Properties of Air and Other Gases

The analysis of dry atmospheric air varies with location, altitude, time of year, and other factors. Table 4.4 gives the molecular weights of the constituents and the volumetric and gravimetric analyses.

For general engineering work for altitudes up to 1500 m it is sufficiently accurate to use the following:

By volume: By weight:

02 = 21 %, N2 = 79 % 02 = 23%, N2 = 77 %

The above values are based on the assumption that argon is combined with ni­trogen, adjusting the molecular weight to 28.16. Other gases present in the atmo­sphere air are normally ignored, as these represent less than 0.003% (by volume, 27.99 ppm). Table 4.5 provides some basic information on these trace gases.

The gases also have other constituents mixed with them, typical ones being: dusts, pollens, bacteria, viruses, mold spores, smoke particles, and the products of industrial activity such as S02, H2, and S. Volcanic activity also adds various gases and dusts to the atmosphere.

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