppm (vol) Mg/m 3
Nitrogen 756,500 8.67 x 10 8
Oxygen 202,900 2.65 x 10 8
Water 31,200 2.30 x 1Q 7
Argon 9,000 1.47 x 10 7
Carbon dioxide 305 5.49 x 10 s
Neon 17.4 1.44 x 10 4
Helium 5.0 8.25 x 10 2
Methane 0,97-1.16 6.35-7.63 x 10 2
Krypton 0.97 3.32 x 10 3
Nitrous oxide 0.49 8.73 x 10 2
Hydrogen 0.49 4.00 x 10 1
Xenon 0.08 4.17 x 10 2
Organic vapors ca. 0.02 —

as ppm, one is in doubt as to whether a volume or weight basis is intended.)
To avoid confusion caused by different units, air pollutant concentrations
in this book are generally expressed as micrograms per cubic meter of air
3
(/x,g/m ) at 25°C and 760 mm Hg, i.e., in metric units. To convert from units
3
of ppm (vol) to /xg/m , it is assumed that the ideal gas law is accurate under
ambient conditions. A generalized formula for the conversion at 25°C and
760 mm Hg is

where MW equals molecular weight. For convenience, conversion units
for common pollutants are shown in Table 2-3.
A minor problem arises in regard to nitrogen oxides. It is common practice
to add concentrations of nitrogen dioxide and nitric oxide in ppm (vol) and
express the sum as "oxides of nitrogen." In metric units, conversion from
3
ppm (vol) to /ig/m must be done separately for nitrogen dioxide and nitric
oxide prior to addition.

III. PARTICULATE MATTER

Neither Table 2-1 nor Table 2-2 lists among the constituents of the air
the suspended particulate matter that it always contains. The gases and
vapors exist as individual molecules in random motion. Each gas or vapor