Page 42 - Fundamentals of Air Pollution

II. Unpolluted Air 21

precise composition of unpolluted air because by the time we had the
means and the desire to determine its composition, humans had been
polluting the air for thousands of years. Now even at the most remote
locations at sea, at the poles, and in the deserts and mountains, the air
may be best described as dilute polluted air. It closely approximates unpol-
luted air, but differs from it to the extent that it contains vestiges of diffused
and aged human-made pollution.
The real atmosphere is more than a dry mixture of permanent gases.
It has other constituents—vapor of both water and organic liquids, and
particulate matter held in suspension. Above their temperature of conden-
sation, vapor molecules act just like permanent gas molecules in the air.
The predominant vapor in the air is water vapor. Below its condensation
temperature, if the air is saturated, water changes from vapor to liquid.
We are all familiar with this phenomenon because it appears as fog or mist
in the air and as condensed liquid water on windows and other cold surfaces
exposed to air. The quantity of water vapor in the air varies greatly from
almost complete dryness to supersaturation, i.e., between 0% and 4% by
weight. If Table 2-1 is compiled on a wet air basis at a time when the water
vapor concentration is 31,200 parts by volume per million parts by volume
of wet air (Table 2-2), the concentration of condensable organic vapors is
seen to be so low compared to that of water vapor that for all practical
purposes the difference between wet air and dry air is its water vapor
content.
Gaseous composition in Tables 2-1 and 2-2 is expressed as parts per
million by volume—ppm (vol). (When a concentration is expressed simply

TABLE 2-1
The Gaseous Composition of Unpolluted Air
(Dry Basis)

ppm (vol) /ig/m 3
Nitrogen 780,000 8.95 x 10 8
Oxygen 209,400 2.74 x 10 8
Water
Argon 9,300 1.52 x 10 7
Carbon dioxide 315 5.67 x 10 5
Neon 18 1.49 x 10 4
Helium 5.2 8.50 x 10 2
Methane 1.0-1.2 6.56-7.87 x 10 2
Krypton 1.0 3.43 x 10 3
Nitrous oxide 0.5 9.00 x 10 2
Hydrogen 0.5 4.13 x 10 1
Xenon 0.08 4.29 x 10 2
Organic vapors ca. 0.02 —