In this chapter we consider those gases present at constant concentrations in air; these are nitrogen, oxygen, and the noble gases (except radon). These gases occur in essentially constant relative proportions everywhere in the atmosphere up to an altitude of about 95 km. The relative proportions of these gases dissolved in seawater are different than in air and vary according to conditions. Oxygen varies because it is biologically produced and consumed, and several physical processes affect the relative concentrations of all of them. Investigation of these variations leads to important insights into several oceanic processes.

Air always contains water vapor. At high altitudes and at such cold high regions as the South Pole the water vapor content may be less than 1 part in 105 (mole fraction < 0.00001),while in the humid tropics it may exceed 1 part in 20 (mole fraction > 0.05).Water vapor often must be accounted for in various calculations dealing with components of air.

Other gases of some interest in the marine environment, such as radon, carbon dioxide, carbon monoxide, methane, ammonia, nitrous oxide, dimethyl sulfide, hydrogen sulfide, etc. are all more or less variable in concentration and some will be discussed in later chapters.

General considerations

The concentrations in air of the six gases considered here (Table 5.1) are listed as the mole fraction for each gas, which is very nearly the fractional composition by volume.